Reaction of siliceous fly ash in blended Portland cementpastes and its effect on the chemistry of hydrate phases and

pore solution

Einfluss der Reaktion von Steinkohleflugasche auf die Chemie der Hydratphasen und derPorenlösung eines flugaschereichen Mischzements

Der Naturwissenschaftlichen Fakultätder Friedrich-Alexander-Universität

Erlangen-Nürnberg

zur

Erlangung des Doktorgrades Dr. rer. nat.

vorgelegt von

Florian Deschneraus Nürnberg

Als Dissertation genehmigtvon der Naturwissenschaftlichen Fakultätder Friedrich-Alexander-Universität Erlangen-Nürnberg

Tag der mündlichen Prüfung: 03. Juli 2014

Vorsitzender des Promotionsorgans: Prof. Dr. Johannes Barth

Gutachter: Prof. Dr. Jürgen Neubauer

Prof. Dr. Christian Kaps

Dr. Frank Winnefeld

I

Acknowledgements

The work for the present thesis was carried out at the Swiss Federal Laboratories for Ma-terial Science and Technology (Empa) in cooperation with Schwenk Zement KG, BASFConstruction Chemicals GmbH, Steag Power Minerals GmbH and the GeoZentrum Nord-bayern, Chair for mineralogy at the Friedrich-Alexander University of Erlangen-Nuremberg.I acknowledge Schwenk Zement KG and BASF construction chemicals GmbH for the fi-nancial and analytical support. Moreover, I want to thank all members of the project fortheir ideas and engagement leading to fruitful discussions.

I want to thank many people who have contributed to the successful outcome of this the-sis.

First, I would like to express my gratitude to my co-supervisor Prof. Jürgen Neubauer forcalling my attention on the research project behind this doctoral thesis and for his manifoldsupport during the last 4 years. I always appreciated his comments and the common dis-cussions with him.

Additionally, I sincerely thank my supervisors at Empa, Dr. Frank Winnefeld and Dr. Bar-bara Lothenbach for guiding me through the doctoral research, teaching me many aspectsof cement related research work and promoting my personal development. I greatly appre-ciated that they always had an open ear for me and time for the discussion of my results.Moreover, I want to thank them for their humour and encouraging optimism.

Special thanks are due to Dr. Barbara Lothenbach for supporting my work with GEMS andteaching me various aspects of the swiss german language.

My sincere thanks are extended to Dr. Beat Münch for his friendly help during the devel-opment of the image analysis program and for sharing his experience in tending cows.

I also want to thank Dr. Wolfgang Kunther, Dr. Belay Zeleke Dilnesa, Dr. Lucia Ferrari, Dr.Mohsen Ben Haha, Dr. Klaartje De Weerdt, Axel Schöler and Dr. Andreas Leemann formany interesting discussions and shared experiences not only about cement or concrete,but also about fishing, music and other important issues of life.

Furthermore I want to thank Boris Ingold for the frequent preparation of microscopy sam-ples and musical support. Luigi Brunetti and Angela Steffen are acknowledged for thetechnical support and ion chromatography measurements. My gratitude is extended to allemployees of the laboratory of concrete and construction chemistry. I enjoyed the friendlyatmosphere in the lab and it was a great pleasure to work with them.

Special thanks are due to Walter Trindler for his motivating engagement, frequent inspira-tions for BBQ or other festivities and outstanding support during great fishing challenges atthe lake Greifensee.

My thanks go also to Tobias Danner, Markus Bernhardt, Dr. Daniel Jansen, Sebastian Dit-trich and and all of my friends from Erlangen and Switzerland for many pub visits, outdooradventures and good friendship.

II

Last but not least I want to thank my family and my beloved Franzi for all the care andsupport during the last 4 years.

III

Abstract

Global warming as a consequence of the anthropogenic emission of greenhouse gases isan issue of global concern. Therefore, the United Nations – Framework Convention onClimate Change has set the goal to keep the increase of the global warming below 2 °C.To realise this goal, the global greenhouse gas emissions need to be reduced to a level of60% of the emissions in 2010, until the year 2050. Due to the fact that about 5-8 % of theglobal CO2 emissions originate from the production of Portland cement clinker, the requestfor lower CO2 emissions is of great concern for the cement industry.Besides process optimisation and the use of alternative fuels, the replacement of Portlandcement clinker by supplementary cementitious materials (SCM) is an efficient way to re-duce the CO2 emissions related to cement production. Siliceous fly ash is one of the mostavailable SCM in Germany. This material is a by-product of the combustion of hard coal inpower plants.

The present thesis studies the effect of siliceous fly ash on the hydration of blended Port-land cement containing 50 wt% of siliceous fly ash. To achieve this goal, a multi-methodapproach using various techniques to characterise the hydration kinetics, solid hydrationproducts, pore solution and the microstructure has been used. To differentiate between theexclusively physical effect of the addition of fillers and the chemical reaction of the fly ash,the investigated samples were compared to a reference containing 50 wt% of quartz pow-der instead of fly ash. The quartz powder has proven to be a suitable reference materialdue to its practically inert behaviour in alkaline solutions. A test of the quartz powder in 0.3mol/l KOH solution (pH ≈ 13.4) with Ca(OH)2 during 90 days has merely shown any poz-zolanic reactivity at 23 °C.Apart from the filler effect, no effect of fly ash on the hydration could be measured before 2days. Afterwards, evidence of the pozzolanic reaction was given by a decrease of the port-landite content and the formation of C-S-H with decreased Ca/Si atomic ratios and in-creased amounts of incorporated Al. The effect of the pozzolanic reaction was reflected inthe pore solution chemistry, which showed decreased concentrations of Ca and increasedAl and Si concentrations.The analysis of the microstructure showed the formation of hydrate phases on the surfaceof fly ash particles within the first days of hydration. The origin of the first hydrate phaseswas however assumed to mainly originate from the hydration of the Portland cement andonly to a minor extent from the dissolution of fly ash. After 7 days, the microstructure of thefly ash particles showed the formation of an inner hydration product, characterised by agel-like consistency with a low density or a high content of water, respectively.

Additionally, a strong impact of the temperature on the hydration of fly ash blended Port-land cement was found. The rate of the fly ash reaction was accelerated with increasingtemperature, and hence the affection of the hydration by the pozzolanic reaction was shift-ed to earlier hydration times. Above 50 °C, the solubility of ettringite is increased, which ledto the destabilisation of this hydrate phase. The thereby released sulphate was enriched inthe pore solution and the C-S-H phase.Ca and Al were precipitated as siliceous hy-

IV

drogarnet, whose formation was kinetically hindered at room temperature and occurredonly at higher temperatures or after very long hydration times. The formation of siliceoushydrogarnet was further enhanced by the fly ash reaction due to the considerable amountof released aluminate from the reaction of the fly ash glass. The temperature relatedchanges in the phase assemblage were confirmed by thermodynamic modelling, per-formed with the help of GEMS (Gibbs energy minimisation software). The change of themicrostructure was similar as in pure OPC. Elevated temperatures led to the formation of amore heterogeneous matrix of hydrates with a higher density of the C-S-H.

To directly investigate the reaction kinetics of fly ash within blended Portland cement animage analysis procedure has been developed using grey level segmentation, grey levelfiltering and morphological filtering to quantify the content of anhydrous fly ash in BSE im-ages. The analysis of 600 images per sample yielded comparable results to conservativetechniques, like selective dissolution using a solution of Ethylenediaminetetraacetic acid(EDTA) and NaOH.

V

Zusammenfassung

Um der globalen Klimaerwärmung und deren Konsequenzen vorzubeugen, haben sich dieVereinten Nationen (United Nations Framework Convention) das Ziel zu Eigen gemacht,den Anstieg der globalen Durchschnittstemperatur auf maximal 2 °C einzuschränken. Umdieses Ziel zu erreichen, soll die Emission von Treibhausgasen bis 2050 auf 60 % des Le-vels von 2010 reduziert werden. Da die Produktion von Portlandzementklinker etwa 5 bis8 % der globalen CO2-Emissionen ausmacht, steht auch die Zementindustrie in der Pflicht,entsprechende Maßnahmen zur Minderung der CO2-Emissionen zu ergreifen.

Neben prozesstechnischen Optimierungen und der Verwendung alternativer Brennstoffe,können die CO2-Emissionen bei der Zementherstellung insbesondere durch die Reduktiondes Klinkergehalts, bei gleichzeitig vermehrtem Einsatz mineralischer Zumahlstoffe, ge-senkt werden. Als Zumahlstoffe werden unter anderem, industrielle Nebenprodukte wieHochofenschlacke oder Flugasche verwendet. Flugaschen, die bei der Verbrennung vonSteinkohle in Kraftwerken anfallen, erweisen sich in Deutschland auf Grund ihrer hohenVerfügbarkeit als besonders geeignete Zumahlstoffe.

Die vorliegende Dissertation zeigt den Effekt von Steinkohleflugasche auf die Hydratationeines Portlandzements mit 50 % Flugascheanteil. Dazu wurden entsprechende Zement-pasten mit 50 % Portlandzement und 50 % Steinkohleflugasche mit verschiedenen Me-thoden untersucht. Die Hydratationskinetik, die Entwicklung der Hydratphasen, die Poren-lösungschemie und die Mikrostruktur wurden charakterisiert. Um zwischen dem rein phy-sikalisch wirkenden Füller-Effekt der Flugasche und dem Effekt der chemischen Reaktio-nen der Flugasche zu unterscheiden, wurden Proben mit 50 % Quarzmehl anstatt Flug-asche als Referenz benutzt. Auf Grund des praktisch inerten Verhaltens des Quarzmehlsin 0,3 molarer KOH Lösung (pH ≈ 13,4) mit Ca(OH)2 über einen Zeitraum von 90 Tagenbei 20 °C, hat sich das Quarzmehl als geeignetes Referenzmaterial erwiesen.

Neben der physikalischen Wirkung als Füllstoff, konnten bei der Hydratation der unter-suchten Zemente während der ersten 2 Tage keine weiteren Einflüsse der Flugasche aufdie Hydratation gemessen werden. Zu späteren Zeiten wurde die puzzolanische Reaktionder Flugasche durch den steten Verbrauch von Portlandit (Ca(OH)2) und die Bildung vonC-S-H Phasen mit reduziertem Ca/Si Atomverhältnissen und einem erhöhten Einbau vonAl aufgezeigt. Die Effekte der puzzolanischen Reaktion spiegelten sich auch in der Ent-wicklung der Porenlösungszusammensetzung wieder: die Konzentration an Ca nahm ab,während die Al und Si Konzentrationen zunahmen.

Die Analyse der Mikrostruktur mittels Rasterelektronenmikroskopie zeigte, dass sich aufden Oberflächen der Flugaschepartikel während der ersten 2 Tage ein Saum aus Hyd-ratphasen bildete. Es wurde allerdings angenommen, dass diese Hydratphasen größten-teils auf die Hydratation des Portlandzementklinkers zurückzuführen waren. Nach 7 Tagenwies die Mikrostruktur der Flugaschepartikel erstmals ein inneres Hydratationsprodukt auf,welches sich durch eine niedrige Dichte, bedingt durch einen hohen Wassergehalt, aus-zeichnete. Während des gesamten Untersuchungszeitraums wurde mit der langsam fort-

VI

schreitenden Auflösung der Flugasche ein steter Zuwachs des inneren Hydratationspro-duktes beobachtet.

Darüber hinaus zeigte sich, dass die Umgebungstemperatur einen bedeutenden Einflussauf das Abbinden der untersuchten Zemente hatte. Bei erhöhten Temperaturen wurde dieFlugaschereaktion beschleunigt, wodurch der Effekt der puzzolanischen Reaktion auf dieZementhydratation früher zum Tragen kam. Die erhöhte Löslichkeit von Ettringit bei an-steigenden Temperaturen führte zur Freisetzung von Sulfat, welches sich in der Porenlö-sung und in C-S-H Phasen anreicherte. Freigesetztes Ca und Al wurde in Silikat-reichenHydrogranat, welcher sich vermehrt bei höheren Temperaturen und steigendem Flug-aschereaktionsgrad bildete, gebunden. Die Temperatur-bedingten Veränderungen derPhasenzusammensetzung konnten mittels thermodynamischer Berechnungen mit GEMS(Gibbs energy minimisation software) modelliert und nachvollzogen werden. Darüber hin-aus wurde mit erhöhter Temperatur eine zunehmend ungleichmäßigere Mikrostruktur undeine erhöhte Dichte der C-S-H Phasen festgestellt.

Um die Reaktionskinetik der Flugasche zu bestimmen, wurde ein Bildanalyse-Verfahrenzur Auswertung von Rasterelektronenmikroskopbildern entwickelt. Bei diesem Verfahrenkann durch Kombination verschiedener Morphologie- und Graustufenfilter, sowie Segmen-tierungen, der Gehalt an nicht-hydratisierten Flugaschepartikeln bestimmt werden. Die Er-gebnisse der Analyse von 600 Bildern pro Probe waren vergleichbar zu konventionellenVerfahren, wie der selektiven Auflösung mit Ethylendiamintetraessigsäure (EDTA) Lösungund NaOH.

VII

List of papers

The thesis includes the following papers:

1. Investigation of a model system to characterize the pozzolanic reactivityof two low Ca fly ashes and a quartz powder

Deschner F., Lothenbach B., Winnefeld F., Schwesig P., Seufert S., Dittrich S.,Neubauer J.Tagungsband der Tagung Bauchemie der GDCh, Hamburg 2011, pp. 127-132.

2. Hydration of Portland cement with high replacement by siliceous fly ash

Deschner, F., Winnefeld F., Lothenbach B., Seufert S., Schwesig P., Dittrich S.,Goetz-Neunhoeffer F., Neubauer J.Cement and Concrete Research 42, pp. 1389-1400.

3. Effect of temperature on the hydration Portland cementblended with 50 wt% of siliceous fly ash

Deschner F., Winnefeld F., Lothenbach B., Neubauer J..Cement and Concrete Research 52, pp. 169-181.

4. Quantification of fly ash in hydrated, blended Portland cement pastesby back-scattered electron imaging

Deschner F., Münch B., Winnefeld F., Lothenbach B.Journal of Microscopy 251, pp. 188-204.

VIII

Content

Abstract............................................................................................................................. III

Zusammenfassung............................................................................................................V

List of papers...................................................................................................................VII

Glossary of notations and terms .................................................................................... IX

1 Introduction............................................................................................................. 1

1.1 Portland cement ................................................................................................... 1

1.2 CO2 emissions related to cement production........................................................ 1

1.3 Measures to mitigate CO2 emissions related to cement production ..................... 2

2 Objective ................................................................................................................. 4

3 Summary of used methods.................................................................................... 5

4 Background............................................................................................................. 7

4.1 Ordinary Portland cement (OPC).......................................................................... 7

4.2 Portland cement blended with siliceous fly ash .................................................... 8

5 Main achievements............................................................................................... 11

6 References ............................................................................................................ 16

IX

Glossary of notations and terms

Oxides

C - CaO S - SiO2 A - Al2O3 F - Fe2O3

$ - SO3

Cement minerals and hydration products

Alite C3S Ca3SiO 5

Belite C2S Ca2SiO4

Aluminate C3A Ca3Al2O6

Brownmillerite C4AF Ca4Al2Fe2O10

Gypsum C$H2 CaSO4 * 2H2O

Ettringite C3A$3H32 Ca6Al2(SO4)3(OH)12 * 26H2O

Monosulphate C4A$H12 Ca4Al2SO4(OH)12 * 6H2O

Methods

BSE Backscattered electron

DTG Differential thermogravimetry

GEMS Gibbs energy minimisation software

ICP-OES Inductively coupled plasma optical emission spectroscopy

SEM Scanning electron microscopy

TGA Thermogravimetric analyses

XRD X-ray diffraction

Materials

OPC / CEM I Ordinary Portland cement

SCM Supplementary cementitious materials

X

Other notations

UNFCCC United Nations – Framework Convention on Climate Change

wt% weight percent

1

1 Introduction

1.1 Portland cement

Cementitious materials and concrete are used since thousands of years by mankind. Theoldest archaeological finds of concrete in the broadest sense (mix of sand, rock fragmentsand cementitious binder) in human history date back to about 5,600 B.C., used in thefloors of huts in Serbia [1]. The romans used mortar and concrete based on lime and poz-zolana, which was the basis of the strength and durability of Roman architecture of whichsome buildings, such as the Panthenon, still stand today [2]. Portland cement was first pa-tented by Joseph Aspdin in 1824 [3] and a modern version of this cement is nowadaysused in numerous applications ranging from various kinds of mortar and concrete overplaster and screed to special products like tile adhesives.Today, Portland cement based concrete is the most used, solid material on earth. In theyear 2011 estimated 3.4 Gt of cement were produced world-wide [4] and the demand forcement is predicted to grow in the medium-term future [5]. Obviously, an industry branchof this size is also involved in the present discussions about global warming caused by an-thropogenic greenhouse gas emissions. Scientific studies show that the global warmingsince 1900 is at least partially due to anthropogenic components like the emission ofgreenhouse gases [6]. As a consequence, the reduction of the greenhouse gas emissionswas decided by the United Nations – Framework Convention on Climate Change (UN-FCCC) conference of the parties in Cancun in 2010 in order to restrict the increase of theglobal average temperature below 2 °C above pre-industrial levels [7]. Scenarios that meetthis 2 °C limit have global emissions in 2050 of a level, which is roughly 40% below theemissions in 1990 and roughly 60% below the emissions in 2010 [8].

1.2 CO2 emissions related to cement production

The direct and indirect CO2 emissions related to the production of cement account forabout 8 % of the global anthropogenic CO2 emissions [9, 10]. For the production of 1 t ofPortland cement clinker about 0.7-1.0 t of CO2 are emitted, depending on technical pa-rameters of the production process and the types of fuels used [11-13]. Portland cementclinker is made of limestone, clay or rocks of similar composition, which are ground andheated up to about 1450 °C. A typical Portland cement clinker has a CaO content of 64-67 wt%. Since the source of CaO is usually carbonatic, 510 kg CO2 are emitted per t ofPortland cement clinker by the decalcification of CaCO3 (Eq. 1).

1161 kg CaCO3 650 kg CaO + 510 kg CO2 (Eq. 1)

Other CO2 emissions are mainly related to the combustion of fuels. Indirect CO2 emissionsoriginate from quarrying, transport processes and the use of electricity in general.

2

1.3 Measures to mitigate CO2 emissions related to cement production

The most obvious approaches to reduce the CO2 emissions are to optimise the energy ef-ficiency of the production process, e.g. by installing a pre-calciner or the usage of alterna-tive fuels, like refuse-derived fuel or sewage slag instead of coal powder or oil. Althoughthese measures constitute an improvement of the energy efficiency in many older cementplants around the world, a modern cement plant with the best available Portland cementmanufacturing technology using alternative fuels, leaves not much potential to improve theenergy efficiency [10].Another way to reduce the CO2 emissions of the cement production is the replacement ofthe Portland cement clinker by supplementary cementitious materials (SCM). Typical SCMare extracted from natural resources, like limestone powder, calcined clays and naturalpozzolans, or from industrial by-products, like blast furnace slag, silica fume or fly ash.The use of SCM within cement produced in Europe is regulated by the European stand-ards EN 197-1 (Fig. 1). The ordinary Portland cement (OPC or CEM I) may contain up to5 wt% of minor constituents. The other cement types are blended with various amounts ofSCM. With a market share of about 55 % in the year 2010 in Europe [14], the CEM II, alsocalled Portland composite cement, is the most sold cement in Europe. The CEM II family issubdivided in CEM II A and CEM II B, which contain 6-20 wt% and 21-35 wt% of SCM, re-spectively. CEM III are blastfurnace slag cements with a clinker replacement of up to95 wt%. CEM IV is called pozzolanic cement with up to 65 wt% of clinker replacement.The cement with the lowest market share in Europe is the CEM V or composite cementwith up to 80 wt% of clinker replacement.

3

Fig. 1. Composition and minimum Portland cement clinker content of cements specifiedaccording to the European standard EN 197-1.

Besides the ecological aspects, blended cements are used due to the beneficial effects ofcertain SCM on the durability of concrete. Especially fly ash, blastfurnace slag and silicafume improve the resistance to sulphate attack and alkali-silica reaction and reduce thepermeability and chloride diffusion of concrete [15-19].

0% 20% 40% 60% 80% 100%

CEMV/B

CEM V/A

CEM IV/B

CEM IV/A

CEM III/C

CEM III/B

CEM III/A

CEM II/B

CEM II/A

CEM I

Minimum content of Portland cement clinker

Maximal content of fly ash, blast furnace slag, silica fume, naturalpozzolana, limestone, burnt shaleMaximal content of fly ash, natural pozzolana, silica fume

Maximal content of granulated blast furnace slag

4

2 Objective

The use of SCM as clinker replacement in Portland cement is always also a question oftheir regional availability. In Germany, the most abundant SCM is siliceous fly ash (Type V,EN 197-1), which is a by-product of the coal combustion process. Especially since the de-cision of the German Federal Government in 2000 to allow no new constructions of nucle-ar power plants and to withdraw slowly from nuclear energy production, several new con-structions of coal-fired power plants were planned. Therefore, the availability of siliceousfly ash and the potential of its use as clinker replacement was from then on about to be fur-ther increased. With this background and the goal of the cement industry to reduce theCO2 emissions of the cement production, the demand for cements blended with increasedamounts of siliceous fly ash increased.

The drawback of the replacement of large amounts of Portland cement clinker by siliceousfly ash is the reduced early age strength [20, 21], which is related to the late onset of thefly ash reaction [22]. Many studies carried out in order to enhance the reactivity of fly ashshowed that the effect of the physical or chemical activation of fly ash is usually too low tocompensate for the decreased early age strength and often also too cost- or energy-intensive, especially when using high replacement levels [23-27].

In order to find new suitable activators, first of all, the details of the reaction of fly ash andits hydration mechanism in blended Portland cement need to be fully understood.

Therefore, the primary goal of this study is to investigate the effect of the fly ash reaction inblended Portland cement on the development and composition of hydrate phases as wellas the pore solution. To achieve this, the focus of the investigations is a system consistingof a CEM I 42,5 R blended with 50 wt% of siliceous fly ash. The high replacement level ofPortland cement was chosen intentionally to maximise the effect of the fly ash reaction. Amulti-method approach to analyse the assemblage and composition of hydrate phases, thechemistry of the pore solution and the reaction degree of fly ash in the fly ash blendedPortland cement is chosen. The four principal scientific topics of this study are the investi-gation of the:

Pozzolanic reactivity of fly ash and quartz powder

Effect of fly ash on the hydration of fly ash blended Portland cement at 23 °C

Effect of temperature on the hydration of fly ash blended Portland cement

Quantification of the reaction degree of fly ash during the hydration of blended Port-land cement by means of image analysis

5

3 Summary of used methods

Compressive strength

Mortar prisms (40x40x160 mm3) were prepared and tested for certain hydration times ac-cording to EN 196-1.

Thermogravimetric analyses (TGA)

The hydrated samples were ground, immersed in isopropanol for 15 minutes, washed withdiethylether, and filtered. Afterwards, the samples were dried for 10 minutes at 40 °C toevaporate the remaining diethylether and analysed in a Mettler Toledo TGA/SDTA 851edevice at a heating rate of 20 K/min in N2-atmosphere. The analysed bound water of allhydrate phases and the crystal water within Ca(OH)2 were determined by the weight lossin the temperature intervals 50-500 °C and 400-470 °C. The exact boundaries for the tem-perature interval of portlandite were read from the derivative curve (DTG). The results areexpressed as percentage of the dry sample weight at 500 °C [28]. Triple preparation andmeasurement of the samples after 1, 2 and 7 days of hydration at 23 °C showed an abso-lute error of up to 2 wt% for the bound water, and up to 0.6 wt% for the water loss relatedto the decomposition of Ca(OH)2 . This includes the errors caused by preparation, meas-urement and sample inhomogeneity.

X-ray diffraction (XRD) measurements

5 mm thick disks of the hardened paste samples with a diameter of 30 mm were slicedfrom the cast samples, immersed in isopropanol for 2 days to stop the hydration andstored under N2-atmosphere until measurement. Measurements were performed in aPANalytical X’Pert Pro MPD diffractometer with attached X’Celerator detector.

Scanning electron microscopy (SEM)The hydration was stopped by cutting the sample into slices of 5 mm thickness with a di-ameter of 30 mm and keeping them for 3 days in isopropanol and drying them for 3 daysat 40 °C. Subsequently, the samples were impregnated with a modified bisphenol-A-epoxy-resin and polished by polycrystalline diamond suspension at grades from 9 μmdown to 1/4 μm. Finally, the samples were carbon-coated and investigated under highvacuum conditions in a Philips ESEM FEG XL 30. EDX measurements of the hydratephases were carried out at a beam voltage of 10 keV and 10 mm working distance.

Analysis of the pore solution

The pore solutions of the hardened samples were extracted by the steel die method [33]using pressures up to 250 N/mm2. The solutions were filtered immediately with nylon filterswith a mesh size of 0.45 μm. The free OH- concentrations of the pore solutions were calcu-

6

lated with the help of pH measurements with a pH electrode, calibrated against KOH solu-tions with known concentrations. The K, Na, Ca, Al, Si and sulphur concentrations weremeasured by means of inductively coupled plasma optical emission spectroscopy (ICP-OES).

7

4 Background

4.1 Ordinary Portland cement (OPC)

Portland cement clinker is a product made of limestone and clay or rocks of similar com-position. The milled raw materials usually pass through a rotary kiln, where they are heat-ed up to 1450 °C. During this process, calcium silicate phases (alite and belite) and meltsare formed by the reaction of raw materials. Upon rapid cooling and due to the incorpora-tion of other ions into the crystal structure, high temperature polymorphs of the calcium sil-icates are maintained as metastable phases [29, 30]. Upon cooling, calcium aluminate,calcium aluminate ferrate (brownmillerite) and other phases, such as alkali sulphates crys-tallize from the melts [31, 32].The content of the main clinker phases in the OPC used in this project is 57.1 wt% C3S,17.2 wt% C2S, 4.0 wt% C3A and 13.0 wt% C4AF. The reaction of the calcium silicates isdescribed by Eq. 2a&b.

C3S + (3+m-n) H Cn-S-Hm + (3-n) CH (Eq. 2a)

C2S + (2+m-n) H Cn-S-Hm + (2-n) CH (Eq. 2b)

with n and m being typically in a range between 1-2 and 1.5-2.5.

Alite reacts quickly with water, leading to the formation of C-S-H and CH. This process isresponsible for the main strength development in OPC during the first 28 days. Belite re-acts at a much slower rate and contributes to the later strength development [29].

The calcium aluminate phase also reacts very quickly with water. To regulate this reaction,calcium sulphate is added as set regulator. In the presence of calcium sulphate, C3A re-acts according to Eq. 3a, resulting in the formation of ettringite. After the depletion of thecalcium sulphate source, ettringite transforms to monosulphate in a reaction with the re-maining C3A and water (Eq. 3b).

C3A + 3 C$H2 + 26 H C6A$3H32 (Eq. 3a)

C6A$3H32 + 2 C3A + 4 H 3 C4A$H12 (Eq. 3b)

The Al in monosulphate may be substituted by Fe and the SO3 by H2O. In the presence ofcarbonate, analogous phases with CO2 instead of SO3 form. The family of these isostruc-tural phases is called AFm (aluminium iron monophases).

The hydration of calcium aluminate ferrate (C4AF) in the presence of gypsum leads to theformation of ettringite with Al partially substituted by Fe [33]. However, since Al-ettringite is

8

more stable than Fe-ettringite [34], Fe-AFm and mainly Fe-rich siliceous hydrogarnet areformed [33, 35, 36].

4.2 Portland cement blended with siliceous fly ash

Fly ash is a by-product of the coal combustion process. Mineral components of the raw,ground coal (e.g. clay minerals, calcite and quartz) melt during the combustion process.Droplets and fragments of this melt, stream upwards together with the combustion gas intothe chimney, get quenched and solidify. These particles are retained as fly ash in the elec-trostatic filters. Fly ash is a silica and alumina rich material with variable chemical composi-tion depending on the type of burnt coal. Depending on the Ca-content, distinction is madebetween Type V (<10 wt% CaO) and Type W (>10 wt% CaO) fly ash, according to the Eu-ropean Standard EN 197-1. Fly ash is a very heterogeneous material in terms of morphol-ogy and chemistry. Although (partially hollow) spherical particles are most abundant, alsomany crystalline and sponge-like or odd shaped particles can be found within fly ash (Fig.2).

Fig. 2. BSE images of raw fly ash particles.

1: spherical particle, 2: crystalline particle, 3: spherical particle with crystalline iron oxide

lamellae, 4: sponge-like, odd shaped particle.

The variable chemistry of most of the glassy fly ash particles is characterised by mainlySiO2 and Al2O3 (network formers) and minor amounts of network modifiers, such as CaOor alkali oxides (Fig. 3). Additionally, special particles rich in CaO, MgO and P2O5 can befound. The principal crystalline phases are mullite, quartz and iron oxides such as hematiteand magnetite. Siliceous fly ash also contains readily soluble alkali and calcium salts,which precipitate from the combustion gas on the surfaces of the fly ash particles.

1

2

34

9

0.00 0.25 0.50 0.75 1.00

0.00

0.25

0.50

0.75

1.00 0.00

0.25

0.50

0.75

1.00

Al2O3+Fe2O3wt.-%

SiO2wt.-%

CaO+MgOwt.-%

75

100

50

25

0 25 50 75 100100 0

0

50

25

75

Fig. 3. Chemical composition of randomly analysed particles of one siliceous fly ash used

in this study.

Due to its heterogeneous particle size distribution and the mainly spherical particle shape,fly ash is a suitable mineral additive to improve the workability of concrete [37, 38]. Its usein concrete has also many other beneficial effects like the improvement of the ultimatestrength and increase of the durability [15-19, 37].The effect of the sole presence of fly ash on the hydration of OPC, just like any other inertfiller material, is referred to as filler effect [28, 39-45]. One aspect of the filler effect is that,by adding a filler and keeping the water-to-solid ratio constant, the effective water-to-cement ratio is increased and more space for the growth of hydrate phases is available.Another aspect is, that the particle surfaces of the filler material act as sites for heteroge-neous nucleation of hydrate phases. These two aspects of the filler effect lead to an in-creased reaction rate of the OPC.Besides its good properties as a filler material, siliceous fly ash (Type V, EN 197-1) is a la-tent hydraulic material, which hydrates in the alkaline pore solution of Portland cement[46]. The pH of the pore solution is governed by the dissolved ionic species in solution. Ini-tially, Ca, alkali and sulphate ions dominate the pore solution and a pH of 12.6-13.0 isreached. After the depletion of the calcium sulphates, the pH rises to about 13.5. In thesealkaline conditions, the aluminosilicate glass network of the fly ash is attacked by OH- andthe hydroxylated silicate reacts with Ca(OH)2, originating from the hydration of the OPC, toform C-S-H.

10

From the dissolution of the fly ash, also aluminate is released and partially incorporated inthe C-S-H or contributing to the formation of AFm phases or siliceous hydrogarnet [47].The dissolution rate of the fly ash glass is dependent on the pH [41, 48]. The onset of thepozzolanic reaction, measured by the consumption of Ca(OH)2, is reported to start after 7days [41, 43, 49] and in individual cases after 3 days [50] or after 28 days of hydration [51].

11

5 Main achievements

The study is focussed on the investigation of the effect of siliceous fly ash on the hydrationof blended cement pastes. The study is subdivided in four parts. In this section the mainachievements of each part are reported and discussed.

1. Investigation of a model system to characterise the pozzolanic reactivity of two low Cafly ashes and a quartz powder

Prior to the investigation of the effect of fly ash in blended Portland cement, the pozzolanicreactivity of the two used fly ashes and the reference quartz powder need to be investigat-ed. To achieve this, model cement pastes consisting of 100 g of fly ash or quartz powder,70 g of portlandite, 10 g of calcite and 180 g of an aqueous solution with 0.3 mol per litreKOH are used. The KOH solution is chosen to mimic the pH of a matured cement pore so-lution. The portlandite and the calcite serve as reactants for the silicate and aluminatecomponents of the fly ash glass.The hydration of these mixes is stopped after 2, 7, 36 and 90 days of hydration. The char-acterisation of the hydration products by TGA, XRD and SEM show the formation of C-S-Hand AFm phases, mainly monocarbonate. The consumption of Ca(OH)2, measured bymeans of TGA, is used as an indicator for the extent of the pozzolanic reaction. The two flyashes show a similar pozzolanic reactivity, only the fine, air separated fraction shows asignificantly higher reactivity. The quartz powder shows no significant pozzolanic reactivityand proves therefore to be a suitable, inert reference material for other studies.

2. Hydration of Portland cement with high replacement by siliceous fly ash

In this part of the study, the reaction of fly ash in blended Portland cement paste and its ef-fect on the chemistry of the solid hydrate phases, the pore solution and the microstructureare investigated. Two different fly ashes are used as SCM replacing 50 wt% of Portlandcement. To distinguish between the pozzolanic reaction and the physical filler effect of thefly ash, a sample with 50 wt% of practically inert quartz powder is used as reference. Theprocess of fly ash hydration within blended Portland cement is shown schematically in Fig.4. Although the pH of the pore solution rises within 1 hour to 13, which is sufficiently highto enable the dissolution of the fly ash glass, no measurable evidence of the fly ash reac-tion is found before the first 2 days of hydration. After 8 hours, hydration products on thesurface of the fly ash are formed, which are supposed to be mainly originating from thehydration of the OPC. Between 10 and 24 hours, the calcium sulphate phases deplete.Due to the ongoing precipitation of ettringite, the sulphate concentration in the pore solu-tion drops, and hence the pH increases. The rise of the pH is supposed to promote thedissolution of fly ash. After 7 days, evidence of the fly ash reaction is found by a decreaseof the portlandite content and the change of the pore solution compared to the referencesample. The analysis of the pore solution shows a decrease of the Ca concentrations dueto the consumption of Ca(OH)2 and an increase of the Al and Si concentrations due to thedissolution of the fly ash. The C-S-H composition changes analogous to the pore solution

12

composition. SEM-EDX analyses of the matrix of cement hydrates reveal lower Ca/Si andhigher Al/Si atomic ratios in the C-S-H as a consequence of the pozzolanic reaction.After 7 days of hydration, the formation of an inner hydration product of the fly ash is ob-served. The hydrate phases in this area are characterised by a low density and a high con-tent of water. The formation of these hydrate phases is related to the low availability of Cawithin the inner hydration product. Depending on the chemistry of the specific fly ash parti-cles, in some cases hydrotalcite or siliceous hydrogarnet can be found within the inner hy-dration product. Liesegang rings within the inner hydration product observed after 550days of hydration demonstrate the gel-like consistency of the hydrate phases in this area.After long hydration times, the pH decreases slightly due to the binding of alkali within theC-S-H. Additionally, the amount of pore solution is decreased and the continuous for-mation of hydrate phases leads to a densification of the matrix. Therefore, the continuoushydration of fly ash is slowed down.

13

Fig. 4. Schematic process of the hydration of a typical aluminosilicate glass fly ash particle

within blended Portland cement.

Al

Fe

Si FA

Al(OH)4-

Si(OH)62-

Ca2+

SO42-

OH-

OH-

Fe(OH)4-

Al

Fe

Si FA

Al(OH)4-

Si(OH)62-

OH-

OH-

Ca2+

SO42-

Al

Si FA

Al(OH)4-

Si(OH)62-

Ca2+

Ca2+

Fe

Fe(OH)4-

Al

Fe

Si FA

Al(OH)4-

Si(OH)62-

OH-

OH-

Fe(OH)4-

Al

Fe

Si FA

Al(OH)4-

Si(OH)62-

OH-

OH-

Ca2+

SO42-

Fe(OH)4-

(Fe-) hydrotalcite(Fe-) Si-hydrogarnet

AFt, AFm C-A-S-H(Al/Si ~ 0.2) Mullite Inner hydration

product

a) < 8 h b) 8-24 h

c) 1-7 d d) > 7 d

C-A-S-H

14

3. Effect of temperature on the hydration of Portland cement blended with siliceous fly ash.

With a similar setup as in part 2 of the thesis, the effect of temperature on the hydration offly ash blended Portland cement compared to the reference sample containing quartzpowder is investigated. Besides the known effect of temperature on the hydration of OPC,a strong impact of temperature on the hydration kinetics of fly ash is found. All effects onthe chemistry of hydrate phases and pore solution related to the pozzolanic reaction of flyash observed in part 2 of the study are shifted to earlier hydration times at elevated tem-peratures. At 50 °C and higher temperatures the pozzolanic reaction of fly ash is found tostart before 1 day of hydration. At 7 °C, no evidence of the pozzolanic reaction is found be-fore 90 days of hydration.At temperatures above 50 °C, ettringite is destabilised due to its increased solubility andthe released Al and sulphate is found to be partially incorporated in C-S-H and further con-tributing to the formation of considerable amounts of siliceous hydrogarnet. This effect ispronounced in systems blended with siliceous fly ash due to the reaction of the aluminatefraction of fly ash.In accordance with the experimental results, the change of the hydrate phase assemblageas a function of temperature is modelled by thermodynamic calculations using GEMS [52]together with the thermodynamic data from the PSI-GEMS database [53] expanded withadditional data for solids that are expected to form under cementitious conditions [33, 35,36, 54].The effect of temperature on the microstructure of fly ash blended Portland cement isfound to be similar as in pure OPC. At elevated temperatures, the heterogeneity andcoarse porosity of the microstructure is increased. The C-S-H phase appears brighter inthe BSE image as a consequence of its lower water content and higher density at elevatedtemperatures.

4. Quantification of fly ash in hydrated, blended Portland cement pastes by backscatteredelectron imaging.

To investigate the effect of parameters like the addition of activators on the reaction kinet-ics of fly ash in blended Portland cement, the aim of this part of the study is to develop amethod quantifying the content of anhydrous fly ash from the segmentation of BSE imag-es. A new image processing routine is developed, which shows how characteristic featuresof fly ash in the microstructure of hydrated cement pastes can be used to segment anhy-drous fly ash particles from BSE images. For the image processing, a combination of greylevel segmentation, grey level filtering and morphological filtering is used. The analysis of600 images per sample at a magnification of 2000 yields the content of anhydrous fly ash,anhydrous clinker, porosity and portlandite. The analysis of the presented dataset revealsa standard deviation of the reaction degree of fly ash of up to 4.3 %. However, the accura-cy of the results is not found to be better than the determination of the fly ash reaction de-gree by conventional techniques, like selective dissolution [55-57].The limitations of the accuracy of the method are due to the morphological and composi-

15

tional heterogeneities of fly ash and similarities between fly ash and hydrate phases.Although the method proves to be promising, the accuracy of the method could be proba-bly improved by the implementation of element mappings and clustering analyses.

16

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Investigation of a model system to characterize the pozzolanic reactivity of twolow Ca fly ashes and a quartz powder

F. Deschner, B. Lothenbach, F. Winnefeld, P. Schwesig, S. Seufert, S. Dittrich, J.Neubauer

Tagunsband der Tagung Bauchemie der GDCh, Dortmund 2010, p.127-132.

ISSN/ISBN 978-3-936028-69-0

Paper I

Investigation of a model system to characterize the pozzolanic reactivity of two low Ca fly ashes and a quartz powder F. Deschner a, B. Lothenbach a, F. Winnefeld a, P. Schwesig b, S. Seufert b, S. Dittrich c, J. Neubauer c

a Empa, Swiss Federal Laboratories for Materials Science and Technology, Dübendorf/CH b BASF Polymer Research, Polymers for Inorganics, BASF Construction Chemicals GmbH, Trostberg c GeoZentrum Nordbayern, Mineralogy, University of Erlangen-Nuremberg Introduction

Fly ash (FA) is increasingly used as clinker replacement in blended Portland cements in order to reduce the CO2 emissions /1/. The reactivity of the FA changes depending on its chemical and mineralogical composition and the pre-treatment, e. g. grinding or leaching of the raw FA /2/. A test method was developed to evaluate and compare the pozzolanic reactivity and the hydration products of different SCM, at conditions similar to the hydration of an OPC.

Materials and Methods

To illustrate the effectiveness of the reactivity test a low Ca FA as delivered (F1-u) and a fine air-separated FA (F1-f), both from the same batch of one coal power plant, were used. Another untreated FA (F2-u) from a different coal power plant, characterized by a higher Ca-content and the presence of lime, periclase and anhydrite, was used to be compared with F1. Finally an untreated and a screened Qz powder (Qz-u & Qz-f) of similar fineness as the two respective FA were used as reference materials. The mineralogical and chemical composition of the raw materials, determined by means of X-ray flourescence and quantitative X-ray diffraction (according to /3/), is shown in table 1. For the experiments 100 g of FA or Qz, respectively, were homogenized together with 70 g of portlandite and 10 g of calcite. The materials were subsequently mixed with 180 g of 0.3 m KOH solution in a vacuum mixer for 2 min. The portlandite and the calcite serve as reactants for the FA to form calcium silicate hydrates (C-S-H) and carbonate aluminium iron monophases (AFm). According to thermodynamic calculations /4/ the used amount of portlandite was enough to react 62 wt% of F1-u, before it is totally consumed. The

0.3 m KOH solution imitated the pH of a typical OPC pore solution after the consumption of the calcium sulphates (pH 13.4). The pastes were finally sealed in polyethylene bottles and stored at 23 °C. After 2 d, 7 d, 36 d and 90 d of hydration, the samples were analyzed by means of thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). To stop the hydration, the samples were ground, immersed in isopropanol for 15 minutes, washed with diethylether and filtrated. The residue of the filtration was dried for 7-8 minutes at 40 °C to evaporate the remaining diethylether to be instantly measured in a Mettler Toledo TGA device at a heating rate of 20 K/min. The XRD investigations were carried out in a PANalytical X’Pert Pro MPD diffractometer with attached X’Celerator detector. For the SEM investigations, pieces of the hardened samples were kept for 2 days in isopropanol and afterwards dried at 40 °C for several days. After impregnating the samples by a modified bisphenol-A-epoxy-resin, they were polished, carbon-coated and investigated under high vacuum conditions in a Philips ESEM FEG XL 30.

Table 1. Chemical and mineralogical composition of the investigated materials.

chemical composition [wt%] mineralogical composition [wt%] F1-u F1-f F2-u Qz-u Qz-f F1-u F1-f F2-u

SiO2 50.9 47.2 45.0 99.7 98.3 Mullite 8.2 6.8 19.5

Al2O3 24.7 29.7 26.5 1.1 1.6 Quartz 7.0 1.8 7.6

Fe2O3 7.3 6.5 8.5 0.5 0.5 Hematite 0.7 0.1 1.2

CaO 3.7 2.6 5.6 0.0 0.1 Magnetite 0.8 <0.01 1.0 MgO 1.8 1.8 2.8 0.1 0.2 Anhydrite 0.0 0.0 0.8

K2O 3.9 5.0 2.0 0.0 0.0 Periclase 0.0 0.0 1.3

Na2O 0.9 1.2 1.3 0.1 0.1 Lime 0.0 0.0 0.9

TiO2 1.1 1.3 1.2 0.1 0.1 amorphous 83.3 91.3 67.7

Mn2O3 0.1 0.1 0.1 0.1 0.1

P2O5 0.8 1.0 1.1 0.0 0.0

SO3 0.4 0.1 0.6 0.2 0.2 LOI 3.5 4.8 4.2 0.1 0.3

Sum 99.1 101.2 98.9 102.0 101.5 100 100 100

BET [m2/g] 1.8 3.1 2.4 0.8 4.4

Results

Fig. 1 shows the XRD diffractograms of the reactivity tests of the FA samples after 7 d and 36 d of hydration. Apart from the initially added portlandite (CH), the major crystalline hydrate phases in the FA

samples were monocarbonate, hemicarbonate and minor amounts of ettringite, which may arise from small quantities of SO3 in the FA. Due to its X-ray amorphous properties no C-S-H could be detected. The qualitative assemblage of hydrate phases did not change between 2 d and 90 d. The reactivity tests of the samples containing quartz powder did not show the development of any crystalline hydration products.

Figure 1. Details of the XRD diffractograms of the reactivity tests of the FA samples after (a) 7 d and (b) 36 d of hydration. E – ettringite, Hc – hemicarbonate, Mc – monocarbonate, CH – portlandite.

Figure 2. Relative weight loss (upper curves) and DTG (lower curves) of the reactivity tests of the investigated samples after 36 d of hydration. Mc – monocarbonate, CH – portlandite, Cc – calcium carbonate.

8 9 10 11 12 13 14 15 16 17 18 19

°2 Theta Cu K-alpha

b) 36 da) 7 d

F1-f

F1-u

F2-u

8 9 10 11 12 13 14 15 16 17 18 19

°2 Theta Cu K-alpha

McMc

Hc

CH

CH

EE

Hc

100 200 300 400 500 600 700 800 90030

40

50

60

70

80

90

100

-0.16

-0.12

-0.08

-0.04

0.00

0.04

0.08

0.12

F1-u F1-f Qz-u Qz-f F2-u

wei

ght l

oss

[wt%

]

temperature [°C]

DT

G [1

/°C

]C-S-H

AFm(Mc)

CH Cc

The plots of the relative weight loss and the differential thermogravimetry (DTG) determined by means of TGA after 36 d of hydration are shown in fig. 2. The DTG curves show the peaks due to the dehydration of AFm (monocarbonate) and portlandite at 175 °C and 455 °C, respectively. It confirms the hydrate phase assemblage determined by means of XRD. The peaks of the dehydration of C-S-H are broad and overlapped by the dehydration of other hydrate phases. It can be seen that also the Qz samples, especially the Qz-f, show a small broad peak between 50 °C and 200 °C, which illustrates the minor development of hydrate phases in these samples.

The amount of portlandite was calculated by the weight loss between 400 °C and 470 °C. The total bound water was determined by the weight loss between 50 °C and 500 °C and subsequent subtraction of the weight loss due to the initial content of portlandite. Fig. 3 shows the development of the portlandite content and the bound water normalized to the amount of paste. The consumption of portlandite and development of bound water by the pozzolanic reaction of the FA /5/ starts before 2 d of hydration and slow down after 36 d of hydration. Later the rate of the pozzolanic reaction seems to decrease. Although their slight differences in the chemical and mineralogical composition no significant differences can be seen in the reactivity of F1-u and F2-u. The comparison of the untreated and the fine FA up to a time of 36 d of hydration shows that the fine FA is 2-3 times more reactive than the untreated FA. At later hydration times it gets more difficult to assess the reactivity of the fine FA, since the portlandite is almost completely consumed and therefore the potential of the pozzolanic reaction has decreased. Regarding the untreated Qz reference powder, no big amounts of portlandite were consumed by the pozzolanic reaction. However during the first 7 d of hydration 1.6 wt% of portlandite were consumed, although no significant amounts of bound water were observed up to 90 d of hydration. The consumption of portlandite in the Qz sample could be due to Al2O3 impurities in the form of clay minerals or amorphous SiO2 reacting with the Ca(OH)2. This effect is pronounced in the finer Qz powder, which also shows a slight pozzolanic reaction after 7 d of hydration. It shows that the particle fineness and the amount of impurities are crucial aspects governing the reactivity of the Qz powders.

Figure 3. Development of the content of portlandite (dashed lines) and bound water (solid lines) during the reactivity test of the untreated and fine FA and Qz. The error of preparation and measurement is estimated to be ± 1 g/100g paste.

Microstructure

A typical image of the microstructure of F1-u is shown in figure 4. It can be seen that the microstructure is characterized by a relatively porous matrix due to the high water-to-solid ratio. The matrix consists mainly of sparse C-S-H phase. Embedded in the matrix the raw materials FA, portlandite and calcite (not shown here) are found. Characteristic features are the radial orientated, fiber-like zones around the FA particles.

Figure 4. BSE image of the microstructure of the reactivity test of F1-u after 90 d of hydration. CH – portlandite, IP – inner hydration product of FA.

0 20 40 60 80 1000

2

4

6

8

10

12

14

16

18

20

0

2

4

6

8

10

12

14

16

18

20 F2-u Qz untreated

Qz fine F1-u F1-f

bo

un

d w

ater

[g

/100

g p

aste

]

time [d]

po

rtla

nd

ite

[g/1

00 g

pas

te]

2 μm

FA

CHIP

Matrix

IP

Conclusions

The presented reactivity test is suitable to evaluate and compare the pozzolanic reactivity of materials. Due to the experimental setup with not only portlandite but also calcite, suitable reactants are provided to form C-S-H and AFm phases, which is an important aspect for testing aluminium-rich materials like FA. However, if one is investigating highly reactive pozzolanic materials, it is recommended to use higher contents of portlandite for the experimental setup. The two different low Ca ashes showed a similar pozzolanic reactivity. However the air-separated, fine FA showed 2-3 times higher pozzolanic reactivity than the untreated one, due to its higher specific surface area and glass content. The untreated Qz-reference material proved to be basically inert under the chosen conditions and consequently a good reference material for other hydration studies. The fine Qz powder reacted slightly in the reactivity test.

Acknowledgements

The authors wish to acknowledge Schwenk Cement KG for the financial and technical support.

Literature

/1/ E. Gartner (2004). Cement and Concrete Research 34(9), 1489-1498.

/2/ F. Blanco, M. P. Garcia, J. Ayala (2005). Fuel 84, 89-96.

/3/ D. Jansen, Ch. Stabler, F. Goetz-Neunhoeffer, S. Dittrich, J. Neubauer. Powder Diffraction 26 (1), 31-38.

/4/ B. Lothenbach, K. Scrivener, R. D. Hooton (2010). Cement and Concrete Research 41(3), 217-229.

/5/ F. Deschner, B. Lothenbach, F. Winnefeld, S. Dittrich, F. Goetz-Neunhoeffer, J. Neubauer (2010). Jahrestagung der Fachgruppe Bauchemie, 11-18

Hydration of Portland cement with high replacement by siliceous fly ash

F. Deschner, F. Winnefeld, B. Lothenbach, S. Seufert, P. Schwesig, S. Dittrich,J. Neubauer

Cement and Concrete Research, 2012, Vol. 42, p. 1389-1400

doi:10.1016/j.cemconres.2012.06.009

Paper II

Hydration of Portland cement with high replacement by siliceous fly ash

Florian Deschner a,⁎, Frank Winnefeld a, Barbara Lothenbach a, Sebastian Seufert b, Peter Schwesig b,Sebastian Dittrich c, Friedlinde Goetz-Neunhoeffer c, Jürgen Neubauer c

a Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for concrete and construction Chemistry, Überlandstrasse 129, 8600 Dübendorf, Switzerlandb BASF Polymer Research, Polymers for Inorganics, BASF Construction Chemicals GmbH, 83308 Trostberg, Germanyc GeoZentrum Nordbayern, Mineralogy, University of Erlangen-Nuremberg, 91054 Erlangen, Germany

a b s t r a c ta r t i c l e i n f o

Article history:Received 10 December 2011Accepted 26 June 2012

Keywords:Hydration (A)Fly ash (D)Blended Cement (D)Pozzolan (D)Filler (D)

The effects of two different low calcium fly ashes on the hydration of ordinary Portland cement (OPC) pastescontaining 50 wt.% of fly ash were investigated over a hydration time of 550 days. The results were comparedwith a reference blend of OPC containing 50 wt.% of inert quartz powder allowing the distinction between"filler effect" and pozzolanic reaction.Until 2 days, no evidence of fly ash reaction wasmeasured and its influence on the hydration is mainly relatedto the “filler effect”. From 7 days on, the effects of the pozzolanic reaction were observed by the consumptionof portlandite, the change of the pore solution chemistry, the formation of a presumably water-rich inner hy-dration product and the change of the C–S–H composition towards higher Al/Si ratio compared to the C–S–Hof neat OPC. Additional strength due to the pozzolanic reaction developed after 28 days of hydration.

© 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The role of fly ash in cement and concrete has been of great inter-est in the past due to its potential to reduce the CO2 emissions [1] andimprove the durability of concrete [2–5]. Different aspects of the reac-tion of siliceous fly ash (type V according to EN 197‐1) as inert filler atearly hydration times, and as active pozzolanic material at later hy-dration times have been reported [6–14]. It has been observed thatthe sole presence of fly ash increases the reactivity of the OPC clinkerdue to the so-called “filler effect” [6–8,11,15–18]. The “filler effect”describes the effect of adding a material which (initially) does notreact itself but causes:

• The provision of additional nucleation sites on the surface of thefiller for hydrates from the OPC (seeding effect).

• The increase of the effective water-to-cement (w/c) ratio, when thetotal water-to-solid ratio is held constant. Thereby the hydration ofOPC is promoted [10,19,20] due to more space for the growth ofhydrates.

• The change of particle packing. Thewide particle size distribution andthe spherical particle shape of certain materials like fly ash may im-prove the packing and the workability of the cement paste.

While the “filler effect” promotes the hydration of OPC at earlyhydration times, the fly ash itself has been reported to show little orno reaction at hydration times up to 7 days [6,8,10]. At later hydra-tion times the pozzolanic reaction of fly ash with portlandite was

observed. By the reaction of silicate from the fly ash with Ca(OH)2,portlandite is consumed [6,9,10,12,13] and C–S–H with a reducedCa/Si ratio is formed [21–23]. Depending on the reactivity of the flyash and the detection limit of the various methods to investigate itsreaction, different results for the onset of the pozzolanic reactionhave been measured. Usually the decrease of the portlandite contentwas measured after 7 days of hydration [8,10,20] and in individualcases after 3 days [24] or after 28 days of hydration [25]. The determi-nation of the fly ash reaction by the pozzolanicity test at 40 °C(according to EN 196–5) was found not to start before a hydrationtime of 2 days [8].

Another recently discussed topic is the combination of fly ashblended cements with limestone [7,14]. In Portland cements, lime-stone addition stabilises monocarbonate in favour of monosulphate[26,27]. Thereby the conversion of ettringite to monosulphate atlater hydration times is hampered. The stabilisation of the volumi-nous, water rich ettringite instead of the less voluminous mono-sulphate gives rise to an increase of the total volume of hydrationproducts [26–28]. In fly ash blended cements, the reaction of the flyash provides additional alumina and thereby lowers the sulphate toalumina ratio of the bulk paste. Hence the conversion of ettringiteto monosulphate is theoretically promoted and the inhibition of thisconversion more important than in neat OPC with limestone. Further-more the carbonate provides, together with calcium, a partner for achemical reaction with the aluminium from the fly ash to formmonocarbonate.

To study the effects of the replacement of OPC by fly ash the di-rect comparison of neat OPC with fly ash blended pastes can bemisleading, since fly ash features two different effects at the same

Cement and Concrete Research 42 (2012) 1389–1400

⁎ Corresponding author. Tel.: +41 58 765 4761; fax: +41 58 765 4035.E-mail address: florian.deschner@empa.ch (F. Deschner).

0008-8846/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.doi:10.1016/j.cemconres.2012.06.009

Contents lists available at SciVerse ScienceDirect

Cement and Concrete Research

j ourna l homepage: ht tp : / /ees .e lsev ie r .com/CEMCON/defau l t .asp

time: (i) the “filler effect” which promotes OPC hydration by theseeding effect and the increase of the effective w/c and (ii) the poz-zolanic reaction which leads to a consumption of portlandite andthe formation of additional C–S–H. Since both effects influence thehydration, the detection of the pozzolanic reaction becomes com-plicated. To clearly see the effect of fly ash, the characteristics ofthe fly ash blend were compared with a similar blend containingan inert quartz powder of similar particle size distribution (PSD) in-stead of fly ash.

This study presents a dataset for the hydration of an OPC blendedwith two different siliceous fly ashes and a practically inert quartzpowder. To illustrate the effect of limestone addition to fly ash blend-ed OPC, additional samples with a replacement of 5 wt.% of fly ash bylimestone were studied. Mechanical testing of the compressivestrength was done on mortar prisms. The hydration was investigatedby means of isothermal heat flow calorimetry, thermogravimetricanalysis (TGA) and chemical shrinkage. Additionally the solid phaseswere characterised by means of X-ray diffraction (XRD), TGA andscanning electron microscopy (SEM) and energy dispersive X-rayspectroscopy (EDX). The pore solutions were extracted and analysedby means of inductively coupled plasma optical emission spectrosco-py (ICP-OES).

2. Materials and methods

2.1. Characterisation of the raw materials

As raw materials an OPC (CEM I 42.5 N), two siliceous fly ashes(type V according to EN 197‐1), a quartz powder (Qz) and a lime-stone powder with a calcite content of >99% were used. The chemicaland mineralogical composition of the OPC and the two fly ashes(F1 and F2) determined by means of X-ray fluorescence (XRF)and quantitative X-ray diffraction [29] is shown in Table 1. The

contents of CO2 and SO3 were determined by combustion of thematerials at temperatures up to 2000 °C under O2-atmosphereand analysis of the exhaust gas by an infrared detector. Additionallythe distribution of alkalis between sulphates and oxides in the cementwas calculated using the measured concentrations of readily solublealkalis in deionised water at a solid/water ratio of 0.1 after an equilibra-tion time of 10 min. These readily soluble alkalis are assumed to corre-spond to the alkali sulphates present in the clinker, while the remainingalkalis are assumed to be present asminor constituents in solid solutionwithin the major clinker phases [21,30]. In the fly ashes the amount ofreadily soluble alkalis is much lower than in the OPC, which showsthat the bulk of the alkalis is incorporated in the glass of the fly ash(Table 1). The average compositions of the fly ash glasses were calculat-ed by subtracting the crystalline phases and the readily soluble alkalisfrom the total composition of the fly ash and normalizing the resultsto 100 wt.%.

The main difference between the two fly ashes is the higher crys-talline fraction in F2 (32.3 wt.% in F2 compared to 16.7 wt.% in F1[31]), which also contains small amounts of reactive phases likelime and anhydrite. Furthermore F2 shows an overall higher amountof CaO and SO3 and lower content of K2O compared to F1.

Fig. 1 shows the thermogravimetric analysis (TGA) of F1 in an at-mosphere of N2 and O2, similar to air (oxidizing conditions), and inHe-atmosphere (protective gas) up to a temperature of 1000 °C. TheTGA was coupled with a mass spectrometer to analyse the exhaustgases. In the air-like atmosphere, an initial weight increase indicatingoxidation processes in F1 is observed. From 520 °C to 800 °C the flyash loses 3% of its mass. The mass spectrometer analysis shows thatthis weight loss is due to the release of CO2 related to the oxidationof elemental carbon in the fly ash. The measured ion current of sul-phate (as SO3) from F1 is below 10−12 A and therefore negligible. InHe-atmosphere, the oxidation of fly ash by air is prevented and theweight loss due to the continued release of CO2 starting from 450 °C

Table 1Chemical and mineralogical composition of the used materials and analysis of readily soluble compounds. The mean fly ash glass composition was estimated by subtracting thecrystalline and the readily soluble phases from the total XRF composition.

XRF-analysisa [wt%] Mineralogical phase compositionb [wt%] Glass composition [wt%]

F1 F2 OPC F1 F2 OPC F1 F2

SiO2 50.9 45.0 19.4 Mullite 8.2 19.5 C3S 57.1 SiO2 54.4 50.3Al2O3 24.7 26.5 5.2 Quartz 7.0 7.6 β C2S 7.9 Al2O3 24.8 22.9Fe2O3 7.3 8.5 3.6 Hematite 0.7 1.2 α' C2S 9.3 Fe2O3 5.7 7.6CaO 3.7 5.6 62.1 Magnetite 0.8 1.0 C4AF 13 CaO 4.6 7.5MgO 1.8 2.8 1.9 Anhydrite 0.8 C3A cubic 2.0 MgO 2.1 2.4K2O 3.9 2.0 1.2 Periclase 1.3 C3A othorh. 2.0 K2O 4.9 2.9Na2O 0.9 1.3 0.3 Lime 0.9 Calcite 0.4 Na2O 1.1 1.9TiO2 1.1 1.2 0.2 Amorphous 83.3 67.7 Periclase 1.0 TiO2 1.4 1.7Mn2O3 0.1 0.1 0.1 Bassanite 2.6 Mn2O3 0.1 0.1P2O5 0.8 1.1 0.6 Anhydrite 2.8 P2O5 0.9 1.6SO3 0.4c 0.6c 3.7 Arcanite 0.5 SO3 0.0 1.2SrO 0.3 Dolomite 0.5LOI 3.5 4.2 1.1 Magnesite 0.3C 2.7d 2.9d Siderite 0.2

Quartz 0.2Ankerite 0.2

Sum 99.1 98.9 99.7 100.0 100.0 100.0 100.0 100.0

Readily soluble alkalis [wt%]

F1 F2 OPC

CaO 0.11 0.41K2O 0.10 0.01 0.89Na2O 0.03 0.01 0.08SO3 0.30 0.27

a Standard deviation of the XRF analyses is b0.1 wt.% for all oxides except for SiO2 (0.5 wt.%).b Standard deviation ranges from 0.1–0.6 wt.%.c SO3-content (not included in LOI) determined by combustion analysis.d C-content (included in LOI) determined by combustion analysis.

1390 F. Deschner et al. / Cement and Concrete Research 42 (2012) 1389–1400

until the final temperature of 1000 °C is only 0.5 wt.%. The release ofthese minor amounts of CO2 is due to redox-reactions in which car-bon in the fly ash is oxidized and other oxides such as SO3 or Fe2O3

are reduced.The PSD of the used mineral additions measured by means of laser

diffraction are shown in Fig. 2. The overall PSD of the fly ashes and Qzare relatively similar, but the fly ashes show a higher fraction ofparticles with a diameter smaller than 4 μm compared to Qz. Due tothe occurrence of hollow and porous particles in fly ashes, theBET-specific surface areas of F1 (1.8 m2/g) and F2 (2.4 m2/g) arehigher than that of Qz (0.8 m2/g). The limestone powder shows a uni-form PSD with a d50 of 2.2 μm and a BET-specific surface of 4.5 m2.

Qz is used as a crystalline inert reference material for the hydra-tion studies. In a test for pozzolanic activity 100 g of Qz was mixedwith 70 g of portlandite, 10 g of calcite and 180 ml of 0.3 m KOH so-lution. The samples were stored under sealed conditions at 23 °C andthe hydration was stopped by solvent exchange with isopropanol andwashing with diethylether. The samples were investigated by meansof TGA in N2-atmosphere 5 min after mixing and after 90 days of re-action (see Fig. 3). The similar TGA and differential thermogravimetry(DTG) diagrams of Qz at 5 min and 90 days show that no portlanditehas been consumed. Therefore Qz is considered to exhibit no pozzola-nic reactivity within 90 days of hydration.

2.2. Hydration studies

The hydration studies were carried out on paste samples of theneat OPC and blends of this OPC containing 50 wt.% of F1 (OPC–F1),50 wt.% of F2 (OPC–F2) and 50 wt.% of the inert reference quartzpowder (OPC–Qz). Additionally, samples with 50 wt.% OPC, 45 wt.%F1 and 5 wt.% of limestone (OPC–F1–L) were studied to investigatethe effect of combining limestone with fly ash.

The cement pastes with a water-to-binder ratio (w/b) of 0.5 wereprepared in a mixer according to EN 196‐3. The samples were cast in500 ml and 60 ml polyethylene flasks, sealed, rotated during the first24 h to avoid segregation, and stored at 23 °C. The 500 ml sampleswere used for the extraction of the pore solution and the 60 ml sam-ples for the analysis of the solid phases. The solid phases wereanalysed by means of TGA in N2-atmosphere, XRD, and scanning elec-tron microscopy (SEM).

For XRD investigations of the hardened paste samples, 5 mm thickdisks with a diameter of 30 mm were sliced from the cast samplesand then immersed in isopropanol for 5–30 min. Measurementswere performed immediately afterwards in a PANalytical X'Pert ProMPD diffractometer with attached X'Celerator detector. For the inves-tigations by means of TGA, pieces of the hydrated samples wereground, immersed in isopropanol for 15 min, washed with dieth-ylether, and filtered. Just before the analysis the samples were driedfor 7–8 min at 40 °C to evaporate the remaining diethylether andanalysed instantly in a Mettler Toledo TGA/SDTA 851e device at aheating rate of 20 K/min in N2-atmosphere. The analysed boundwater of the hydrate phases (H) and the Ca(OH)2 content (CH) areexpressed as percentage of the dry sample weight at 500 °C (w500)[7]:

H ¼ w50−w500

w500

and

CH ¼ w400−w470

w500⋅M Ca OHð Þ2� �

M H2Oð Þ

where M is the molar mass and wn is the dry sample weight at thetemperature n °C.

The exact boundaries for the temperature intervals were readfrom the derivative curve (DTG). Triple preparation and measure-ment of the samples after 1, 2 and 7 days of hydration showed a rel-ative error of 5–10%. This includes the errors caused by preparation,measurement and heterogeneities within one sample.

100 200 300 400 500 600 700 800 900 100097.5

98.0

98.5

99.0

99.5

100.0

100.5

1

2

3

4

5

6

7

mass (protective gas) mass (oxidzing conditions)

Mas

s [w

t%]

Temperature [°C]

CO2 current

(protective gas)

CO2 current

(oxidizing conditions)

Ion

curr

ent [

10-1

1 A]

Fig. 1. TGA of F1 and the corresponding CO2 ion current measured by means of massspectrometry.

0.1 1 10 100 1000

0.1 1 10 100 1000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Rel

ativ

e fr

eque

ncy

[vol

%]

Size [μm]

F1 F2 Qz

0

10

20

30

40

50

60

70

80

90

100

Pas

sing

vol

ume

[vol

%]

Fig. 2. Particle size distributions of the blended materials measured by means of laserdiffraction. The solid lines show the relative frequency of particles in dependence ofthe diameter and the dashed lines show the volume of the particles smaller than acertain diameter.

100 200 300 400 500 60070

75

80

85

90

95

100

-0.20

-0.16

-0.12

-0.08

-0.04

0.00

0.04

0.08

0.12

0.16

0.20

0.24

Qz 5 min Qz 90 d

Wei

ght l

oss

[wt%

]

Temperature [°C]

DT

G [1

/°C

]

CH

Fig. 3. TGA of Qz in the reactivity test with portlandite and calcite in a 0.3 m KOH so-lution after 5 min and 90 days. The upper curves correspond to the relative weightloss and the lower ones to its derivation (DTG). CH — portlandite.

1391F. Deschner et al. / Cement and Concrete Research 42 (2012) 1389–1400

For the SEM investigations disks of the hardened samples wereobtained in the same matter as for the XRD investigations, kept for3 days in isopropanol and afterwards dried at 40 °C for 3 days. Thepreparation for the SEM was done by impregnating the samples bya modified bisphenol-A-epoxy-resin. After polishing, the sampleswere carbon-coated and investigated under high vacuum conditionsin a Philips ESEM FEG XL 30. For energy dispersive X-ray spectroscopy(EDX) analyses of the hydrate phases, focussed on the characterisa-tion of C–S–H, a beam voltage of 10 kV was chosen to reduce theinteraction volume of the electron beam with the sample. Accordingto Monte-Carlo simulations, this setting results in a penetrationdepth of 0.75–1.5 μm and a lateral distribution of non-adsorbedX-rays of 0.8–1.4 μm, depending on the chemistry of the analysedphase [32]. Thus the maximal excitation volume of one EDX pointanalysis at 10 kV is 1.6 μm3.

The pore solutions of the hardened samples were extracted by thesteel die method [33] using pressures up to 250 N/mm2, and the solu-tions were filtered immediately. The pH of the pore solutions wasanalysed with a pH electrode, calibrated against KOH solutions withknown concentrations, and the free OH− concentrations were calcu-lated. The K, Na, Ca, Al, Si and sulphur concentrations were measuredby means of inductively coupled plasma optical emission spectrosco-py (ICP-OES). The standard deviation of the measured elemental con-centrations ranged between 5 and 10%.

Chemical shrinkage was measured according to procedure A de-scribed in ASTM Standard C 1608‐07 in a water bath at 23 °C [34].

The heat flow of the hydration was measured by a TAM Air iso-thermal calorimeter. For each measurement 15 g of the dry cementsample were mixed by hand with a spatula for 1 min with 7.5 g ofdeionised water. Afterwards 3 cells of the calorimeter were each filledwith 3 g (1.4 cm3) of cement paste.

For the investigation of the compressive strength two mortarprisms (40×40×160 mm3) for each testing age were prepared andtested according to EN 196‐1.

3. Results and discussion

3.1. Kinetics of hydration

3.1.1. CalorimetryFig. 4 shows the development of the specific heat flow of the hy-

dration of OPC, OPC–Qz, OPC–F1, OPC–F2 and OPC–F1–L normalizedto the OPC content. Each curve represents the average of three, wellreproducible measurements. The error of the specific heat flow is±0.1 mW/g for hydration times between 30 min and 48 h.

In the neat OPC and in the OPC–Qz blend the induction periodstarts after 1 h of hydration. The acceleration period of the OPC andthe OPC–Qz, which is associated with the accelerated formation rateof C–S–H [35], begins after 2.5 h and the second heat flow maximumreaches 3.8 mW/g OPC. During the subsequent deceleration period, a“shoulder” in the specific heat flow of the OPC–Qz blend is observed.It correlates to the depletion of the calcium sulphate phases andis assigned to a second aluminate reaction presumably resultingin the formation of ettringite or AFm (aluminium iron mono-)phase [13,36–40]. The fact that this “shoulder” is present in theQz-blended OPC as well as in the fly ash blended OPC showsthat it is related to the “filler effect”, i.e. the seeding effect offillers. Slight differences in the extent of the “shoulder” relatedto the second aluminate reaction might be explained by differentseeding effects due to the different fineness of the three blends[17]. In the case of OPC–F2, also the increased content of avail-able Ca and sulphate in F2, due to the presence of 0.8 wt.%anhydrite in the anhydrous F2 (Table 1), may be responsible forthe higher specific heat flow between 12 and 48 h.

The main heat flow peak after 9 h of hydration correlating to thesilicate reaction shows only a slight increase of the heat flow per gof OPC by the addition of Qz. This illustrates that the seeding effectof Qz and fly ash is affecting the aluminate reaction more than thesilicate reaction. This is supported by another study, which showsthat the released heat due to the “filler effect” is related to a fastersecond C3A hydration [40].

The fly ash blends show a different behaviour; the inductionperiods last 2 h longer compared to the other samples, and the heatflow during the induction period is reduced by 0.3 mW/g OPC. Thesecond heat flow maximum of the fly ash blends occurs 3.5 h laterthan in the samples without fly ash. This retarding effect of fly ashon the OPC hydration has been reported previously [8,10,41–47].The retardation is dependant on the type of cement used and is pro-longed by increasing levels of fly ash replacement [48]. It has beenproposed that this effect is produced by the reduced OPC contentdue to the fly ash replacement [41]. However, the reference blendwith Qz, which is diluted in the same manner as the blends with flyash, does not show this retardation. Therefore the change of the effec-tive w/c ratio alone is not sufficient to explain the observed hydrationkinetics.

Another effect of the addition of fly ash is the introduction of addi-tional readily soluble compounds (Table 1). However, Fajun et al. [44]showed that the addition of the leachate of fly ash or the readilysoluble compounds, respectively, has no influence on the cement hy-dration. An alternative explanation for the longer induction period infly ash blended OPC was given by showing its correlation to lower Caconcentrations in the pore solution [44]. It is suggested that the flyash surface acts as a Ca sink, which is caused by a reaction of the alu-minate in the fly ash with the Ca from the solution and/or chemisorp-tion of Ca ions on the fly ash surface [44,49]. This would retard theformation of C–S–H nuclei and thereby delay the end of the inductionperiod.

Fig. 5 shows the cumulative heat normalized to the amount of OPCstarting from 0.5 h of hydration. OPC–Qz starts to release more heatcompared to the OPC after 10–12 h of hydration, which is related tothe promotion of the clinker reaction by the seeding effect of thefiller. This effect is also observed in the fly ash blends, although thecurves of the released heat are delayed for 3.5 h due to the longerand more pronounced induction period.

OPC–F1 shows no significantly higher heat of hydration after2 days compared to OPC–Qz. However, additional heat is released inOPC–F2 compared to OPC–Qz after 28 h. This is related to the higherspecific heat flow during the deceleration period between 12 and48 h due to the enhanced aluminate reaction.

The replacement of 5 wt.% of F1 by limestone has no significantinfluence on the heat released during the first 2 days of hydration.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160

1

2

3

4

52040

Spe

cific

hea

t flo

w [m

W/g

OP

C]

Time [h]

0 5 10 15 20 25 30 35 40 450

1

2

3

4

52040 OPC

OPC-Qz

OPC-F1

OPC-F2

OPC-F1-L

Spe

cific

hea

t flo

w [m

W/g

OP

C]

Time [h]

Fig. 4. Specific heat flow normalized to the mass of OPC at 23 °C with a w/b ratio of 0.5.

1392 F. Deschner et al. / Cement and Concrete Research 42 (2012) 1389–1400

3.1.2. Chemical shrinkageTo monitor the process of hydration for longer hydration times,

chemical shrinkage was measured and normalized to the mass ofOPC in each sample (see Fig. 6). The promotion of the OPC by the“filler effect” can be seen after about 1 day of hydration, resultingin more chemical shrinkage in the Qz blend and the fly ash blends.This is related to the seeding effect of the filler materials, whichenhances mainly the aluminate reaction and increases of the effec-tive w/c ratio.

The determination of the onset of the pozzolanic reaction bymeans of chemical shrinkage is difficult due to the uncertainty ofthe measurement. Higher chemical shrinkage in OPC–F2 comparedto OPC–Qz indicates the effect of the fly ash reaction after already2–3 days. However, a significant increase of chemical shrinkage inOPC–F1 was found after 8 days of hydration. The chemical shrinkageof OPC–F2 is tending towards higher values compared to OPC–F1. Thisis consistent with the additional heat released during the first 2 days,related to the enhanced aluminate reaction. However, the differencein chemical shrinkage is not significant.

The replacement of 5 wt.% F1 by limestone results in more chem-ical shrinkage compared to OPC–F1 starting from 3 days of hydration.This might be explained by the stabilisation of ettringite by limestone.

3.2. Portlandite and bound water content

Fig. 7 shows the development of portlandite and bound watercontents normalized to the mass of dry OPC. The comparisonbetween the neat OPC and the OPC–Qz shows slightly highercontents of bound water in the Qz-blend after 7 days of hydra-tion. A similar development is observed for the portlandite,which is a confirmation of the promotion of the OPC hydrationby the “filler effect” of Qz, as it is also observed by means ofchemical shrinkage (Fig. 6).

OPC–F1 and OPC–F2 show higher contents of bound water after7 days and 2 days of hydration compared to OPC–Qz. The por-tlandite contents in both fly ash blends show a maximum at7 days and start to decrease afterwards, due to the pozzolanic reac-tion which consumes portlandite. The starting consumption of por-tlandite in OPC–F1 is indicated between 2 and 7 days due to thereduced portlandite content in OPC–F1 at 7 days of hydration com-pared to OPC–Qz.

The replacement of 5 wt.% F1 by limestone does not result in a sig-nificant change of the portlandite contents. However, the developmentof additional bound water at hydration times after 90 days has beenobserved,which is probably related to the stabilisation of water rich hy-dration products, i.e. ettringite and monocarbonate.

0 5 10 15 20 25 30 35 40 450

50

100

150

200

250

300

350 OPC

OPC-Qz

OPC-F1

OPC-F2

OPC-F1-L

Rel

ease

d he

at [J

/g O

PC

]

Time [h]

Fig. 5. Released heat normalized to the amount of OPC at 23 °C with a w/b ratio of 0.5.

5 10 15 20 250.00

0.05

0.10

0.15 OPC

OPC-Qz

OPC-F1

OPC-F2

OPC-F1-L

Che

mic

al s

hrin

kage

[ml/g

OP

C]

Time [d]

Fig. 6. Chemical shrinkage of OPC, OPC–F1, OPC–F2 and OPC–F1 with limestone, nor-malized to the mass of OPC. The error represents the standard deviation of threemeasurements.

0.1 1 10 100

0

5

10

15

20

25

30

35

40 OPC

OPC-Qz

OPC-F1

OPC-F2

OPC-F1-L

Bou

nd w

ater

[g/1

00 g

OP

C]

Time [d]

0.1 1 10 100

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28 OPC

OPC-Qz

OPC-F1

OPC-F2

OPC-F1-L

Por

tland

ite [g

/100

g O

PC

]

Time [d]

b)

a)

Fig. 7. Content of bound water (a) and portlandite (b) determined by TGA andnormalized to the OPC content in the samples after hydration times up to550 days. The relative error due to preparation and measurement is between±5% and 10%.

1393F. Deschner et al. / Cement and Concrete Research 42 (2012) 1389–1400

3.3. Characterisation of the hydration products

3.3.1. XRDDetails of themeasured XRD diffractograms of the samples after 2 to

90 days are shown in Fig. 8. The identified crystalline phases areettringite, monosulphate (C4AsH12) and an AFm phase, which is mostlikely a solid solution of hemicarbonate and highly OH−-substitutedmonosulphate [50]. In the sample with replacement of 5 wt.% F1 bylimestone, hemicarbonate andmonocarbonate were also found. The re-flections of the AFm phases result in relatively broad peaks due to thepoor crystallinity of AFm and due to the presence of solid solutions aris-ing from the partial replacement of OH− by SO4

2− and to aminor extentby CO3

2−.Qualitatively no significant differences can be seen between the

four samples without limestone.

The replacement of 5 wt.% F1 by limestone results in the stabilisationof hemicarbonate instead ofmonosulphate from early ages. After 28 daysthe hemicarbonate content starts to decrease in favour of the formation ofmonocarbonate.

Fig. 9 shows details of the diffractograms of the investigated sam-ples after 250 and 550 days. It shows the presence of hydrogarnet inthe neat OPC and the OPC–Qz after 250 days of hydration. Hydrogarnetwas also found in OPC–F1 after 550 days of hydration.

3.3.2. TGAThe DTG curves of the investigated samples after 90 days of hydra-

tion show peaks due to weight losses related to the dehydration ofettringite, AFm and portlandite at 105 °C, 175 °C and 455 °C, respective-ly (Fig. 10). The broad peak at 350 °C probably indicates traces ofhydrotalcite. In the sample containing 5 wt.% of limestone, the peak of

°2 Theta Cu K-alpha

7 d 28 d 90 d

F

E

MS

AFmss

MC

HCE

MS

AFmss

MCHC

E

MS

AFmss

HC

FF

8 9 10 11 12 8 9 10 11 12 8 9 10 11 128 9 10 11 12

E

MS

AFmss

F

OPC-F2

OPC-F1

OPC-Qz

OPC

OPC-F1-L

2 d

HC

Fig. 8. Details of the XRD diffractograms of the investigated samples after 2, 7, 28 and 90 days. F — ferrite, E — ettringite, MS — monosulphate (C4AsH12), AFmss — solid solution ofhemicarbonate and OH− substituted monosulphate, HC — hemicarbonate, MC — monocarbonate.

OPC-F2

OPC-F1

OPC-Qz

OPC

OPC-F1-L

19181716151413121110988 9 10 11 12 13 14 15 16 17 18 197 7

°2 Theta Cu K-alpha

250 d 550 d

E

MS

AFmss

F

HC MC

EMu

HG

CH

E E

MS

AFmss

F

HC MC

EMu

HG

CH

E

Fig. 9. Details of the XRD diffractograms of the investigated samples after 250 and 550 days. F — ferrite, E — ettringite, MS — monosulphate (C4AsH12), AFmss — solid solution ofhemicarbonate and OH− substituted monosulphate, HC — hemicarbonate, MC — monocarbonate, Mu — mullite, HG — hydrogarnet.

1394 F. Deschner et al. / Cement and Concrete Research 42 (2012) 1389–1400

the AFm phase is shifted to lower temperatures due to the presenceof monocarbonate instead of monosulphate [27,51]. Additionallytwo peaks at 670 °C and 730 °C are observed. They correspond tothe release of CO2 from calcite. In the samples containing fly ash,minor weight losses at temperatures higher than 850 °C are ob-served due to the oxidation of elemental carbon and release ofCO2 (Fig. 1).

3.3.3. Analysis of C–S–H composition by SEM-EDXSEM-EDX analyses of the hydrate phases of OPC and fly ash blend-

ed OPC were carried out in order to characterise the C–S–H phase,which is mainly X-ray amorphous. EDX point analyses were targetedat the outer product in the matrix of hydrates and the inner productof fly ash or clinker. Due to the interaction volume of electrons withthe specimen and the intergrowth of hydrates, the detected X-raysof one spot often consist of the signal of a mix of two or more phases[52]. To derive the composition of a phase, element ratio plots like Al/Caversus Si/Ca were used [53,54]. These are shown in Figs. 11 and 12 forOPC and OPC–F1 after hydration times of 250 and 550 days.

The Al/Si ratio of C–S–H is determined by the slope of a line, whichis drawn through the points with the lowest Al/Ca ratio and repre-sents mixed analyses of portlandite and C–S–H without AFm orettringite. The range of Si/Ca ratios of the C–S–H is represented by the

bulk of data points along this line. Measurements with lower Si/Caratios representmixed analyses of C–S–Hand portlandite. The fewmea-surementswith higher Si/Ca ratios are due to the error ofmeasurement,which may arise from too low intensities of the EDX spectra when e.g.analysing a spot near a pore. The bulk of EDX analyses of the C–S–H inthe neat OPC sample after 250 and 550 days gave Ca/Si values in therange between 1.6 and 2.0. This variance is in agreementwith transmis-sion electron microscopy EDXmeasurements, showing a high variationof the Ca/Si ratio of the C–S–H, evenwithin the inner product of a singleclinker grain [55]. The C–S–H in the OPC after 250 and 550 days of hy-dration has a Ca/Si ratio of 1.8±0.2 and an Al/Si ratio of 0.07±0.02.

The data points of the fly ash blends are more scattered as the hy-drates are often analysed together with fly ash particles of variablecomposition in the vicinity, or below the sample surface. The scatterof the data points complicates the exact determination of thecomposition of the C–S–H. However, an increase of the Al/Si ratios(to 0.17±0.03) and the tendency of decreased Ca/Si ratios (1.3±0.2)in the F1 blend compared to the pure OPC are observed. This is consis-tent with earlier studies on fly ash blended cements [22,23,43,56], andconfirmed by the measurements of the other investigated samples(Table 2). The changes of the C–S–H composition within one sampleduring hydration times between 28 and 550 days are not significantand within the error of the measurements. However, the comparisonof the atomic ratios in the fly ash blended samples after 28 and550 days of hydration shows the tendency of decreasing Ca/Si and in-creasing Al/Si ratios with advancing hydration.

The lower Ca/Si ratios in the fly ash blended OPC are due to the ad-ditional Si, provided by the fly ash dissolution, which is incorporatedin the C–S–H and results in longer silicate chain lengths [23,57,58].This in turn leads to an increased uptake of aluminium, which hasbeen reported to be incorporated in the bridging tetrahedra of thesilicate chains [59,60].

100 200 300 400 500 600 700 800 90040

50

60

70

80

90

100

-0.16

-0.12

-0.08

-0.04

0.00

0.04

0.08

0.12

OPCOPC-QzOPC-F1OPC-F1-L

Wei

ght l

oss

[wt%

]

Temperature [°C]

DT

G [1

/°C

]

E

AFm

CH

Cc

C-S-H

Fig. 10. TGA of the neat OPC, OPC–Qz, OPC–F1 and OPC–F1–L after 90 days of hydra-tion. The upper curves correspond to the relative weight loss and the lower ones toits derivation (DTG). E — ettringite, CH — portlandite, Cc — calcium carbonate.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

OPC 250 d

OPC 550 d

Ettr

Jen Tob

AFm

CH

Si/Ca of OPC C-S-H

Si/Ca atomic ratio

Al/C

a at

omic

rat

io

Al/Si~0.07

Fig. 11. Plot of Al/Ca versus Si/Ca atomic ratios of the EDX-measurements with 10 kVfocussed on hydrate phases in the OPC after 250 days and 550 days of hydration. CH— portlandite; Jen — jennite; Tob — tobermorite; Ettr — ettringite.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

OPC-F1 250d

OPC-F1 550d

Ettr

Jen

AFm

CH

Tob

Si/Ca of OPC-F1 C-S-H

Si/Ca atomic ratio

Al/C

a at

omic

rat

io

Al/Si~0.17

Fig. 12. Plot of Al/Ca versus Si/Ca atomic ratios of the EDX-measurements with10 kV focussed on hydrate phases in the OPC–F1 after 28 days and 90 days of hy-dration. CH — portlandite; Jen — jennite; Tob — tobermorite; Ettr — ettringite.

Table 2Ca/Si and Al/Si ratios of the EDX analyses of the C–S–H in the OPC and fly ash blendedOPC pastes after 28 to 90 days and 250 to 550 days.

OPC OPC–F1 OPC–F2

28–90days

250–550days

28–90days

250–550days

28–90days

250–550days

Ca/Si 1.7±0.2 1.8±0.2 1.4±0.3 1.3±0.2 1.6±0.4 1.4±0.2Al/Si 0.07±0.02 0.07±0.02 0.15±0.03 0.17±0.03 0.16±0.03 0.17±0.03

1395F. Deschner et al. / Cement and Concrete Research 42 (2012) 1389–1400

3.4. Analysis of the pore solution chemistry

3.4.1. Neat OPCThe development of OPC pore solutions has been described by

various studies [61–63]. The elemental concentrations of the poresolutions of the neat OPC from this study are shown in Fig. 13. Dueto the fast dissolution of the alkali sulphates of the OPC and thepresence of the calcium sulphate phases, the solution is dominatedby K, Na, Ca, sulphur (S) and OH− during the first 8 h. At this timethe concentrations in the pore solution remain more or less constant,since they are controlled by the consumption of calcium sulphatephases and the formation of portlandite. The pH is measured to be13.1±0.1. Between 8 h and 1 day, the chemistry of the pore solutionof the OPC starts to change significantly due to the depletion of thecalcium sulphates. The Ca and S concentrations decrease while theOH–, Si and Al concentrations rise. This correlates with the secondaluminate reaction as seen in Fig. 4. In this period, the remaining sul-phate is consumed by the reaction with the aluminate phase and theprecipitation of ettringite or AFm [13,38,39,64]. Due to the resultingincreased dissolution of C3A, additional alkalis are introduced to thepore solution and the pH rises to 13.7±0.1. Due to the high pH, theCa concentrations in the solution decrease as they are limited by theportlandite solubility. Furthermore the solubilities of Si and Al areincreased. The continuous reaction of the clinker phases provides aconstant input of alkalis, which are partially bound in the C–S–Hphase and partially remain in the pore solution. Consequently the al-kali concentrations and the pH are slightly increasing even after longhydration times.

3.4.2. Blended cementsFig. 14 shows a comparison of the pore solution chemistry of the

samples OPC–F1 and OPC–Qz. The development of the pore solution inOPC–Qz is similar to that of the neat OPC. Only the concentrations of al-kalis, OH− and the initial sulphate concentrations are lower. This canalso be seen in the fly ash blends and is due to the fact that all the blendscontain 50 wt.% less OPC and less readily soluble alkalis. The pH of theOPC–Qz pore solution after 1 h of hydration is at 13.0±0.1. After thedepletion of the calcium sulphates it rises to a value of 13.4±0.1.

The development of the pore solutions of fly ash blended OPC(Figs. 14 and 15) shows generally the same trends, however, someimportant differences are observed. These differences are shown bythe comparison between OPC–F1 and OPC–Qz (Fig. 14). The Ca con-centrations in the pore solution of the fly ash blends decrease after7 days of hydration compared to the reference sample OPC–Qz. Thisis related to the consumption of portlandite by the pozzolanic reac-tion with fly ash. Additionally the alkali and OH− concentrations

tend to decrease slightly after 7 days of hydration. This is due to thebinding of alkalis in the C–S–H, which is enhanced by the additionalamounts of C–S–H generated by the pozzolanic reaction [22]. In addi-tion, C–S–Hwith a lower Ca/Si ratio, as it was found in fly ash blendedcements by means of SEM-EDX, binds more alkalis [65]. The decreaseof the alkali concentrations after 7 days of hydration is in agreementwith the data published by Diamond [61].

0.1 1 10 1000.01

0.1

1

10

100

1000

Con

cent

ratio

n [m

mol

/l]

K

OH-

S

Na

Ca

Si

Al

Time [d]

Fig. 13. Development of the elemental concentrations in the pore solution of the neat OPC.

0.1 1 10 1000.01

0.1

1

10

100

1000

Con

cent

ratio

n [m

mol

/l]

Time [d]

OH-

SAl

OH-

SAl

OPC-Qz OPC-F1

0.1 1 10 1000.01

0.1

1

10

100

1000

Con

cent

ratio

n [m

mol

/l]

Time [d]

KNaCaSi

KNaCaSi

OPC-Qz OPC-F1

b)

a)

Fig. 14. Development of the (a) S, Al, OH− and (b) K, Na, Ca, Si concentrations in thepore solutions of OPC–Qz and OPC–F1.

0.1 1 10 1000.01

0.1

1

10

100

1000

Con

cent

ratio

n [m

mol

/l]

KOH-

SNaCa

SiAl

Time [d]

Fig. 15. Development of the elemental concentrations in the pore solution of OPC–F2.

1396 F. Deschner et al. / Cement and Concrete Research 42 (2012) 1389–1400

Another important effect of the fly ash reaction on the chemistryof the pore solution is the increasing Al and Si concentrations after7 days of hydration compared to the reference sample blended withQz. This development is due to the introduction of Al and Si to thepore solution by the dissolution of the fly ash glass. Also the changeof the C–S–H composition towards lower Ca/Si ratios and higherAl/Si ratios is related to the decreased Ca and increased Al and Siconcentrations in the pore solution, assuming that the concentra-tions of these elements are governed by the solubility of C–S–H.The observed tendency of increasing Al and Si concentrations at latehydration times agrees with the results from Luke and Lachowski[66], who observed elevated Al and Si concentrations in 20 year oldfly ash blended OPC compared to neat OPC.

The replacement of 5 wt.% F1 by limestone does not result in asignificant change of the chemistry of the pore solutions (Fig. 16).This is in agreement with previous observations in OPC and OPC‐flyash blends [7,27].

3.4.3. Saturation indicesFrom the measured elemental concentrations in the pore solution

and the corresponding activities, the saturation indices (SI) of thesolid phases can be calculated. The SI with respect to the solids aregiven by log(IAP/KS0), where KS0 is the theoretical solubility productof the respective solid, while the IAP is the ion activity product calcu-lated from the measured concentrations. A positive SI implies over-saturation, a negative value undersaturation. As the use of SI can bemisleading when comparing phases which dissociate into a differentnumber of ions, effective SI were calculated by dividing the SI bythe number of ions participating in the reactions to form the solids,as described by Lothenbach et al. [67].

The effective SI of relevant hydrate phases in the investigatedsamples are shown in Table 3. Due to the depletion of the calcium sul-phates between 16 h and 1 day, the SI of the sulphate containingphases decrease. With respect to gypsum the solution turns fromnear saturation to a highly undersaturated level. Also the effective SIof ettringite and monosulphate decrease from a high level to a lowlevel of oversaturation. The solution keeps a slight oversaturationwith respect to portlandite in the samples without fly ash over thewhole hydration time. However, in the fly ash blended OPC, the satu-ration with respect to portlandite starts to decrease after 7 days ofhydration and becomes undersaturated between 28 and 90 days(Fig. 17). This indicates the consumption of CH by the pozzolanicreaction. The solutions in neat OPC are usually undersaturated withrespect to strätlingite. However, the pore solutions of fly ash blendedOPC become slightly oversaturated with respect to strätlingite after28 days and longer. This is consistent with the consumption of

portlandite and the additional Al and Si supplied by the reaction ofthe fly ash.

For OPC blended with F1 and 5 wt.% of limestone, the effective SIof hemi- and monocarbonate were calculated, assuming that thepore solution is saturated with respect to calcite. It can be seen thatthe pore solution is slightly oversaturated with respect to hemi- andmonocarbonate at all investigated hydration times greater than 4 h.After the depletion of the calcium sulphates after 1 day, the solutionshows a higher effective SI for hemi- and monocarbonate than formonosulphate (Table 3).

0.1 1 10 1000.01

0.1

1

10

100

1000

Con

cent

ratio

n [m

mol

/l]

K

OH-

S

Na

Ca

Si

Al

Time [d]

Fig. 16. Development of the elemental concentrations in the pore solution of OPC–F1–L.

Table 3Effective saturation indices of relevant hydrate phases, calculated by the elemental con-centrations in the pore solutions. CH — portlandite, Ettr — ettringite, Strätl — strätlingite,MS— monosulphate, HC — hemicarbonate; MC— monocarbonate.

Time [d] Effective saturation indices

CH Gypsum Ettr Strätl MS HC MC

OPC 1 h 0.21 0.11 0.53 −0.76 0.09 – –

4 h 0.19 0.05 0.45 −0.88 0.00 – –

8 h 0.11 −0.05 0.39 −0.85 −0.05 – –

16 h 0.19 −0.80 0.15 −0.74 −0.10 – –

1 day 0.21 −1.21 0.20 −0.19 0.12 – –

2 days 0.16 −1.33 0.04 −0.44 −0.06 – –

7 days 0.16 −1.29 0.06 −0.45 −0.05 – –

28 days 0.00 −1.38 0.03 −0.33 −0.05 – –

90 days 0.11 −1.22 0.21 −0.10 0.14 – –

250 days 0.21 −1.08 0.24 −0.24 0.14 – –

550 days 0.16 −1.01 0.27 −0.18 0.15 – –

OPC–Qz 1 h 0.12 0.01 0.56 −0.49 0.16 – –

4 h 0.08 −0.04 0.51 −0.52 0.11 – –

8 h 0.04 −0.06 0.63 −0.16 0.28 – –

16 h 0.12 −0.55 0.38 −0.38 0.11 – –

1 day 0.12 −0.94 0.32 −0.13 0.17 – –

2 days 0.18 −1.39 0.14 −0.21 0.09 – –

7 days 0.12 −1.06 0.25 −0.20 0.12 – –

28 days 0.15 −1.34 0.12 −0.29 0.06 – –

90 days 0.12 −1.24 0.15 −0.28 0.06 – –

250 days 0.02 −0.96 0.25 −0.22 0.09 – –

550 days −0.01 −0.93 0.26 −0.18 0.09 – –

OPC–F1 1 h 0.07 0.06 0.46 −0.79 0.01 – –

4 h 0.16 0.04 0.48 −0.82 0.03 – –

8 h 0.14 0.05 0.50 −0.75 0.06 – –

16 h 0.14 −0.32 0.30 −0.89 −0.08 – –

1 day 0.16 −0.95 0.22 −0.42 0.05 – –

2 days 0.18 −1.42 0.13 −0.20 0.10 – –

7 days 0.18 −1.43 0.12 −0.21 0.08 – –

28 days 0.02 −1.12 0.31 0.14 0.22 – –

90 days −0.05 −1.07 0.35 0.33 0.26 – –

250 days −0.22 −1.31 0.14 0.18 0.07 – –

550 days −0.38 −1.31 0.07 0.14 −0.03 – –

OPC–F2 1 h 0.10 0.01 0.55 −0.50 0.14 – –

4 h 0.08 −0.02 0.50 −0.56 0.09 – –

8 h 0.07 −0.05 0.65 −0.15 0.30 – –

16 h 0.11 −0.26 0.49 −0.38 0.16 – –

1 day 0.12 −0.81 0.28 −0.36 0.08 – –

2 days 0.18 −1.40 0.13 −0.23 0.08 – –

7 days 0.13 −1.07 0.24 −0.20 0.12 – –

28 days 0.01 −1.46 0.10 −0.04 0.07 – –

90 days −0.09 −1.41 0.11 0.06 0.06 – –

250 days −0.26 −1.07 0.19 0.09 0.04 – –

550 days −0.20 −0.95 0.26 0.10 0.10 – –

OPC–F1–L 1 h 0.09 −0.03 0.52 −0.54 0.11 −0.08 −0.044 h 0.07 −0.04 0.50 −0.54 0.09 −0.10 −0.058 h 0.06 −0.04 0.65 −0.14 0.30 0.09 0.1516 h 0.11 −0.33 0.46 −0.36 0.15 0.00 0.051 day 0.14 −0.89 0.27 −0.32 0.09 0.05 0.102 days 0.17 −1.31 0.15 −0.26 0.08 0.12 0.177 days 0.14 −1.04 0.22 −0.30 0.08 0.07 0.1228 days −0.02 −1.31 0.15 −0.04 0.08 0.09 0.1790 days −0.11 −1.31 0.17 0.15 0.11 0.11 0.20250 days −0.24 −1.25 0.15 0.16 0.05 0.03 0.13550 days −0.20 −1.08 0.24 0.17 0.12 0.07 0.16

1397F. Deschner et al. / Cement and Concrete Research 42 (2012) 1389–1400

3.5. Microstructure

Fly ash is a heterogeneous material, which is characterised by ahigh variance of particles in terms of chemical composition, crystal-linity and porosity. Consequently the illustration of just a few parti-cles can hardly be representative for the whole fly ash. However, inorder to illustrate different stages of the fly ash hydration, BSE imagesof polished sections showing relevant features of the microstructureafter 7, 90 and 550 days of hydration have been selected (Fig. 18).

At 7 days of hydration (Fig. 18a) a layer of hydration products hasbeen found around the fly ash particles. These hydration products aremost likely formed by the hydration of OPC. However, a minor contri-bution of fly ash cannot be excluded. Furthermore, at 7 days of hydra-tion a very fine and dark layer between the first layer of hydrates andthe fly ash particle already indicates the formation of an inner hydra-tion product (IP).

After 90 days of hydration (Fig. 18b) an IP was observed betweenthe first dense layer of hydration products at the original surface of

0.1 1 10 100

-1.0

-0.5

0.0

0.5

Effe

ctiv

e sa

tura

tion

inde

x

Time [d]

OPCOPC-QzOPC-F1

OPCOPC-QzOPC-F1

SI: strätlingiteSI: portlandite

Fig. 17. Effective saturation indices of portlandite and strätlingite calculated by the el-emental concentrations in the pore solutions.

Fig. 18. SEM BSE images of polished sections of OPC–F1 after (a) 7 days, (b) 90 days and (c–f) 550 days of hydration. 1— barely reacted fly ash particles; 2— partially reacted fly ashparticles; 3 — completely reacted fly ash particles.

1398 F. Deschner et al. / Cement and Concrete Research 42 (2012) 1389–1400

the fly ash and the remaining unreacted fly ash particle. The relativelylow grey values of the IP indicate that it is characterised by a highly po-rous area, which is assumed to arise from the presence ofwater-rich hy-dration products within the IP. Additionally, fine fibres of presumablyC–S–H and sometimes somewhat denser areas of hydrates were foundin the IP.

After 550 days of hydration (Fig. 18c and d) partially and fully hy-drated fly ash particles typically show a microstructure with a denserim of hydration products at their original surface and the IP withmainly fibril hydration products. A clear identification of the phasesin the IP by SEM-EDX is not possible due to the relatively large inter-action volume of the electron beam with the sample. Similar struc-tures were described in a blend of synthetic tricalcium silicate with30 wt.% of fly ash [68]. The study included TEM investigations,which showed that the IP of the fly ash consists of fibril C–S–H anddense crystalline material, which was identified as hydrogarnet.

Some spherical fly ash particles show concentric, unequally spacedrings in the IP (see Fig. 18c and e). These structures, also known asLiesegang rings [69], are related to periodic precipitations in gels, cau-sed by species that diffuse from different directions and start to precip-itate when a critical supersaturation is reached [70]. Therefore, theLiesegang rings are another indication of the gel-like character of theIP of fly ash. Liesegang rings were also observed by Rodger and Groves[68] in the IP of fly ash particles in tricalcium silicate-fly ash pastes.

Fig. 18d shows that mullite crystals, which can be located inside thefly ash glass, remain unchanged and do not participate in the hydrationreaction. This finding is in agreement with other studies [6,68].

3.7. Compressive strength

Fig. 19 shows the results of the compressive strength testing. Thedifference in the compressive strength between neat OPC and theblended samples is especially large at early ages. After 2 days thecompressive strength of the blends is about 25% of the compressivestrength of the neat OPC, while after 28 days the strength of theblends reaches 60–75% of the neat OPC. The comparison of the flyash blends with the Qz reference shows after 1 day of hydrationslightly less strength in the fly ash samples.

A significant enhancement of the compressive strength due to thepozzolanic reaction in the fly ash blended mortars compared to theQz reference mortars is observed in OPC–F2 after 28 and in OPC–F1after 90 days of hydration. After 90 days the fly ash blends exhibit10–17 MPa more strength than the Qz blend. The strength gain relat-ed to the pozzolanic reaction occurs later than the consumption ofportlandite and the increase of chemical shrinkage, which starts al-ready after 7 days of hydration. This inconsistency might be related

to various effects. First, the hydration of fly ash in a mortar samplemay differ from a paste sample. Second, the shape of Qz is differentand the grains might be better interlocked with the matrix than thefly ash. Third, the hydrates of the fly ash reaction, especially the IP,might be weak resulting in a poor connection of anhydrous fly ashparticles to the matrix.

The replacement of 5 wt.% of F1 by limestone results in 20% highercompressive strength values compared to OPC–F1 after 28 days andlonger hydration times.

4. Conclusions

The presented work shows the effect of the replacement of50 wt.% of OPC by siliceous fly ash on the hydration kinetics, thechemistry of hydrate phases, pore solution and the microstructureof blended cement. The comprehensive investigation of the solidand liquid phases during the hydration of two blended cementswith different fly ashes allows the qualitative description of the hy-dration mechanism of typical glassy fly ash particles in blended OPC:

At early hydration times up to 2 days, the contribution of fly ash tothe hydration is marginal. Readily soluble compounds from the fly ashare dissolved in the pore solution. Between 12 h and 2 days, the hy-dration is influenced by the so called “filler effect” of fly ash. By theprovision of additional nucleation sites on the surface of the fly ashand the increase of the effective water-to-cement ratio, the reactionof the OPC is promoted. After the first 12 h, hydrate phases precipi-tate on the surface of fly ash resulting in the formation of a layer ofhydration products. These hydration products are most likely due tothe hydration of OPC. Theoretically there could also be a minor contri-bution of the fly ash to this first layer of hydration products since thepH of the pore solution is already high enough for the dissolution offly ash [6]. After the consumption of the calcium sulphates the pHrises to 13.4±0.1, which accelerates the dissolution of fly ash. Indica-tions of fly ash reaction between 2 and 7 days are given by increasedchemical shrinkage, bound water and reduced portlandite contents at7 days of hydration.

From 7 days on, the release of Al and Si from the fly ash into thepore solution leads to the formation of additional hydration productsand the change of the C–(A–)S–H composition towards lower Ca/Siand higher Al/Si atomic ratios. Additionally, upon the ongoing disso-lution of fly ash, an inner hydration product (IP) is formed. Due tothe low Ca content of anhydrous fly ash, the Ca availability in the IPof fly ash is low and the dissolution of fly ash leads to the formationof a gel-like and water-rich IP. However, due to the continuous supplyof Ca from the pore solution, C–(A–)S–H and possibly hydrogarnetform inside the IP with advancing hydration. After long hydrationtimes, the pozzolanic reaction is slowing down due to the bindingof alkalis in the C–(A–)S–H and the related reduction of the pH inthe pore solution.

Additionally it can be concluded that the reaction (observed bychemical shrinkage and bound water) and the strength developmentof OPC–F2 blend is faster than that of OPC-F1. This is probably relatedto the slightly finer particle size of F2.

Finally, the replacement of 5 wt.% fly ash by limestone has showna positive effect on the compressive strength after 28 days. This is re-lated to the stabilisation of ettringite and CO2-containing AFm phasesand the corresponding increase of the bound water content.

Acknowledgments

The authors wish to acknowledge Schwenk Cement KG for theanalytical and financial support. Thanks are also due to Klaartje DeWeerdt and Mohsen Ben Haha for helpful discussions and comments.Thanks are extended to Boris Ingold for the preparation and polishingof the SEM cross sections and Luigi Brunetti for the technical support.

0

10

20

30

40

50

60

70

1 2 28 91

σ com

p[M

Pa]

Time [d]

OPC

OPC-Qz

OPC-F1

OPC-F2

OPC-F1-L

Fig. 19. Compressive strength of the mortar samples. The positive effect on the devel-opment of compressive strength by limestone addition to the sample OPC–F1 is indi-cated by the double arrow.

1399F. Deschner et al. / Cement and Concrete Research 42 (2012) 1389–1400

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1400 F. Deschner et al. / Cement and Concrete Research 42 (2012) 1389–1400

Effect of temperature on the hydration of Portland cement blended withsiliceous fly ash

F. Deschner, B. Lothenbach, F. Winnefeld, J. Neubauer

Cement and Concrete Research, 2013, Vol.52, p. 169-181

doi:10.1016/j.cemconres.2013.07.006

Paper III

Effect of temperature on the hydration of Portland cement blended withsiliceous fly ash

Florian Deschner a,⁎, Barbara Lothenbach a, Frank Winnefeld a, Jürgen Neubauer b

a Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Concrete and Construction Chemistry, Überlandstrasse 129, 8600 Dübendorf, Switzerlandb GeoZentrum Nordbayern, Mineralogy, University of Erlangen-Nuremberg, 91054 Erlangen, Germany

a b s t r a c ta r t i c l e i n f o

Article history:Received 14 March 2013Accepted 9 July 2013Available online xxxx

Keywords:Fly ash (D)Blended cement (D)Temperature (A)Hydration (A)Modelling (E)

The effect of temperature on the hydration of Portland cement pastes blendedwith 50 wt.% of siliceous fly ash isinvestigated within a temperature range of 7 to 80 °C.The elevation of temperature accelerates both the hydration of OPC and fly ash. Due to the enhanced pozzolanicreaction of the fly ash, the change of the composition of the C–S–H and the pore solution towards lower Ca andhigher Al and Si concentrations is shifted towards earlier hydration times. Above 50 °C, the reaction offly ash alsocontributes to the formation of siliceous hydrogarnet. At 80 °C, ettringite and AFm are destabilised and the re-leased sulphate is partially incorporated into the C–S–H. The observed changes of thephase assemblage indepen-dence of the temperature are confirmed by thermodynamic modelling.The increasingly heterogeneousmicrostructure at elevated temperatures shows an increaseddensity of the C–S–Hand a higher coarse porosity.

© 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Cementitious materials are exposed to a vast range of temperaturesdue to factors like local climatic conditions, externally applied heat inprecast concrete or heat due to the exothermic nature of the hydrationprocess. Several studies investigated the impact of temperature on thehydration kinetics, the hydrate assemblage, microstructure and durabil-ity of ordinary Portland cement (OPC) in a temperature range between5 and 60 °C [1–8]. In accordancewith the Arrhenius law, the augmenta-tion of temperature leads to the increase of the hydration rate of OPCand therefore to increased early compressive strength [1]. The temper-ature sensitivity of the hydration rate is indicated by the apparent acti-vation energy, which was determined for OPC and cements blendedwith fly ash, slag and other supplementary cementing materials [9,10].At later hydration times, the rate of strength development diminishes,so that after a certain time span, the compressive strength at high tem-peratures is lower than for samples hydrated at lower temperatures.This effect is related to the microstructure development of the cementpaste [1–3,5,8,11]. The rapidly forming initial hydration products atelevated temperatures are distributed heterogeneously, the density ofthe C–S–H is increased and at above 47 °C, ettringite and monocar-bonate are destabilised to monosulphate and calcite. This leads to ahigher coarse porosity and reduced compressive strength [2,3,12,13].At lower temperatures, the hydration rate is reduced allowing thedissolved ions more time for diffusion before the hydrates precipitate.This leads to the formation of a less polymerised C–S–H with higher

gel-porosity [13–15] and a cement paste with a more even distributionof the hydrate phases. Hence, the coarse porosity is reduced and thecompressive strength increased [1,7].

Furthermore, the chemistry of the pore solution is affected by thetemperature due to the change of the solubility of the hydrate phases.For example, the increase of the temperature in neat OPC leads to ahigher solubility of ettringite and thus to an increase of the sulphateconcentrations in the pore solution [3,4,16,17].

Although the effect of temperature on the hydration of neat OPC iswell studied, there are few studies about fly ash blended OPC at differ-ent temperatures [11,15]. The present study shows a consistent set ofdata of the hydration of a Portland cement containing 50 wt.% of sili-ceous fly ash between 7 and 80 °C. The hydrate phase assemblage, thecomposition of the C–S–H, the development of the microstructure andthe composition of the pore solution at hydration times between 1and 180 days were investigated. The results are compared to the analy-sis of reference samples containing 50 wt.% of quartz powder instead offly ash to allow the observation of the effect of temperature on the reac-tion of fly ash.

2. Materials and methods

2.1. Raw materials

AnOPC (CEM I 42.5 N) and a siliceousfly ash (type V according to EN197-1) have been used as rawmaterials. The chemical and mineralogi-cal compositions of thesematerials were determined bymeans of X-rayfluorescence (XRF) and quantitative X-ray diffraction [18] (Table 1,reproduced from [19]). The average composition of the fly ash glass

Cement and Concrete Research 52 (2013) 169–181

⁎ Corresponding author. Tel.: +41 58 765 4535; fax: +41 58 765 4035.E-mail address: florian.deschner@gmail.com (F. Deschner).

0008-8846/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.cemconres.2013.07.006

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was calculated by subtracting the crystalline phases and the readily sol-uble alkalis from the total composition of the fly ash and normalizingthe results to 100 wt.%. As reference material, a quartz powder, whichshowed no pozzolanic activity at 23 °C up to 90 days of hydration wasused [19]. The particle size distribution of the quartz powder is similarto that of the fly ash (graph shown in [19]).

2.2. Preparation of cement pastes

All rawmaterials andmixing tools used for the preparation of the ce-ment pastes were equilibrated at the respective temperature for 1 daybefore use. Cement paste samples containing 50 wt.% of OPC and50 wt.% of fly ash (OPC-FA) or 50 wt.% of quartz powder (OPC-Qz), re-spectively, were prepared according to EN 196-3 with a water-to-binder ratio (w/b) of 0.5. The samples were cast in 500 ml and 60 mlpolyethylene flasks, sealed and stored at 7, 23, 40, 50 and 80 °C. The500 ml samples were used for the extraction of the pore solution andthe 60 ml samples for the analysis of the solid phases. The solid phaseswere analysed by means of TGA in N2-atmosphere, XRD, and scanningelectron microscopy (SEM).

2.3. Thermogravimetric analyses (TGA)

The hydrated samples were crushed, immersed in isopropanol for15 min,washedwith diethylether, andfiltered. Afterwards, the sampleswere dried for 10 min at 40 °C to evaporate the remaining diethyletherand analysed in a Mettler Toledo TGA/SDTA 851e device at a heatingrate of 20 K/min in N2-atmosphere. The analysed bound water of allhydrate phases (H) and the crystal water within Ca(OH)2 (HCH) weredetermined by the weight loss in the temperature intervals 50–500 °Cand 400–470 °C. The exact boundaries for the temperature interval ofportlandite were read from the derivative curve (DTG). The results areexpressed as percentage of the dry sample weight at 500 °C [20]. Triplepreparation and measurement of the samples after 1, 2 and 7 days ofhydration at 23 °C shows an absolute error of up to 2 wt.% for theboundwater, and up to 0.6 wt.% for thewater loss related to the decom-position of Ca(OH)2. This includes the errors caused by preparation andmeasurement.

2.4. X-ray diffraction (XRD) measurements

5 mm thick discs of the hardened paste samples with a diameter of30 mm were sliced from the cast samples, immersed in isopropanolfor 2 days to stop the hydration and stored under N2-atmosphereuntil measurement. Measurements were performed in a PANalyticalX'Pert Pro MPD diffractometer with attached X'Celerator detector.

2.5. Analysis of the pore solution

The pore solutions of the hardened samples were extracted by thesteel die method [21] using pressures up to 250 MPa. The solutionswere filtered immediately with nylon filters with a mesh size of0.45 μm. The free OH− concentrations of the pore solutions were calcu-lated from pH measurements with a pH electrode, calibrated againstKOH solutions with known concentrations. The K, Na, Ca, Al, Si and sul-phur concentrations were measured by means of inductively coupledplasma optical emission spectroscopy (ICP-OES). The standard devia-tion of the measured elemental concentrations ranges between 5 and10%.

2.6. Scanning electron microscopy (SEM)

Thehydrationwas stopped by cutting the sample into slices of 5 mmthickness with a diameter of 30 mm and keeping them for 3 days inisopropanol and drying them for 3 days at 40 °C. Subsequently, thesamples were impregnated with a modified bisphenol-A-epoxy-resinand polished by polycrystalline diamond suspension at grades from9 μm down to 1/4 μm. Finally, the samples were carbon-coated and in-vestigated under high vacuum conditions in a Philips ESEM FEG XL 30.EDX measurements of the hydrate phases were carried out at a beamvoltage of 10 keV and 10 mm working distance. The content of coarsepores (pore size N 0.6 μm)was determined bymeans of image analysisof 600 images per sample atmagnification of 2000. The upper thresholdfor the grey level segmentation of pores was determined by the mini-mumpoint between the peak of pores and the brighter hydration prod-ucts peak in the histogram of the sum of all 600 images [22,23]. Theerror of the quantification of the coarse porosity is estimated to be±3%.

Table 1Composition and density of the raw materials.

XRF-analysisa [wt.%] Mineralogical phase compositionb [wt.%] Average composition ofthe fly ash glass [wt.%]

FA OPC FA OPC FA

SiO2 50.9 19.4 Mullite 8.2 C3S 57.1 SiO2 54.1Al2O3 24.7 5.2 Quartz 7.0 β C2S 7.9 Al2O3 23.4Fe2O3 7.3 3.6 Hematite 0.7 α′ C2S 9.3 Fe2O3 7.1CaO 3.7 62.1 Magnetite 0.8 C4AF 13.0 CaO 4.6MgO 1.8 1.8 Amorphous 83.3 C3A cubic 2.0 MgO 2.2K2O 3.9 1.2 C3A orthorh. 2.0 K2O 5.0Na2O 0.9 0.3 Calcite 0.4 Na2O 1.1TiO2 1.1 0.2 Periclase 1.0 TiO2 1.4Mn2O3 0.1 0.1 Bassanite 2.6 Mn2O3 0.1P2O5 0.8 0.6 Anhydrite 2.8 P2O5 0.9SO3 0.4c 3.7 Arcanite 0.5 SO3 b0.1SrO 0.3 Dolomite 0.5LOI 3.5 1.1 Magnesite 0.3C 2.7d Siderite 0.2

Quartz 0.2Ankerite 0.2

Sum 99.1 99.5 100 100 100

a Standard deviation of the XRF analyses is b0.1 wt.% for all oxides except SiO2 (0.5 wt.%).b Standard deviation ranges from 0.1 to 0.6 wt.%.c SO3 content determined by combustion analysis.d C-content (part of the LOI) determined by combustion analysis.

170 F. Deschner et al. / Cement and Concrete Research 52 (2013) 169–181

2.7. Compressive strength of mortar samples

Mortar prisms (40 × 40 × 160 mm3) were prepared according toEN 196-1 and cured at 7, 20, 40, 50 and 80 °C. After demoulding, twomortar prisms per testing age were stored inside sealed boxes filledwith distilled water at the appropriate temperature until testing. Thecompressive strength was tested according to EN 196‐1.

2.8. Thermodynamic modelling

The stable phase assemblage of the investigated cementitious sys-tems at thermodynamic equilibrium was calculated in dependence ofthe temperature. The calculations were performed with the help ofthe Gibbs free energy minimisation programme GEMS [24,25] togetherwith the thermodynamic data from the PSI-GEMS database [26] ex-panded with additional data for solids that are expected to form undercementitious conditions [4,27–29]. For the sake of simplicity, the crys-talline phases in the fly ash were considered as inert and the glassphase of the fly ash was assumed to dissolve uniformly.

GEMSwas also used to calculate the activities of the aqueous speciesin the investigated pore solutions by the extended Debye–Hueckelequation [30]. From the activities, the saturation indices (SI) of thesolid phases were calculated. The SI with respect to the solids aregiven by log(IAP/KS0), where KS0 is the theoretical solubility productof the respective solid, while IAP is the ion activity product calculatedfrom themeasured concentrations. A positive SI implies oversaturation,a negative value undersaturation. As the use of SI can be misleadingwhen comparing phases which dissociate into a different number ofions, effective SI were calculated by dividing the SI by the number ofions participating in the reactions to form the solids, as described byLothenbach et al. [4].

3. Results and discussion

3.1. Compressive strength

Fig. 1a shows a positive influence of elevated temperature on theearly compressive strength of the reference sample OPC-Qz. After28 days, the OPC-Qz samples hydrated at 7 to 40 °C show similarstrength. At 50 °C, a late increase of strength is observed compared toOPC-Qz at 40 °C. And at 80 °C, a strong increase of compressive strengthfrom 23.4 to 71.9 MPa is detected between 7 and 180 days. This effect isprobably related to the temperature induced pozzolanic reaction of thequartz powder.

The development of the compressive strength of OPC-FA between 1and 180 days is shown in Fig. 1b. Between 7 and 40 °C, the compressivestrength shows a positive correlation with the temperature throughoutthe whole hydration time. After 1 day of hydration at 50 and 80 °C, the

strength is higher than at lower temperatures. Afterwards however, therate of strength increase at these high temperatures is lower, and there-fore the late strength values are lower than at 40 °C. At 80 °C, this effectis pronounced: the high early strength of 21.8 MPa after 1 day of hydra-tion is followed by a slow increase of strength resulting in 36.8 MPaafter 180 days. In contrast to OPC-Qz, no relevant increase of the com-pressive strength is found after 7 days of hydration at 80 °C.

In comparison to the reference sample OPC-Qz (Fig. 1a), the com-pressive strength of the fly ash blended cement pastes at 20 °C is clearlyhigher after 28 days. This additional strength is related to the pozzolanicreaction of the fly ash, while the quartz powder in the reference showsno significant pozzolanic reactivity at this temperature [31]. From 7 to50 °C, the rise of compressive strength in OPC-FA compared to OPC-Qzis shifting to earlier hydration times and thereby indicating a faster hy-dration of fly ash at higher temperatures.

3.2. Content of portlandite and bound water

Fig. 2a–e shows the portlandite contents (CH) and boundwater of allhydrate phases other than portlandite (H*), determined by means ofTGA. H* is calculated by subtracting the content of crystal water ofportlandite in the sample from the amount of total bound water (H).For each temperature CH andH* in OPC-FA is compared to the referencesample OPC-Qz. The beginning of the portlandite consumption relatedto the pozzolanic reaction is indicated by the relative decrease of theportlandite content in OPC-FA compared to OPC-Qz. It shows that thestart of thepozzolanic reaction is shifting to earlier hydration times at el-evated temperatures. The onset of portlandite consumption in OPC-FA isfound at 7 °C after 90 days, at 23 °C after 7 days, at 40 °C after 1 day, at50 °C after 16 h, and at 80 °C before 16 h of hydration. The differencebetween H* in OPC-FA and H* in OPC-Qz is related to the amount of hy-dration products formed by the pozzolanic reaction of fly ash. The pointat which H* of OPC-FA is starting to be higher than H* in OPC-Qz isshifting towards earlier hydration times at elevated temperatures andcorrelates well with the beginning decrease of portlandite.

The decrease of the portlandite content in the OPC-Qz samples ob-served at 40, 50 and 80 °C indicates a pozzolanic reaction of the quartzin the reference samples. The rate of the quartz reaction increaseswith temperature. Surprisingly at 80 °C, the reaction of the quartz con-tinues while the fly ash reacts much slower after the first hours. Thiscould be related to a hindrance of the further fly ash reaction at thehigh dissolved Al concentrations observed in these samples.

3.3. Characterization of hydration products

3.3.1. X-ray diffraction (XRD)Figs. 3 and 4 show details of the XRD patterns of the samples after 1

and 180 days of hydration. In addition to anhydrous Portland cement

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Fig. 1. Development of the compressive strength of (a) OPC-Qz and (b) OPC-FA at temperatures ranging from 7 to 80 °C.

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clinker phases and fly ash minerals, the diffractograms of OPC-FA showthe formation of portlandite, ettringite, monosulphate and an AFmphase, which is most likely a ternary solid solution of AFm phases con-taining sulphate, carbonate and hydroxide [32,33].

After 1 day of hydration (Fig. 3) the diffractograms of OPC-Qz andOPC-FA are similar. At 7 and 23 °C the main hydrate phases are port-landite and ettringite. The increase of the temperature to 40 or 50 °Cleads to the formation of monosulphate in addition to ettringite andportlandite. At 80 °C ettringite is not stable and an AFm phase is formedinstead.

After 180 days of hydration at 7 and 23 °C, portlandite, ettringite,monosulphate, a solid solution of AFm phases and monocarbonate arepresent in both samples (Fig. 4). At 40 and 50 °C, ettringite andportlandite persist in both samples. However, while OPC-FA shows thepresence of monosulphate and other AFm phases, OPC-Qz shows only

minor amounts of AFmphases. Due to the relatively high content of sul-phate and magnesium in the Portland cement, the majority of Al isbound within ettringite and possibly hydrotalcite in OPC-Qz. In OPC-FA, the fly ash dissolution leads to the release of Al and the increase ofthe Al-to-sulphate ratio and thereby promotes the formation of AFmphases. At 80 °C, OPC-FA and OPC-Qz do not show any more reflectionsrelated to the presence of ettringite and AFm phases.

The presence of siliceous hydrogarnet is found after 180 days of hy-dration in OPC-FA and OPC-Qz. While the diffractograms at 50 °C andlower temperatures showonlyminor amounts of siliceous hydrogarnet,at 80 °C siliceous hydrogarnet is one of themain hydrate phases. This isrelated to the kinetic hindrance of the formation of hydrogrossularbelow 50 °C.

The change of the stable phase assemblage from ettringite andmonosulphate to siliceous hydrogarnet is related to the temperature

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Fig. 2. Comparison of the content of portlandite (dashed lines) and bound water (solid lines) in OPC-FA and OPC-Qz at (a) 7 °C, (b) 23 °C, (c) 40 °C, (d) 50 °C and (e) 80 °C.

172 F. Deschner et al. / Cement and Concrete Research 52 (2013) 169–181

dependent solubility of these phases and the availability of Al, Ca, sul-phate and hydroxide [4].

3.3.2. Analysis of the C–S–H composition by SEM-EDXEDX point analyses were targeted at the outer product in thematrix

of hydrates and the inner product of fly ash or clinker in order to char-acterize the composition of the C–S–Hphase. Due to the interaction vol-ume of electrons with the specimen and the intergrowth of hydrates,the detected X-rays of one spot often consist of the signal of a mix oftwo or more phases [34]. To derive the composition of a phase, elementratio plots are used [35,36].

The Al/Ca versus Si/Ca plot of OPC-FA hydrated for 180 days at 7,50 and 80 °C is shown in Fig. 5. The scatter of data points towardshigh Si/Ca and high Al/Ca ratios is due to the presence of fly ash particleswithin the excited sample volume of some EDXpoint analyses. The Al/Siratio of C–S–H is determined by the slope of a line, which is drawnthrough the points with the lowest Al/Ca ratio and represents mixedanalyses of portlandite and C–S–H without AFm or ettringite. The rangeof Si/Ca ratios of the C–S–H is represented by the bulk of data pointsalong this line. OPC-FA at 7 °C shows a range of Si/Ca values between0.5 and 0.7 or Ca/Si ratios between 1.4 and 2.0. This variance is in

agreement with transmission electron microanalyses of C–S–H in flyash blended white Portland cement [37].

The Ca/Si, Al/Si and S/Si atomic ratios of OPC-FA and OPC-Qz after180 days of hydration at temperatures ranging from 7 to 80 °C aregiven in Table 2. The C–S–H composition of OPC-Qz hydrated for180 days at 80 °C shows a significant reduction of the Ca/Si ratio com-pared to lower temperatures. This is on one hand related to the reducedsolubility of Ca(OH)2 at elevated temperatures. On the other hand thequartz powder is being dissolved at high temperatures and the addi-tionally provided Si is probably incorporated into the C–S–H in a similarway as during the hydration of silica fume blended OPC [15,38,39].

Compared to the reference sample, the C–S–H of the fly ash blendedPortland cement is characterised by lower Ca/Si and higher Al/Si ratios.This change of the C–S–H composition is related to the additional Si,provided by the fly ash dissolution, which is incorporated in the C–S–Hand results in longer silicate chain lengths [37,40,41]. This effect andthe provision of Al from the dissolution of fly ash leads to an increaseduptake of aluminium, which is substituting the Si atoms located in thebridging tetrahedra of the silicate chains [42–44].

The comparison of the C–S–H compositions of OPC-FA hydrated atdifferent temperatures shows that the Ca/Si ratio decreases with in-creasing temperature and the Al/Si ratio of the C–S–H increases from 7

8 10 12 14 16 18 20 22 24 26 28 308 10 12 14 16 18 20 22 24 26 28 30

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Fig. 3.Comparison ofXRDdata of (a)OPC-Qz and (b)OPC-FA after 1 day of hydration at 7, 23, 40, 50 and 80 °C. E—ettringite,Ms—monosulphate, AFmss—solid solution of AFmphases, CH—portlandite, F—ferrite, Mu—mullite, Qz—quartz.

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E E E E

HGHGMuAFmss

Fig. 4. Comparison of XRD data of (a) OPC-Qz and (b) OPC-FA after 180 days of hydration at 7, 23, 40, 50 and 80 °C. E—ettringite, Ms—monosulphate, AFmss—solid solution ofhemicarbonate and OH− substituted monosulphate, CH—portlandite, HG—hydrogarnet, F—ferrite, Mu—mullite, Qz—quartz.

173F. Deschner et al. / Cement and Concrete Research 52 (2013) 169–181

to 50 °C. This is probably related to the enhanced reactivity of fly ash athigher temperatures, the increased release of Si and Al ions from the flyash dissolution and the increased degree of polymerisation of the C–S–Hat higher temperatures [13–15,45].

The uptake of sulphate in the C–S–H has been shown to be depen-dent on the temperature and time of hydration [12,46]. The S/Si ratiosof the C–S–H in OPC-FA hydrated at 7 to 50 °C are about 0.04–0.05(Table 2). However, at a hydration temperature of 80 °C, the S/Si ratioof the C–S–H in OPC-FA is significantly increased to about 0.09. This isrelated to the increased solubility of ettringite at 80 °C and the therebyincreased availability of sulphate in the pore solution [17]. This effectagrees with investigations on the sulphate concentrations of C–S–H inplain OPC at elevated temperatures [12,46]. In OPC-Qz the S/Si ratio in-creases slightly, in agreementwith the less distinct increase of dissolvedsulphate in the pore solution. A similar observation has beenmade in anunpublished study including microanalyses of the C–S–H of Portlandcement pastes blended with 50 wt.% silica fume hydrated at 80 °C(personal communication by Dr. B. Lothenbach).

To show the relation between the amount of Al and S incorporated inthe C–S–H and the concentration of these elements in the pore solution,the measured Al/Si or S/Si ratios of the C–S–H are plotted against themeasured concentrations of the pore solution (Fig. 6). The presenteddata is expanded by published data from other studies dedicated tothe characterisation of synthesized C–S–H [44,46]. It shows that the in-creased amount of Al and S incorporated in the C–S–H agrees with the

measured increased concentrations of dissolved Al and S in the poresolution.

3.4. Analysis of the pore solutions

The development of the elemental concentrations in the pore solu-tion of fly ash blended OPC has been described by various studies[19,20,47,48]. On one hand, the consumption of portlandite by the poz-zolanic reaction causes a decrease of the Ca concentrations in the poresolutions. On the other hand, the release of Al and Si from the dis-solving fly ash leads to the increase of the Al and Si concentrations.Additionally, the hydroxide concentrations tend to decrease due tothe enhanced binding of alkalis in the C–S–H formed by the pozzola-nic reaction [45,49–51].

The progress of the Ca, Al, Si, alkali and sulphate concentrations inOPC-Qz and OPC-FA at temperatures ranging from 7 to 80 °C is shownin Fig. 7. All elemental concentrations of the investigated pore solutionsare also given in the appendix in Tables A.1 and A.2.

Fig. 7a shows that the Ca concentrations in the reference sampleOPC-Qz are slightly decreased at elevated temperature, which agreeswith the decreased solubility of portlandite. In OPC-FA, a significantdrop of the Ca concentrations related to the pozzolanic reaction is ob-served. The drop of the Ca concentrations is shifting to earlier hydrationtimes by increasing the temperature from 7 to 50 °C due to the fasteronset of the pozzolanic reaction and the faster consumption of Ca(OH)2.

At 80 °C, the effect of the faster onset of the pozzolanic reaction is ob-served in the low measured Ca concentration at 1 day of hydration. Af-terwards the Ca concentration at 80 °C is higher than at 40 and 50 °Cand the Al and Si concentrations are equal to those of OPC-FA at 40 and50 °C. This is related to the fast reaction of fly ash at 80 °C up to 1 dayand a relatively low rate of reaction at later hydration times, as observedby the rate of portlandite consumption measured by TGA (Section 3.2)and the development of compressive strength (Section 3.1).

Fig. 7b shows the Al concentrations of the pore solution in OPC-Qzand OPC-FA. Up to 2 days of hydration the Al concentrations in OPC-Qz are higher at 40–80 °C compared to lower temperatures. This isdue to the higher ettringite solubility at higher temperature [17]. Thedecrease at later ages could be related to the formation of siliceoushydrogarnet and Al uptake by C–S–H.

Fig. 5. Plot of Al/Ca versus Si/Ca atomic ratios of the EDXmeasurements focussed on hydrate phases in OPC-FA hydrated for 180 days at 7, 50 and 80 °C. Ettr—ettringite, CH—portlandite,C2ASH8—strätlingite, C3AH6—katoite, C3ASH4—hydrogrossular, Jen—jennite, Tob—tobermorite.

Table 2Ca/Si, Al/Si and S/Si ratios of the EDX analyses of the C–S–H in OPC-FA and OPC-Qz after180 days of hydration at 7, 23, 40, 50 and 80 °C.

Temperature 7 °C 23 °Ca 40 °C 50 °C 80 °C

OPC-FACa/Si 1.7 ± 0.3 1.3 ± 0.2 1.4 ± 0.3 1.4 ± 0.3 1.1 ± 0.2Al/Si 0.14 ± 0.03 0.17 ± 0.03 0.19 ± 0.02 0.19 ± 0.02 0.19 ± 0.02S/Si 0.05 ± 0.02 0.04 ± 0.02 0.04 ± 0.02 0.04 ± 0.02 0.09 ± 0.02

OPC-QzCa/Si 1.8 ± 0.2 1.6 ± 0.3 1.7 ± 0.3 1.7 ± 0.3 1.2 ± 0.2Al/Si 0.07 ± 0.02 0.08 ± 0.02 0.08 ± 0.02 0.08 ± 0.02 0.05 ± 0.02S/Si 0.05 ± 0.02 0.05 ± 0.02 0.05 ± 0.02 0.06 ± 0.02 0.06 ± 0.02

a Data at 23 °C acquired from 250 day old samples (reproduced from [19]).

174 F. Deschner et al. / Cement and Concrete Research 52 (2013) 169–181

In OPC-FA, the increase of the Al concentrations related to the reac-tion of fly ash is shifting towards earlier times at higher temperaturesdue to the faster onset of the pozzolanic reaction, which is expectedto lead to an increased Al uptake in C–S–H. The acceleration of the poz-zolanic reaction by higher temperature is also observed for the Si con-centrations (Fig. 7c).

The progress of the alkali concentrations in OPC-FA is shown inFig. 7d. The effect of temperature on the alkali concentrations is notstraight forward compared to the other elements. There are several fac-tors which influence the concentration of alkalis in the pore solution:

• The rate of the clinker reaction and the related release of alkalis intothe pore solution

• The rate of the fly ash reaction and the related release of alkalis intothe pore solution

• The alkali binding capacity of the hydrate phases, which is related tothe amount and composition of the C–S–H [50]

• The decrease of the volume of pore solution.

At early hydration times, the enhanced clinker dissolution leads toincreased alkali concentrations at elevated temperatures. Thereforethe highest alkali concentrations are found at 80 °C and the lowest at7 °C. As long as thefly ash or the quartz powder is not reacting, the alkaliconcentrations in the solution are increasingdue to the ongoing dissolu-tion of clinker minerals and the reduction of the pore solution volume.This effect is clearly observed in OPC-FA at 7 °C where the reaction offly ash is just slowly starting between 90 and 180 days. At higher tem-peratures, a reduction of the alkali concentrations is observed due tothe enhanced binding of alkalis on the increased amounts of C–S–Hdur-ing themain reaction of thefly ash. Hence, the reduction of alkalis in thepore solution is related to the temperature dependent reactivity of flyash. A similar effect is observed for OPC-Qz when the quartz powderstarts to react at temperatures of 40 °C and higher. At 80 °C, the concen-tration of alkalis in OPC-Qz after 7 and 28 days of hydration is signifi-cantly lower than in OPC-FA. This may be related to a higher C–S–Hcontent, and hence enhanced binding of alkalis in OPC-Qz comparedto OPC-FA.

Fig. 7e shows the sulphate concentrations in the pore solutions ofOPC-Qz and OPC-FA. At elevated temperatures an increase of sulphateis observed due to the increased solubility of ettringite at higher tem-perature [17]. A similar increase of sulphate concentrations with tem-perature was also observed in silica fume blended and neat Portlandcements [3,15,45,52]. The lower sulphate concentrations in OPC-Qzcompared to OPC-FA agree with the sulphate concentrations found inthe C–S–H.

The effective saturation indices of portlandite, ettringite, monosul-phate and strätlingite, calculated from the elemental concentrations inthe pore solutions, are shown and discussed in Appendix B.

3.5. Microstructure

The effect of temperature on the microstructure of the investigatedsamples is studied by back-scattered electron (BSE) images of OPC-FAandOPC-Qz after 180 days of hydration (Fig. 8). Themain difference be-tween the samples hydrated at different temperatures is found regard-ing the porosity of the matrix and the grey level of the C–S–H.

Fig. 8a and b shows a very low matrix porosity in the samples hy-drated at 7 °C.Most of the visible pores in themicrostructure are hollowshell, so called “Hadley” grains [53] or internal fly ash pores. At highertemperatures, themicrostructure ismore heterogeneous and the poros-ity is higher compared to lower temperatures as shown by the exampleimages of OPC-FA and OPC-Qz at 50 and 80 °C (Fig. 8c–f).

Another temperature dependent parameter of the microstructureis the grey level of the C–S–H in BSE images. In all investigated samplesat 7 °C, lower grey levels of the C–S–H are observed compared to highertemperatures. The intensity of the grey level in the BSE image of apolished sample is dependent on the electron density of the analysedsample volume, which is mainly related to the atomic number of theanalysed elements and the density or microporosity, respectively.Since the C–S–H at 7 °C has a higher Ca content compared to the C–S–H of the samples hydrated at higher temperatures (as shown inFig. 5), the decreased grey levels have to be related to a highermicropo-rosity or lower density, respectively. Similar findings have been report-ed for the hydration of neat OPC mortars [2,8] and recently also for flyash blended Portland cement [11].

The relation between the microstructure of the paste samplesand the compressive strength of the mortar samples is not yet fullyunderstood. Fig. 9 shows the compressive strength of the mortar sam-ples plotted versus the coarse porosity (pores N 0.6 μm). Although thegraph shows a general trend of increased strength with decreased po-rosity, the compressive strength of samples with low coarse porosityvaries significantly depending on the temperature and time of hydra-tion. This effect can also visually be seen from the comparison of theBSE images shown in Fig. 8a and c. Although OPC-FA after 180 days ofhydration at 50 °C has the highest compressive strength, it shows ahigher coarse porosity than the same sample hydrated at 7 °C. This ef-fect is pronounced in OPC-Qz: At 80 °C, the OPC-Qz mortar shows byfar the highest compressive strength after 180 days, although the coarseporosity is clearly lower than at 7 °C. On one hand, this may indicate,

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175F. Deschner et al. / Cement and Concrete Research 52 (2013) 169–181

that in addition to the content of coarse pores other factors such as theinterfacial transition zone at the aggregates in mortars, the hydratephase assemblage and the mechanic properties of the hydration prod-ucts play an important role. On the other hand, the variance of compres-sive strength values at low porosity is similar to other studies and isprobably related to the uncertainty caused by the fact that fine capillarypores with a diameter of less than 0.6 μm are not identified by thisimage analysis procedure [8].

4. Modelling of the hydrate phase assemblage atdifferent temperatures

The effect of temperature on the hydrate phase assemblage of OPC-Qz and OPC-FA after 180 days of hydration is simulated by using a

simplified model with constant reaction degrees of C3S, C2S, C3A andC4AF. The assumed reaction degrees after 180 days of hydration (100%for C3S, 86% for C2S, 93% for C3A and 90% for C4AF) are based on an exper-imental study showing only slight variations of the OPC reaction degreeafter 180 days within a temperature range between 5 and 40 °C [11].

In order to calculate the phase assemblage the following aspects areincluded in the model:

• The variance of the Ca/Si ratio of the C–S–H is accounted for by using asolid-solution model with jennite (C1.67–S–H2.1) and tobermorite(C0.83–S–H1.33) as end-members. The Al and sulphate uptake in theC–S–H is adapted to themeasured Al/Si and S/Si ratios (Section 3.3.2).

• The formation of siliceous hydrogarnet is restricted. Significant quan-tities of this phase have been reported only for cements hydrated at

1 10 100

7 °C 23 °C 40 °C 50 °C 80 °C

Time [d]

OPC-FAOPC-Qz

OPC-FAOPC-Qz

OPC-FAOPC-Qz

a) Calcium

1 10 1000.1

1

10

100 7 °C 23 °C 40 °C 50 °C 80 °C

Ca

[mM

]

Time [d]

1 10 100

7 °C 23 °C 40 °C 50 °C 80 °C

Time [d]

b) Aluminium

1 10 1000.01

0.1

1

10 7 °C 23 °C 40 °C 50 °C 80 °C

Al [

mM

]

Time [d]

1 10 100

7 °C 23 °C 40 °C 50 °C 80 °C

Time [d]

c) Silicium

1 10 1000.01

0.1

1

10 7 °C 23 °C 40 °C 50 °C 80 °C

Si [

mM

]

Time [d]

Fig. 7. Development of the (a) Ca, (b) Al, (c) Si, (d) alkali and (e) sulphate concentrations in OPC-FA and OPC-Qz hydrated at 7, 23, 40, 50 and 80 °C.

176 F. Deschner et al. / Cement and Concrete Research 52 (2013) 169–181

higher temperatures [4,29,54–56] or in the presence of Fe(OH)3[57,58]. Since the formation of Al siliceous hydrogarnet (hydro-grossular) is kinetically hindered at room temperature, it is exclud-ed from the calculations at temperatures up to 50 °C. However, Fesiliceous hydrogarnet (hydroandradite) may form also at lowertemperatures.

With these implementations, the development of the phase assem-blage after approximately 180 days of hydration is calculated as a func-tion of temperature and expressed as percentage of the original volumeof the raw materials and water (Figs. 10 and 11).

In the reference system OPC-Qz, the TGA experiments (Section 3.2)show the consumption of portlandite by the pozzolanic reaction of thequartz powder at temperatures between 40 and 80 °C. This is takeninto account in the model by the adaption of the reaction degree ofthe quartz powder at a given temperature to fit the experimentallymeasured portlandite content.

Fig. 10 shows the calculated change of the phase assemblage, exclud-ing the formation of hydrogrossular up to 50 °C. At low temperatures,C–S–H, hydrotalcite, ettringite, monocarbonate, hydroandradite andportlandite are predicted to form. At higher temperatures the pozzolanicreaction of the quartz powder leads to the consumption of portlanditeand the formation of C–S–H. Due to the binding of Al in the C–S–H, thecontent of monocarbonate is reduced with increasing temperature untilthis phase disappears at 50 °C, in agreement with the XRD resultsshown in Fig. 4. Ettringite is predicted to be stable up to 72 °C.Hydrotalcite persists throughout the modelled temperature range.

In OPC-FA, the calculated average composition of the glass phaseis used as input for the model, since the crystalline fraction of the flyash shows no reaction [19,59,60]. The reaction degree of fly ash at a

given temperature is adapted to fit themeasured portlandite content(Section 3.2).

Fig. 11a shows the calculated change of the phase assemblage inOPC-FA up to 50 °C. Due to the increased amount of reacted fly ashglass, the elevation of temperature leads to the consumption of port-landite and the formation of additional C–S–H. Additionally, aluminateis provided by the fly ash dissolution and contributes to the forma-tion of monosulphate starting from 30 °C. This process leads to thedestabilisation of ettringite at 50 °C.

For themodelling of the temperature interval between 50 and 90 °C,the formation of hydrogrossular is allowed in addition to hydro-andradite (Fig. 11b). The presence of hydrogrossular affects the amountof ettringite and AFm. Due to the high Al content of the siliceoushydrogarnet, the ratio of available sulphate to aluminate is increased,and hence ettringite persists up to 74 °C while no more AFm phasesare predicted to form. Hydrotalcite persists throughout the modelledtemperature range.

Comparing the modelled development of hydrate phases in the twosystems shows lower amounts of C–S–H inOPC-FA compared toOPC-Qzat temperatures above 50 °C. This may be responsible for the observedlower compressive strength of OPC-FA at 80 °C compared to OPC-Qz(Fig. 1). As a consequence of the lower C-S-H content in OPC-FA, less al-kali are bound within the C-S-H, which agrees well with the observedhigher alkali concentrations in the pore solution (Section 3.4). As at80 °C, in the OPC-FA and the OPC-Qz where neither ettringite normonosulphate is present, the sulphate is distributed between the poresolution and the C–S–H. Hence, the lower content of C–S–H in OPC-FAis responsible for the observed increased S/Si ratio in the C–S–H(Section 3.3.2) and the higher sulphate concentrations in the pore solu-tion (Section 3.4).

1 10 100

7 °C 23 °C 40 °C 50 °C 80 °C

Time [d]1 10 100

100

200

300

400

500

600

7 °C 23 °C 40 °C 50 °C 80 °C

Na+

K [m

M]

Time [d]

OPC-FAOPC-Qz

OPC-FAOPC-Qz

d) Sodium and potassium

1 10 100

7 °C 23 °C 40 °C 50 °C 80 °C

Time [d]1 10 100

0.1

1

10

100

7 °C 23 °C 40 °C 50 °C 80 °C

SO

4 [m

M]

Time [d]

e) Sulphate

Fig. 7 (continued).

177F. Deschner et al. / Cement and Concrete Research 52 (2013) 169–181

5. Conclusions

This study shows the effect of temperature on the hydration kinetics,the change of the phase assemblage and the microstructure in Portlandcement blendedwith 50 wt.% of siliceous fly ash in comparison to a ref-erence sample with 50 wt.% of quartz powder.

First of all, the temperature is affecting the OPC hydration, whichleads to enhanced early compressive strength at elevated temperatures.Besides the well-known effects of temperature on the OPC hydration, astrong influence of temperature on the reactivity of fly ash is observed.The beginning pozzolanic reaction of thefly ash, as indicated by the con-sumption of portlandite, and the change of the pore solution chemistryis shifting towards earlier hydration times at elevated temperatures.The increase of the temperature from 7 to 23 °C shifts the start of thepozzolanic reaction from 90 days to 7 days. At 50 °C the fly ash reactionstarts after 1 day of hydration.

20 μm

a) OPC-FA 7 °C

20 μm

b) OPC-Qz 7 °C

20 μm

d) OPC-Qz 50 °C

20 μm

c) OPC-FA 50 °C

20 μm

e) OPC-FA 80 °C

20 μm

f) OPC-Qz 80 °C

Fig. 8. BSE images of the microstructure of OPC-FA and OPC-Qz after 180 days of hydration at 7, 50 and 80 °C.

0 10 20 30 400

10

20

30

40

50

607 °C20 °C40 °C50 °C80 °C

Com

pres

sive

str

engt

h [M

Pa]

Coarse porosity [%]

Fig. 9. Compressive strength of OPC-FA plotted versus the coarse porosity. The error of thecoarse porosity is estimated to be ±3%.

178 F. Deschner et al. / Cement and Concrete Research 52 (2013) 169–181

At advanced hydration times the effect of temperature on the hydra-tion is governed by the enhanced pozzolanic reaction of the fly ash. Thetypical effects of the pozzolanic reaction on the hydration, like theconsumption of portlandite, the release of Al and Si into the pore solu-tion and the formation of additional C–S–H are accelerated at elevatedtemperatures. Al and alkalis are partially incorporated into the C–S–Hand the remaining Al contributes to the formation of AFm phases andto the formation of Al containing siliceous hydrogarnet at highertemperatures.

At 80 °C, Al containing siliceous hydrogarnet forms while ettringiteand AFm phases are destabilised. The released sulphate is partiallyenriched in the pore solution and partially incorporated into C–S–Hand siliceous hydrogarnet.

The microstructure of the fly ash blended Portland cement is affect-ed in a similar way as OPC by the change of temperature. At 7 °C homo-geneously distributed hydration products and a low content of coarsepores (pores N 0.6 μm) are found. The C–S–H phases show relativelylow grey values due to their highmicroporosity. The increase of temper-ature leads to a more heterogeneous distribution of the hydrationproducts and the formation of a C–S–H with a lower microporosity,and hence higher grey levels. The amount of coarse porosity increaseswith temperature.

The correlation between the coarse porosity and the compressivestrength is not perfectly clear and indicates that other factors, such as

the interfacial transition zone at the aggregates in mortars, the hy-drate phase assemblage and the mechanic properties of the hydrationproducts, play an important role for the development of compressivestrength.

Acknowledgements

The authors wish to acknowledge the BASF Construction ChemicalsGmbH and Schwenk Zement KG for the analytical and financial support.STEAG Power Minerals GmbH is acknowledged for the provision of thefly ash. Thanks are extended to Julien Keraudy for the technical supportand Boris Ingold for the preparation and polishing of the samples for theSEM investigations.

Appendix A. Elemental concentrations of the pore solutions

The elemental concentrations of the pore solution in OPC-Qz andOPC-FA are shown in Tables A.1 and A.2.

Table A.1Elemental concentrations in the pore solutions of OPC-Qz at 7, 23, 40, 50 and 80 °C.

Temperature Time [d] K Na Ca Si S Al Fe Cl OH−

mM

7 °C 1 192 26 8.5 0.1 30.4 0.02 0.003 3.66 1662 184 42 4.4 0.2 3.0 0.14 0.004 1.67 2457 228 47 3.9 0.2 5.3 0.17 0.002 1.27 303

28 251 64 3.4 0.2 5.9 0.15 0.005 0.51 29690 276 93 2.8 0.3 2.0 0.24 0.004 0.33 352

180 295 92 1.5 0.1 7.7 0.15 0.003 0.15 38123 °C 1 228 45 3.7 0.1 4.8 0.15 0.002 n.a. 283

2 289 74 3.6 0.1 0.9 0.12 0.002 n.a. 3117 263 70 2.9 0.1 4.4 0.12 0.002 n.a. 332

28 322 97 2.5 0.1 1.8 0.10 0.003 n.a. 32390 303 95 2.2 0.1 3.1 0.11 0.003 n.a. 317

250 205 56 2.1 0.1 7.5 0.13 0.005 n.a. 27740 °C 1 216 53 2.3 0.2 8.9 0.59 0.003 1.33 308

2 222 73 2.1 0.2 5.6 0.53 0.007 1.33 2967 249 66 2.3 0.1 11.7 0.14 0.002 0.69 327

28 207 59 2.1 0.1 17.5 0.10 0.003 0.42 24490 186 68 1.9 0.2 20.6 0.11 0.004 0.33 231

180 178 58 1.0 0.2 25.9 0.11 0.002 0.34 21450 °C 1 227 58 2.6 0.2 17.4 0.50 0.003 1.23 285

2 226 76 2.3 0.2 14.8 0.36 0.007 0.95 2747 246 67 2.2 0.2 36.0 0.10 0.002 n.a. 251

28 213 59 2.2 0.1 43.2 0.13 0.004 0.78 21690 236 86 3.2 0.3 50.8 0.11 0.005 0.38 190

180 172 57 1.4 0.2 42.3 0.09 0.002 0.23 16980 °C 1 248 67 2.7 0.2 71.7 0.65 0.006 0.70 88

2 185 65 2.6 0.2 62.1 0.21 0.003 0.40 1737 209 57 4.0 0.1 83.8 0.06 0.003 0.40 151

n.a. = not available.

anhydrous OPCglass fraction of FA

C-S-H

pore solution

portlandite

ettringite

monocarbonate

a) b)

hydrotalcite

monosulphate siliceous hydrogarnet

crystalline fraction of FA

% o

f orig

inal

vol

ume

Temperature [°C]

100

80

60

40

20

010 20 30 40 50 50 60 70 80 90

Fig. 11.Modelled change of thephase assemblage inOPC-FA after approximately 180 daysas a function of temperature. (a) Calculation excluding the formation of hydrogrossularbelow 50 °C and (b) including hydrogrossular above 50 °C.

% o

f orig

inal

vol

ume

Temperature [°C]

anhydrous OPCanhydrous Qz

C-S-H

pore solution

portlandite

ettringite

monocarbonate

a) b)

hydrotalcite

siliceous hydrogarnet

100

80

60

40

20

010 20 30 40 50 50 60 70 80 90

Fig. 10.Modelled changeof thephase assemblage inOPC-Qz after approximately 180 daysas a function of temperature. (a) Calculation excluding the formation of hydrogrossularbelow 50 °C and (b) including hydrogrossular above 50 °C.

Table A.2Elemental concentrations in the pore solutions of OPC-FA at 7, 23, 40, 50 and 80 °C.

Temperature Time [d] K Na Ca Si S Al Fe Cl OH−

mM

7 °C 1 177 31 10.3 0.1 34.2 0.01 0.004 3.84 1942 171 47 4.0 0.2 5.2 0.43 0.005 2.30 2547 211 53 3.8 0.2 5.4 0.16 0.003 1.62 303

28 221 60 3.6 0.1 6.0 0.12 0.005 0.70 29690 275 95 1.9 0.2 1.9 0.28 0.004 0.70 352

180 297 98 0.5 0.5 7.8 1.75 0.002 0.30 38123 °C 1 262 68 3.7 0.1 5.8 0.02 0.002 1.24 276

2 294 86 3.4 0.1 0.9 0.13 0.002 0.75 3117 322 104 3.0 0.1 1.1 0.13 0.002 0.97 270

28 231 72 1.7 0.3 5.5 1.36 0.004 0.47 28590 202 84 1.5 0.7 8.8 3.75 0.005 0.74 234

250 254 97 0.5 1.2 13.9 3.57 0.003 0.10 236

(continued on next page)

179F. Deschner et al. / Cement and Concrete Research 52 (2013) 169–181

Appendix B. Effective saturation indices

The effective saturation indices (SI) of portlandite, ettringite, mono-sulphate and strätlingite in OPC-Qz andOPC-FA at temperatures rangingfrom 7 to 80 °C are shown in Table B.1. The effective SI of portlandite inthe reference sample OPC-Qz at different temperatures shows a minoreffect of the temperature. At higher temperatures after 180 days of hy-dration, the effective SI of portlandite is slightly reduced compared tolower temperatures. Also the SI of ettringite and monosulphate showlower values at elevated temperatures. This is related to a faster hydra-tion kinetic at higher temperatures. Thepore solutions are thus nearer tothe thermodynamic equilibrium.

At 80 °C, the pore solution is undersaturated with respect to et-tringite and monosulphate, which agrees with the disappearance of

these phases from the XRD patterns (Section 3.3.1). The effective SI ofsträtlingite indicate that it is not expected to precipitate.

In OPC-FA, the effective SI of portlandite (Table B.1) changes fromoversaturation to undersaturation with ongoing hydration at 7–50 °C.This effect is due to the consumption of portlandite by thepozzolanic re-action of fly ash. When the rate of portlandite consumption by the poz-zolanic reaction overcomes the formation rate of portlandite by thereaction of the OPC, the effective SI yields negative values. The time ofthe decrease of the effective SI of portlandite therefore depends on thereaction rate of OPC and fly ash, and hence on the temperature duringhydration. The data agrees with the portlandite consumptionmeasuredby means of TGA (Fig. 2).

The effective SI of ettringite andmonosulphate in OPC-FA are affect-ed in a similarway as in OPC-Qz. The reaction of fly ash shows no signif-icant effect on the effective SI of these phases.

The effective SI of strätlingite turns positive with ongoing hydration.This effect is typically observed in fly ash blended samples due to theconsumption of Ca(OH)2 and the provision of Al and Si by the reactionof the fly ash. Due to the enhanced reactivity of fly ash at higher temper-atures, the increase of the effective SI of strätlingite is shifted towardsearlier hydration times. In fact, OPC-FA does not show the formationof strätlingite due to the presence of siliceous hydrogarnet, as shownby XRD (Section 3.3.1) and thermodynamic modelling (Section 4).

References

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H2O system at 50 °C and 85 °C, Cem. Concr. Res. 22 (1992) 1179–1191.[17] R.B. Perkins, C.D. Palmer, Solubility of ettringite Ca6[Al(OH)6]2(SO4)3 ∗ 26H2O at 5–

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Table A.2 (continued)

Temperature Time [d] K Na Ca Si S Al Fe Cl OH−

mM

40 °C 1 235 64 2.4 0.2 8.4 0.49 0.005 1.82 3082 180 73 1.1 0.3 5.7 0.54 0.004 1.77 3087 199 62 0.6 1.2 20.8 5.09 0.002 2.81 251

28 179 59 0.7 1.1 25.5 4.01 0.003 2.67 23490 200 84 0.4 1.3 23.3 2.30 0.004 2.65 250

180 222 81 0.2 1.5 38.9 3.49 0.004 3.18 24150 °C 1 207 57 1.7 0.3 13.5 0.66 0.003 1.79 275

2 221 84 0.7 0.8 30.0 2.73 0.013 2.71 2557 194 61 0.6 1.3 46.6 3.38 0.003 3.38 215

28 212 68 0.7 0.8 50.9 3.37 0.007 5.15 20090 217 89 0.5 1.4 49.7 2.40 0.004 3.08 213

180 239 86 0.3 1.5 63.0 3.20 0.002 2.96 21480 °C 1 268 86 1.4 0.7 154.6 3.89 0.006 3.06 227

2 271 109 1.3 1.0 149.0 2.60 0.006 3.96 1047 303 98 1.3 1.1 166.9 3.12 0.003 5.89 124

28 305 100 1.6 0.6 174.9 2.96 0.005 5.08 98

Table B.1Effective saturation indices of selected hydrate phases in OPC-Qz and OPC-FA at 7, 23, 40,50 and 80 °C calculated from the respective elemental concentrations in the pore solution.CH—portlandite, Ettr—ettringite, Ms—monosulphate, Strätl—strätlingite.

Time[d]

Effective saturation indices

OPC-Qz OPC-FA

CH Ettr Ms Strätl CH Ettr Ms Strätl

7 °C 1 0.11 0.65 0.23 −0.38 0.12 0.65 0.21 −0.462 0.10 0.50 0.25 −0.04 0.07 0.59 0.34 0.127 0.12 0.52 0.28 −0.05 0.10 0.52 0.27 −0.05

28 0.12 0.50 0.25 −0.10 0.11 0.50 0.25 −0.1390 0.12 0.39 0.22 −0.01 0.07 0.33 0.17 −0.03

180 0.03 0.36 0.13 −0.19 −0.19 0.24 0.09 0.0923 °C 1 0.12 0.32 0.17 −0.13 0.16 0.22 0.05 −0.42

2 0.18 0.14 0.09 −0.21 0.18 0.13 0.10 −0.207 0.12 0.25 0.12 −0.20 0.18 0.12 0.08 −0.21

28 0.15 0.12 0.06 −0.29 0.02 0.31 0.22 0.1490 0.12 0.15 0.06 −0.28 −0.05 0.35 0.26 0.33

250 0.02 0.25 0.09 −0.22 −0.22 0.14 0.07 0.1840 °C 1 0.06 0.19 0.15 −0.03 0.09 0.18 0.14 −0.05

2 0.07 0.12 0.11 −0.04 −0.06 0.01 0.00 −0.117 0.09 0.12 0.05 −0.27 −0.22 0.09 0.07 0.21

28 0.03 0.12 0.01 −0.31 −0.23 0.12 0.08 0.2190 −0.01 0.12 0.00 −0.28 −0.25 0.00 −0.03 0.06

180 −0.13 0.03 −0.10 −0.36 −0.36 −0.05 −0.09 0.0350 °C 1 0.09 0.15 0.14 −0.07 0.02 0.08 0.08 −0.07

2 0.09 0.10 0.09 −0.16 −0.13 0.05 0.06 0.087 0.05 0.08 0.00 −0.35 −0.26 0.05 0.03 0.13

28 0.00 0.12 0.02 −0.31 −0.20 0.10 0.08 0.1390 0.08 0.17 0.07 −0.26 −0.25 −0.02 −0.03 0.03

180 −0.12 0.01 −0.11 −0.38 −0.34 −0.08 −0.09 0.0080 °C 1 0.05 0.01 0.08 −0.18 −0.40 −0.03 −0.02 0.07

2 −0.01 −0.06 −0.03 −0.32 −0.34 −0.17 −0.16 −0.217 0.00 −0.04 −0.07 −0.47 −0.31 −0.04 −0.01 0.06

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181F. Deschner et al. / Cement and Concrete Research 52 (2013) 169–181

Quantification of fly ash in hydrated, blended Portland cement pastes bybackscattered electron imaging

F. Deschner, B. Münch, F. Winnefeld, B. Lothenbach

Journal of microscopy, 2013, Vol.251, p. 188-204

doi:10.1111/jmi.12061

Paper IV

Journal of Microscopy, Vol. 251, Issue 2 2013, pp. 188–204 doi: 10.1111/jmi.12061

Received 14 January 2013; accepted 24 May 2013

Quantification of fly ash in hydrated, blended Portland cementpastes by backscattered electron imaging

F . D E S C H N E R , B . M U N C H , F . W I N N E F E L D& B . L O T H E N B A C HEmpa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Concrete andConstruction Chemistry, Uberlandstrasse, Dubendorf, Switzerland

Key words. Blended cement, electron microscopy, fly ash, image analysis,quantification, reaction degree.

Summary

An automated image analysis procedure for the segmenta-tion of anhydrous fly ash from backscattered electron imagesof hydrated, fly ash blended Portland cement paste is pre-sented. A total of six hundred backscattered electron imagesper sample are acquired at a magnification of 2000. Char-acteristic features of fly ash particles concerning grey level,shape and texture were used to segment anhydrous fly ash bya combination of grey level filtering, grey level segmentationand morphological filtering techniques. The thresholds for thegrey level segmentation are determined for each sample bysemiautomatic histogram analysis of the full image stack ofeach sample. The analysis of the presented dataset reveals astandard deviation of the reaction degree of fly ash of up to4.3%. The results agree with a selective dissolution method toquantify the reaction degree of fly ash showing the potentialof the presented image analysis procedure.

Introduction

Low-calcium siliceous fly ash (FA) is widely used as supple-mentary cementitious material as partial replacement of ordi-nary Portland cement (OPC) in order to reduce the CO2 emis-sions related to cement production (Gartner, 2004) and dueto its beneficial effects on the durability of concrete (Chenet al., 1993; Torii & Kawamura, 1994; Shayan et al., 1996).In combination with OPC, fly ash reacts at a high pH of atleast 13.0 together with portlandite formed by the hydrationof OPC. The effects of this so-called pozzolanic reaction havebeen studied intensively in the past (Fraay et al., 1989; Duch-esne & Berube, 1995; Taylor, 1997; Rahhal & Talero, 2004;Baert et al., 2008; Lothenbach et al., 2011; Deschner et al.,2012).

Correspondence to: Florian Deschner, Empa, Swiss Federal Laboratories for Materi-

als Science and Technology, Laboratory for Concrete and Construction Chemistry,

Uberlandstrasse 129, 8600 Dubendorf, Switzerland. Tel.: +41-58-765-4535; fax:

+41-58-765-4035; e-mail: florian.deschner@gmail.com

In order to evaluate and compare the reactivity of differ-ent fly ashes, techniques like isothermal heat flow calorime-try, chemical shrinkage and thermogravimetric analyses havebeen used (Sakai et al., 2005; Deschner et al., 2012). Althoughthese methods are suitable to show relative differences betweenvarious samples, none of them is capable of directly quantify-ing the degree of fly ash reaction. In addition, these methodsmeasure the amount of hydrates and hence variations in thecomposition and proportions of hydrates strongly affect theresults and make the comparison between different activatorsor fly ashes difficult. Moreover, the use of activators usuallyaffects both, the hydration of fly ash and the hydration ofthe OPC. Since methods like calorimetry and the determina-tion of the chemical shrinkage evaluate the complete system,the differentiation between fly ash and OPC reaction is onlypossible when analyzing additional reference samples withsuitable inert reference materials like quartz powder instead offly ash. However, perfect reference materials are rarely foundand in the end, the comparison of reference sample and fly ashblended sample allows only relative comparisons of the fly ashreactivity. To show the effect of chemical activators on the flyash hydration kinetics, a direct method quantifying the reac-tion degree of fly ash is required. Furthermore, the reaction de-gree of fly ash is needed as input for thermodynamic modellingof the hydration of fly ash blended OPC (De Weerdt et al., 2011).

In order to directly obtain the absolute reaction degree of flyash in blended OPC, researchers have applied selective disso-lution techniques (Gutteridge, 1979; Li et al., 1985; Ohsawaet al., 1985), 27Al and 29Si MAS NMR spectroscopy (Poulsenet al., 2009) and microscopic analyses (Feng et al., 2004; BenHaha et al., 2010) in the past. It was shown that a coupleof selective dissolution techniques, including salicylic acid,ethylenediaminetetraacetic acid and picric acid are inaccu-rate in quantifying the reaction degree of fly ash, as a signif-icant part of the fly ash was dissolved while some hydratesand Portland cement remained (Ben Haha et al., 2010). NMRtechniques are usually not feasible due to the high Fe contentin most fly ashes.

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Another approach is to use microscopic analyses based onthe investigation of backscattered electron (BSE) images ac-quired from polished and coated samples in a scanning elec-tron microscope under high vacuum conditions. One of theproposed methods is a point counting procedure involving ahuman operator counting 3000 points per sample and dis-tinguishing between anhydrous cement, unreacted fly ash orothers (Feng et al., 2004). Apart from the very high time con-sumption of this method, it has been reported to quantify thehydration degree of fly ash with a standard deviation of 4.6–5%. A less time-consuming approach is the automatic seg-mentation and analysis of BSE images, which has successfullybeen used to quantify the porosity (Scrivener et al., 1987; Zhao& Darwin, 1992; Lange et al., 1994; Wang & Diamond, 1995;Wong et al., 2006) and pore structure (Lange et al., 1994;Diamond & Leeman, 1995), anhydrous cement (Scriveneret al., 1987; Wang & Diamond, 1995), aggregates (Yang &Buenfeld, 2001), degree of alkali silica reaction (Ben Haha etal., 2007) and microcracks (Ammouche et al., 2000). Also,the application of image analysis techniques to segment theunreacted fly ash in an OPC paste blended with up to 35 wt%of fly ash and limestone has recently been shown by Ben Hahaet al. (2010). However, up to now, a detailed description of theworkflow to segment unreacted fly ash and a discussion of thereproducibility and the error of the method has not yet beenreported. This is, however, highly essential since the type ofevaluation has a tremendous impact on the achieved results.

This study presents a new image analysis procedure for thesegmentation of fly ash from BSE images. The aim of the presentapproach is the quantitative and reproducible determinationof the content of anhydrous fly ash in an OPC blended with50 wt% of fly ash (OPC-FA).

The designated objective of this work is the quantitative as-sessment without any subjective user interventions, which aredepending on human perception and thus inevitably involv-ing considerable scattering. The principles of the segmentationand the full process are documented in detail. Additionally, thestatistics and systematic errors are determined and the sourcesof deficiencies in the segmentation of fly ash discussed.

Experimental Methods

Raw materials

An OPC (CEM I 42.5 N) and a siliceous fly ash (type V accordingto EN 197–1) have been used as raw materials. The chemicaland mineralogical compositions of these materials were deter-mined by means of X-ray fluorescence and quantitative X-raydiffraction (Jansen et al., 2011; Table 1; reproduced from De-schner et al., 2012). The densities were measured by meansof a helium pycnometer. The particle size distribution of thefly ash determined by laser diffraction is shown in Figure 1(Deschner et al., 2012).

Table 1. Composition and density of the raw materials.

XRF-analysisa (wt%) Mineralogical phase compositionb (wt%)

FA OPC FA OPC

SiO2 50.9 19.39 Mullite 8.2 C3S 57.1Al2O3 24.7 5.15 Quartz 7 β C2S 7.9Fe2O3 7.3 3.63 Hematite 0.7 α’ C2S 9.3CaO 3.7 62.1 Magnetite 0.8 C4AF 13MgO 1.8 1.84 Amorphous 83.3 C3A cubic 2.0K2O 3.9 1.19 C3A othorh. 2.0Na2O 0.9 0.28 Calcite 0.4TiO2 1.1 0.23 Periclase 1.0Mn2O3 0.1 0.05 Bassanite 2.6P2O5 0.8 0.56 Anhydrite 2.8SO3 0.4c 3.72 Arcanite 0.5SrO n. d. 0.3 Dolomite 0.5LOI 3.5 1.1 Magnesite 0.3C 2.7d Siderite 0.2

Quartz 0.2Ankerite 0.2

Sum 99.1 99.5 100 100Density [g/cm3]

FA 2.29OPC 3.14

aStandard deviation of the XRF analyses is < 0.1 wt% for all oxides exceptSiO2 (0.5 wt%).bStandard deviation ranges from 0.1 to 0.6 wt%.cSO3 content (not included in LOI) determined by combustion analysis.dC-content (included in LOI) determined by combustion analysis.n. d., not determined.

Preparation of cement pastes

Cement paste samples containing 50 wt% of OPC and 50 wt%of fly ash (OPC-FA) were prepared in a mixer according to EN196–3 with a water-to-binder ratio of 0.5. The samples werecast in 60 mL polyethylene flasks, sealed, rotated slowly alongthe longitudinal axis during the first 24 h to avoid segregationand stored at 23◦C.

Content of air voids in cement pastes

The content of air voids (cair) in the paste caused by the sam-ple preparation was needed to determine the initial volumefraction of clinker and fly ash. It was estimated by relating thecalculated mass of paste in the 60 mL flask without air voids(mcalc) to the measured sample mass (mmeas). The mass of thepaste without air voids (mcalc) was calculated from the productof the volume fraction (vi), the total volume and the density(δi) of each component i (OPC, fly ash and H2O) as shown inEq. (1).

mcalc =∑

i

vi · Vtot · δi (1)

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Fig. 1. Particle size distribution of fly ash. The solid line shows the relativefrequency of particles in dependence of the diameter and the dashed lineshows the volume of the particles smaller than a certain diameter.

The content of air voids was then calculated according toEq. (2).

cair = mcalc − mmeas

mcalc(2)

Sample preparation for scanning electron microscopy

The hydrated samples were cut into slices of 5 mm thicknessand immersed for 3 days in isopropanol to stop the hydra-tion by solvent exchange. Subsequently, the slices were driedfor 3 days at 40◦C, impregnated with a modified bisphenol-A-epoxy-resin and polished by water-free polycrystalline di-amond suspension at grades from 9 μm down to 0.25 μm.Finally, the samples were carbon coated.

Acquisition of BSE images

The polished and coated samples (Fig. 2) were investigatedunder high vacuum conditions in a Philips ESEM FEG XL 30at a beam voltage of 15 keV, spot size of 3, lens aperture of100 μm and a working distance of 10 mm. A total of 600BSE images (8 bit) were captured at a magnification of 2000.Each image contains 1024 × 800 pixels with a pixel spacingof 0.116 μm. The field of view of each image is 119 μm ×93 μm. The contrast and brightness was adapted such thatporosity was at the lower and Fe-rich fly ash at the upper limitof the grey scale (Fig. 4). With the help of the EDAX GENE-SIS software (version 6.02), a regular grid of images was ac-quired over the cross-section of each cement slice sample. Theposition of the rectangular grid was chosen carefully, cover-ing areas from the outer part of the sample as well as from thesample core in order to be representative. Examples of suitablegrid positions are shown in Figure 2. The image processing

Fig. 2. Photo of an impregnated and polished sample. The quadrants a,b, c and d show examples of suitable sections for the image acquisition.

and quantification of fly ash was achieved by MATLAB scriptusing the image processing toolbox.

Determination of fly ash reaction degree by selective dissolution

A selective dissolution procedure based on ethylenediaminete-traacetic acid, as described by (Ben Haha et al., 2010), wasapplied to assess the reaction degree of fly ash by an alterna-tive method. Similar procedures were used previously for flyash-gypsum-Ca(OH)2 (Ohsawa et al., 1985) or slag blendedsystems (Luke & Glasser, 1987). The method aims to dissolveselectively the hydrates and the unhydrated clinkers with-out dissolving the unreacted fly ash. By taking the content ofbound water in the sample into account, the reaction degree offly ash was calculated. However, the selective dissolution pro-cedure was not working satisfactorily for fly ash blended Port-land cements. Mg-rich clinker and hydrate phases partiallyremained unsolved while parts of the fly ash were dissolvedduring the procedure (Luke & Glasser, 1987; Ben Haha et al.,2010). Depending on the assumptions made to correct theseerrors, the reaction degree of the investigated fly ash varied byup to 9 wt% (Ben Haha et al., 2010).

Image analysis

Introduction

The main fraction of siliceous fly ash particles in the BSE imageis characterized by spherical particles with intermediate greylevels. The brightness of a phase in the BSE image of a pol-ished sample is a function of the atomic number of the excitedelements (Scrivener et al., 1987). Due to the heterogeneouscomposition of fly ash, its grey levels in the BSE image varyextremely and overlap with those of hydrate or clinker phases.This can be seen in the example image of OPC-FA after 7 daysof hydration in Figure 3(a). The segmentation of pixels with

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Fig. 3. (a) BSE image of the microstructure of OPC-FA after 7 days of hydration and (b) the correlating histogram. (c) Segmentation of pixels with gi yieldsa mask of certain fly ash (FA) particles and portlandite (CH).

the selected grey levels gi, highlighted in the histogram (Fig.3b), yields a mask with objects attributed to fly ash and port-landite (Fig. 3c). The two phases cannot be distinguished dueto their similar grey levels. Other fly ash particles may alsoshow similar grey levels as AFm/AFt (aluminium iron mono-and tri-) phases or clinker. Therefore, the segmentation of an-hydrous fly ash by grey level segmentation techniques onlyis not possible. To obtain a reasonable quantification of thefly ash, a combination of grey level segmentation, grey levelfiltering and morphological filtering is required.

Selection of thresholds for grey level segmentation

By defining suitable threshold levels, grey level segmentationcan be used to segment porosity, anhydrous clinker and themajor fraction of anhydrous fly ash. The thresholds can bedefined from histograms, which are representative for the fulldataset of images of each sample. Such a histogram is ob-tained by summing up the frequencies of each grey level of allimages of the image stack. Figure 4 shows such histogramsafter hydration times between 1 and 550 days. Up to 7 days ofhydration, the histograms show specific maxima correlatingto porosity, AFm/AFt, the major part of anhydrous fly ash,portlandite, anhydrous clinker and fly ash with high iron con-tent. Between 2 and 90 days also a broad hump between thegrey levels of porosity and AFm/AFt is observed. This hump ismainly related to calcium-silicate-hydrate (C-S-H) formationin the porous matrix. At hydration times of more than 7 days,the clinker has almost reacted completely and the related max-imum in the histogram disappears. Additionally, the ongoingreaction of the clinker and fly ash leads to the formation ofadditional hydrates and hence the densification of the matrixand decrease of porosity. Therefore, the maximum associatedwith porosity decreases and also the grey levels of C-S-H in thematrix shift to higher values. These effects can be observed inFigure 4(d–g). After 550 days of hydration (Fig. 4g), the max-imum of AFm/AFt and the broad maximum of C-S-H havemerged with the maximum associated with the major parts ofanhydrous fly ash.

The histograms show characteristic points like maxima andminima, which are related to the afore-mentioned distribu-tion of anhydrous and hydrated phases. These points are usedfor the definition of thresholds for grey level segmentation ofporosity, the majority of anhydrous fly ash partially mixedwith hydrate phases, anhydrous clinker and fly ash with highiron content. By determining these points for each image stackagain, the thresholds are adapted to the individual contrastand brightness settings of each sample.

A qualitative description of the workflow of a procedure toautomatically determine the characteristic points is given bythe example of OPC-FA after 28 days of hydration in Figure5. After applying a running average filter to remove smallfluctuations of the histogram, the maxima and minima aredetermined. The characteristic points are suggested automat-ically by specific selection criteria, e.g. the last maximum, char-acterized by a higher frequency than the frequencies of theneighbouring 80 grey levels, is associated with the majority ofanhydrous fly ash.

The threshold for clinker segmentation cannot be specifiedby the minimum between portlandite and clinker due to theconsumption of anhydrous clinker and the disappearance ofthe correlating maximum. Therefore, the triangle algorithm(Zack et al., 1977) was applied, using the last minimum asstarting and the last big maximum (h3) as end point, to identifythe right base of the main fly ash maximum.

In special cases, the automatic selection may be incorrect,e.g. for OPC-FA after 1 day of hydration the automaticallydetermined minimum (grey level = 116) between the first twomaxima is not suitable as threshold for the segmentation ofthe porosity. However, the user has the choice to use othercharacteristic points instead, e.g. the right base of the porositymaximum, determined by the triangle method between h5

and h1.

Fly ash segmentation

As shown in Figure 3, simple grey level segmentation is notsuitable to segment anhydrous fly ash due to its heterogeneity

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Fig. 4. Histograms of full image stacks of various samples acquired at hydration times ranging from 1 to 550 days.AFm/Aft, aluminium iron mono- and triphases; C-S-H, calcium silicate hydrate; FA, major fraction of anhydrous fly ash; CH, portlandite; Fe-FA, Fe-richfly ash.

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Fig. 5. Flow chart of the processing of the histogram of a full image stack in order to obtain characteristic points needed for grey level segmentation,shown by the example of OPC-FA after 28 days of hydration.

of grey levels and overlap with hydrate phases and clinker.Therefore, a procedure treating each single image of the ac-quired image stack by a variety of processing routines withdifferent structures and parameters is chosen to segment an-hydrous fly ash. Each image processing routine can be assignedto categories dedicated to the segmentation of:

� Fly ash from smoothed image.� Fly ash from entropy segmented image.� Porous fly ash.

Each image processing routine comprises a sequence of greylevel filtering, grey level segmentation and morphological fil-tering operations resulting in a binary image, called mask.Combining the fly ash masks of all image processing routinesyields the final mask showing the anhydrous fly ash in theoriginal image. The following chapters shall explain the prin-ciple of each section of the program. The detailed workflowshowing every single performed operation and all parametersis shown in Appendix F.

Segmentation of fly ash from the smoothed image

Fly ash particles that show a good contrast to the surroundingmatrix can be segmented by a procedure using the smoothed

image. This is shown for the example of an image of OPC-FAafter 90 days of hydration in Figure 6. Smoothening of theoriginal image (for details, see Appendix B) is used to homog-enize the distribution of grey levels and improve the stabilityof the grey level segmentation. After this step a grey level seg-mentation with the previously determined thresholds (see thesection on ‘Selection of thresholds for grey level segmenta-tion’) is carried out. The thresholds are based on the charac-teristic points of the histogram (h1–h5) of the image stack andsometimes modified by fixed offsets, which are automaticallyadapted to the contrast settings (Appendix A). For the proce-dure described in Figure 6, h3 – 15 and h4 (see Fig. 5) are usedas thresholds. The thereby segmented range of grey levels gi

is highlighted in the histogram of Figure 6(a) and the result-ing binary mask of pixels attributed to gi is shown in Figure6(b). The mask shows objects corresponding to anhydrous flyash, hydrate phases or fragments of the matrix. The variationof grey levels within some particles leads to the formation ofholes after the grey level segmentation. Therefore, a hole fillingoperation yielding the mask shown in Figure 6(c) is carried out.Small, remaining fragments of the matrix and bridges betweenobjects are removed by morphological erosion (Fig. 6d, for de-tails, see Appendix C). The residual objects are differentiatedby shape criteria like roundness or size (Appendix D). Roundparticles with a low perimeter-to-area ratio are attributed to

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Fig. 6. Scheme of an image processing routine to segment fly ash basedon the smoothed, original image shown for the example of a BSE imageof OPC-FA after 90 days of hydration. The highlighted area (*) in Figure6(a) shows some of the FA particles, which are not segmented by thisprocedure.

anhydrous fly ash, while hydrates (fragments) usually showa higher perimeter-to-area ratio. Several combinations of sizeand roundness criteria to select objects corresponding to an-hydrous fly ash were determined by trial and error. To obtainthe fly ash mask from this image processing routine (Fig. 6e),a morphological dilation (Appendix C) is used, to restore theparticle shape prior to the previous erosion.

Segmentation of fly ash from the entropy segmented image

Fly ash particles in the direct vicinity of hydrate phases ofsimilar grey levels cannot be segmented by the combinationof smoothening, grey level segmentation and morphologicalfiltering. This can be seen from Figure 6, which shows that flyash particles within portlandite (highlighted area in the upperleft of Figure 6(a) are not segmented by the procedure basedon the smoothed image.

To increase the contrast between fly ash and the surround-ing hydrates, an approach using the local entropy in order toremove grain boundaries and parts of the matrix is employed(for details, see Appendix E). The procedure is demonstratedon the example image of OPC-FA after 90 days of hydration inFigure 7. An entropy image is created in which the grey level ofeach pixel represents a measure of the entropy of the surround-ing pixels in the original BSE image. The entropy image of theoriginal example image is shown in Figure 7(b). Grain bound-aries and large fractions of the matrix show high entropy, whilebulk phases like pores and fly ash exhibit low entropy. By set-ting an appropriate threshold for the grey level segmentationof the entropy image with the help of Otsu’s method (Otsu,1979), areas of high entropy are blanked out (see Fig. 7c).Thereafter, grey level segmentation (Fig. 7d) and morpholog-ical filtering (Fig. 7e & f) are used in a similar way as describedin Section ‘Segmentation of fly ash from the smoothed image’to generate a mask of anhydrous fly ash (Fig. 7f).

For the described segmentation of fly ash by entropy seg-mentation, grey level segmentation and morphological filters,various combinations of parameters are used, as shown inAppendixes F and H.

Segmentation of porous fly ash

Fly ash particles with a highly heterogeneous texture are notsegmented by the previously described procedures. Amongthose are a lot of porous fly ash particles (see upper and lowerleft part of Fig. 8a). The low grey levels inside of the fly ashpores and the relatively high grey levels at the pore boundaries,which are caused by the uneven sample surface and the relatededge effect, create a heterogeneous texture. The high varianceof grey levels causes the fragmentation of the fly ash objectduring the entropy or grey level segmentation and therebyprevents their segmentation.

Therefore, a different approach using the content of char-acteristically round pores within the fly ash as segmentationcriteria is used. The mask of the characteristically round flyash pores (Fig. 8b) needed for this approach is obtained by greylevel segmentation and morphological filtering (for details, seeAppendix G).

The image processing routine to segment porous fly ashstarts in a principally similar way to the previously describedprocedures. First, a grey level segmentation of the originalimage (Fig. 8a) is carried out. The resulting mask (Fig. 8c)

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Fig. 7. Scheme of an image processing routine to segment fly ash fromthe entropy segmented image, shown for the example of a BSE image ofOPC-FA after 90 days of hydration.

is processed by morphological filtering, i.e. closing and open-ing (Appendix C), yielding the mask shown in Figure 8(d).Afterwards, all holes are filled and the round pores withinfly ash (Fig. 8b) are subtracted from the mask (Fig. 8e). Inthis mask, all objects with a certain area of closed porosityare attributed to fly ash. These particles are selected by theirincrease of area when performing a hole filling operation.Thereby (parts of) the porous fly ash particles in Figure 8(a)

Fig. 8. Scheme of an image processing routine to segment porous flyash shown for the example of a BSE image of OPC-FA after 90 days ofhydration.

are segmented (Fig. 8f), although their shape and texture areirregular.

Assemblage of final fly ash mask

Each image processing routine yields a binary mask solelyshowing certain parts of fly ash in the original image. Thesebinary masks are finally assembled by a logical ‘OR’ operation

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Fig. 9. Comparison of (a) original example image with (b) final fly ash mask, assembled by superposition of all fly ash masks of the subroutines based on(I) the smoothed image, (II) the entropy segmented image and (III) the mask of porous fly ash. Multiplication of original image by final fly ash mask yieldsa grey-scale image of the segmented fly ash (c).

to a final fly ash mask. Although most of the fly ash masksfrom the different image processing routines show a lot ofoverlap, each of them contributes to a certain part to thefinal fly ash mask. Figure 9(b; I–III) shows how the masksfrom the processing of the smoothed image (I), the entropysegmented image (II) and porous fly ash (III) are assembledto the final fly ash mask. The multiplication of the mask ofsegmented areas (Fig. 9b) with the original image (Fig. 9a)yields a grey-scale image (Fig. 9c), which can be used toverify the quality of the segmentation. The segmentation ofanhydrous fly ash seems to be reasonable in this exampleimage. Nevertheless, the segmentation of fly ash may as wellshow deficiencies, whose sources are discussed in the followingsection.

Deficiencies in fly ash segmentation

The deficiencies in fly ash segmentation are mainly caused by:

� Missing or only partial identification and segmentation ofanhydrous fly ash particles with a heterogeneous textureand/or odd particle shape.

� Missing identification of small fly ash particles with a diam-eter of less than 1 μm.

� Identification of parts of hydrate phases as anhydrous flyash.

The comparison of original and segmented BSE images ofOPC-FA after 90 days of hydration in Figure 10 shows ex-amples of incorrect segmentations. The highlighted areas inFigure 10(a–d) show fly ash particles, which are not detected as

such by the segmentation routine. Especially, the large porousparticles in Figure 10(a; lower left) and Figure 10(b; lowerright) are hard to be distinguished from the matrix of cementhydrates. Due to their heterogeneous texture, these particlesare fragmented by grey level or entropy segmentation. Thosefragments are only partially detected as fly ash, since most ofthem do not fulfil the shape criteria (primarily roundness) tobe identified as anhydrous fly ash.

Another class of fly ash particles with a heterogeneous tex-ture are hollow spherical particles (Fig. 10c, left). If those par-ticles are at the edge of the BSE image, the segmented fly ashobject is not closed. Therefore, the cavity cannot be filled bya hole filling operation and it will not be segmented as flyash. The same effect may appear when the shell of the hollowsphere particle is not segmented completely.

Additionally, small fly ash particles of less than 1 μm aresorted out by the object selection due to their similarity tofragments of hydrate phases. The fraction of these particlesin the raw fly ash is 10 vol%, as shown by the particle sizedistribution in Figure 1.

The errors due to the missing identification of fly ash par-ticles may be reduced by adapting the parameters of the seg-mentation procedure, e.g. by reducing the requirements of sizeand roundness for the object selection of fly ash. However, thiswould in turn lead to the increased identification of hydratephases as anhydrous fly ash, as shown by the highlighted ar-eas in the example images of Figure 10(e & h). This is due tothe high similarity of certain fly ash particles to the matrixof hydrates or anhydrous clinker and can hardly be avoidedwhen choosing the parameters for the segmentation proce-dure. Due to the varying amount, porosity and composition

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Fig. 10. (a–d) Example images of OPC-FA after 90 days of hydrationand (e–h) the correlating images showing the segmented fly ash. Thehighlighted areas indicate errors in the segmentation of fly ash.

of the hydrates, the systematic error induced by the wrongidentification of hydrates as anhydrous fly ash varies over thehydration time. To avoid this, the parameters are chosen inorder to minimize the amount of hydrates identified as an-hydrous fly ash, but in turn to omit certain ‘problematic’ flyash particles. Thereby, the main systematic error is ideally re-duced to a certain underestimation of the fly ash content inthe sample. However, with ongoing hydration, the content ofsmall anhydrous fly ash particles decreases and therefore thesystematic error is not constant. Additionally, the overall mi-crostructure changes: the porosity decreases and larger areasof C-S-H are formed. In case that these areas show a homoge-neous texture, they may be misinterpreted as anhydrous flyash.

Fig. 11. Volume fractions of fly ash, clinker, portlandite and pores inOPC-FA hydrated at 23◦C determined by image analysis. Additionally,the quantities of portlandite determined by means of thermogravimetricanalyses are plotted. The error bars represent the standard deviation ofvi, which was determined after 1 day by analyses of 8 individual samplesand after 28 and 550 days by analyses of three individual samples fromone batch.

Results

Quantification of anhydrous fly ash, clinker and pores

The volume fraction vi of each component i is defined by theratio of masked pixels to the entire number of pixels (1024 ×800). The volume fraction of clinker, portlandite and porositywas analyzed according to Scrivener et al. (1987). The progressof the volume fractions of fly ash, clinker, portlandite and poresin OPC-FA is shown in Figure 11. Additionally, the quanti-ties of portlandite determined by means of thermogravimetricanalyses are also plotted (reproduced from Deschner et al.,2012). The portlandite contents determined by thermogravi-metric analyses and image analysis are in good agreementwith each other.

The standard deviation of the volume fraction of fly ash wasdetermined after 1 day by the analysis of eight individual sam-ples from four batches and after 28 and 550 days by analysesof three individual samples from one batch (see Table 2). Theresulting standard deviations of the absolute volume fractionsare 0.8 vol% after 1 day, 0.3 vol% after 28 days and 0.5 vol%after 550 days of hydration. On the one hand, this deviation iscaused by the instability of the image quality, which is relatedto slight variations of:

� The quality of the surface polish.� The focusing over the analyzed area of the sample.� The contrast settings during image acquisition.� Instrumental parameters of the microscopic system.

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Table 2. Quantified volume fraction of anhydrous fly ash (vFA) after 1, 28and 550 days of hydration, used to calculate the reaction degree of fly ash(rFA), the corrected reaction degree (r’FA) and the corresponding standarddeviations.

Time [d] Sample vFA [%] rFA [%] r’FA

1 Sample 1 Disc 1 18.3 25.0 5.1Disc 2 16.7 31.5 13.3Disc 3 18.6 23.7 3.5

Sample 2 Disc 1 18.5 24.1 3.9Disc 2 18.0 26.1 6.4Disc 3 18.4 24.5 4.5

Sample 3 Disc 1 18.6 23.8 3.6Sample 4 Disc 1 19.7 19.2 -2.2Mean 18.4 24.8 4.8SD 0.8 3.4 4.3

28 Sample 1 Disc 1 17.6 28.0 8.8Sample 2 Disc 1 18.2 25.5 5.7Sample 3 Disc 1 17.7 27.5 8.2Mean 17.8 27.0 7.6SD 0.3 1.3 1.7

550 Sample 1 Disc 1 14.1 42.3 27.0Sample 2 Disc 1 13.3 45.3 30.8Sample 3 Disc 1 14.2 41.7 26.2Mean 13.9 43.1 28.0SD 0.5 1.9 2.4

On the other hand, the deviation is caused by compositionalheterogeneities within the samples, which arise from segrega-tion effects during the setting of the cement paste. This can beseen from the analysis of different discs from the sample 1 af-ter 1 day of hydration (Table 2), which shows results varyingbetween 16.7 and 18.6 vol% of anhydrous fly ash.

Minimal size of datasets required

In order to investigate if the number of analyzed images is suf-ficient to provide satisfactory statistics, auto-correlation func-tions between the data points of vFA subtracted by the meanof vFA in a dataset of images are plotted in Figure 12 for threedifferent samples of OPC-FA after 1, 28 and 550 days. Withincreasing lag, the envelope of the auto-correlation functiontends towards zero. At a lag of 550, the distance between theupper and the lower envelope is in all samples below 0.01.The low auto-correlation of values at 550 images indicatesthat the analysis of more images is not expected to furtherimprove the final result. Therefore, 550 images are consideredto be sufficient to provide satisfactory statistics.

Reaction degree of fly ash

The anhydrous fraction fi and the reaction degree ri of a com-ponent i are calculated by the ratio of the components volumefraction vi and its initial volume fraction vini,i in the sample

Fig. 12. Auto-correlation functions between the data points of vFA sub-tracted by the mean of vFA in datasets of images from three differentsamples of OPC-FA after 1, 28 and 550 days.

according to Eq. (3).

ri = 1 − fi = 1 − vi

vi ni,i(3)

vini,i is calculated by the initial mass fraction in the paste (w0,i),the measured density (δi) and the content of entrained air voids(cair = 2.4%), normalized to 100% Eq. (4).

vi ni,i =wo,i

δi∑i

wo,i

δi

· (1 − cair ) (4)

The progress of the reaction degree of fly ash rFA in OPC-FAbetween 1 and 550 days of hydration is shown in Figure 14.It shows a reaction degree of 24.8% of fly ash after 1 day ofhydration with a standard deviation of 3.4%, which was cal-culated from the reaction degrees of eight individual samples(see Table 2). This value is clearly overestimated regardingthat fly ash shows no significant reaction at 23◦C up to 2days of hydration (Weng et al., 1997; Rahhal & Talero, 2004;Sakai et al., 2005; Deschner et al., 2012). The reason for theoverestimation is due to the systematic error of the image anal-ysis, which is mainly related to the missing identification of fly

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Fig. 13. Reaction degree of fly ash as determined by image analysis (rFA)and corrected for the systematic error (r’FA) of OPC-FA over a hydrationtime of 1–550 days.

ash particles (see the section on ‘Assemblage of final fly ashmask’). This error may be corrected assuming similar reactionrates for all fly ash particles. This assumption is very muchsimplified, since the reactivity of fly ash particles depends ontheir chemical composition and crystallinity. The correctedreaction degree r’FA is determined according to Eq. (5).

r ′F A = 1 − f ′

F A = 1 − f F A

1 − e(5)

where e is the systematic error related to the overestimationof the fly ash reaction degree. An estimate of e is the reactiondegree of fly ash (rFA) after 2 days assuming that no fly ash hasreacted at this time. Figure 13 shows the corrected reactiondegree r’FA for a given systematic error of 21%. The errorbars represent the calculated standard deviations shown inTable 2.

Comparison of image analysis with selective dissolution

Figure 14 shows the fly ash reaction degree determined byselective dissolution compared to the corrected fly ash reactiondegree determined by image analysis. Except for the valueafter 28 days, the differences between the two methods arewithin the error range. The graph also shows the portlanditeconsumption by the pozzolanic reaction of the fly ash as aqualitative comparison for the hydration kinetics of fly ash. Itis determined by the difference between the portlandite contentof OPC-FA and a sample blended with practically inert quartzpowder instead of fly ash as reference (details are given inDeschner et al., 2012). The portlandite consumption indicatesa similar progress of the reaction rate of fly ash as the resultsfrom the image analysis up to hydration times of 90 days.

Fig. 14. Comparison of reaction degree of fly ash determined by selectivedissolution and image analysis, corrected for a systematic error of 21%(IA corrected).

Conclusions

An image analysis program for the segmentation of fly ash inBSE images and for the determination of its reaction degree hasbeen developed. The procedure is carried out automatically ona stack of at least 550 images per sample at a magnification of2000. The evaluation procedure is based on various combina-tions of grey level filtering, grey level segmentation, entropysegmentation and morphological filters.

Features of the procedure are:

� At the magnification of 2000, the analysis of at least 550images is considered to yield satisfactory statistics.

� The thresholds for the grey level segmentation are deter-mined by semiautomatic analysis of the histogram of thecorresponding image stack. Thereby, the thresholds areadapted to the contrast and brightness settings during theimage acquisition.

� The standard deviation of the reaction degree of fly ash inthe presented dataset is up to 4.3%.

The drawbacks of this image analysis technique are:

� Many parameters for filters and thresholds for segmentationtechniques are manually optimized and need to be adaptedfor different fly ashes.

� The analysis shows a systematic error, which is supposedto be mainly related to the misidentification of certain flyash particles exhibiting for example highly heterogeneoustexture, irregular shape or similar grey level and texture ashydrate phases.

� The systematic error of the analysis is probably not constantover the hydration time due to the changing microstructureand decrease of the amount of fly ash particles with less than1 μm.

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The presented image analysis procedure shows on one hand,how characteristic features of anhydrous fly ash in BSE imagescan be used to segment fly ash. On the other hand, it shows howmorphological and compositional heterogeneities of fly ashand similarities between fly ash and hydrate phases affect theaccuracy of the result. Probably, the accuracy of the methodmay be improved by the implementation of element mappingsand clustering analyses, like it has been already done for thecharacterization of anhydrous Portland cement (Stutzman,2004) and fly ash (Chancey et al., 2010). The comparison ofthe analyzed fly ash reaction degree to the selective dissolutiontechnique based on ethylenediaminetetraacetic acid showssimilar results and supports the conclusion that the presentedimage analysis procedure is a promising method to quantifythe reaction degree of fly ash in cement pastes.

Acknowledgements

The authors wish to acknowledge Schwenk Zement KG for thefinancial support and STEAG Power Minerals GmbH for thesupply of fly ash. Thanks are also due to Mohsen Ben Hahafor helpful discussions and comments. Thanks are extended toBoris Ingold for the preparation and polishing of the samplecross-sections.

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Appendix A: Adaption of grey level threshold parameters tocontrast settings

The thresholds for grey level segmentation are based on thesemiautomatically determined characteristic points of the his-togram of the image stack of one sample. For most thresholds,a characteristic point of the histogram is modified by manu-ally determined, fixed offsets, e.g. h3 – x and h3 + y, where xand y are the offsets. The offsets are dependent on the contrastsettings during the image acquisition. Therefore, a measure ofcontrast was defined by the difference of h5 and h1, as shownexemplarily for OPC-FA after 7 and 28 days in Figure A1

The contrast in each measured sample s can then be relatedto the contrast of the sample at which the fixed values weredetermined by manual optimization (OPC-FA after 7 days) by

the factor c (Eq. (A1)).

cs = (h5 − h1)s

(h5 − h1)O PC −F A,7d(A1)

To approximately account for variations in the contrast, thefixed values are multiplied by the factor cs.

Appendix B: Smoothening filter

Smoothening is applied by a low-pass filter to achieve noiseremoval while keeping strong gradients. A pixel-wise adaptiveWiener method (Lim, 1990) based on statistics estimated froma local neighbourhood of each pixel is used. The grey level ofthe filtered pixel b (at the coordinates x and y) is calculated byEq. (B1)

b(x, y) = μ+ σ 2 − v2

σ 2· (a (x, y) − μ) (B1)

where μ is the estimated local mean, σ the local variancearound the pixel, ν the noise of the image and a is the originalgrey level of the pixel.

Appendix C: Morphological erosion, dilation, closing andopening

Removal of small or branched fragments of hydrates andbridges between fly ash particles and hydrates is achieved byan erosion filter (Vandenboomgaard & Vanbalen, 1992) witha disk-shaped kernel of different radii. In order to restore theoriginal shape of the objects after erosion, dilation at the samekernel is carried out.

The morphological closing operation is a dilation followedby erosion, using the same structuring element for both oper-ations. The counterpart to this operation, the morphologicalopening operation, is erosion followed by a dilation.

Fig. A1. Illustration of the difference between h5 and h1 as measure for the contrast, shown for OPC-FA after 7 and 28 days of hydration.

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Appendix D: Selection of objects according to shape criteria

Objects (e.g. remaining after the erosion) are differentiated byshape criteria like perimeter, size and roundness. The value ofroundness (r) is determined via the ratio of size (s; equallingthe number of pixels of a 2-D object) and perimeter (p) of theobject according to Eq. (D1)

r = 4πsρ2

(D1)

This term yields 1 for a circle and low values close to 0 forobjects with a highly branched structure.

Unreacted fly ash particles are classified by their roundnessand their size. Combinations of roundness and size criteriahave to be determined manually to filter fly ash particles. Bigfly ash particles are classified by a high minimum size and lowminimum roundness values to be distinguished from otherobjects. Small fly ash particles are classified by low minimumsizes and relatively high values of roundness to be differenti-ated from fragments of hydrates with similar size. In additionto size and roundness criteria, the eccentricity of an ellipse thathas the same second moments as the objects may be used aswell.

Appendix E: Entropy segmentation

Fly ash particles close to hydrate phases of similar grey levelscan not be easily differentiated from its surroundings by meansof grey level thresholds. To improve the contrast, areas of highlocal entropy, which are characteristic for grain boundariesand hydrate phases with a high fluctuation of grey levels, aremasked out.

Consequently, a procedure that determines the entropy ofthe local environment (defined by a kernel) of each pixel(Gonzalez et al., 2009) is being used. The used kernel wasdefined by a disk of either 3 or 4 pixels. The entropy E ofthe kernel around the centre pixel was calculated by Eq. (E1;Gonzalez et al., 2009)

E =L−1∑

i=0

P (Z i ) log2 P (zi ) (E1)

where zi is a variable indicating intensity, p(zi) is the histogramof the intensity levels in the local environment and L is thenumber of possible intensity levels.

Figure E1 shows the workflow of the entropy segmentationby an example image of OPC-FA after 90 days of hydration(Fig. E1a). In the entropy image (Fig. E1b), the grey level in-dicates the measure of entropy of the surrounding pixels inthe original BSE image. With the help of Otsu’s method (Otsu,1979), a grey level threshold to segment foreground (highentropy) and background (low entropy) of the entropy imagewas automatically determined (see histogram of Fig. E1b). Tosegment areas of different entropies, the threshold is slightlymodified by fixed offsets, e.g. 0.02 or 0.04. The grey levels,which are lower than the threshold, yield a binary mask (Fig.

Fig. E1. Schematic illustration of the entropy filter used in this study,showed by the example of a BSE image of OPC-FA after 90 days of hy-dration.strel, structuring element; dx, disk with radius x pixel; Otsu, leveldetermined by Otsu method.

E1c), which can be multiplied by the original image to showthe areas of low entropy (Fig. E1d).

The effect of the entropy segmentation depends on the shapeand size of the kernel and the adaption of Otsu’s thresholdlevel. For fly ash particles that are strongly interlocked withthe matrix of hydrates, the surrounding of a disk of a radiusof 4 pixels is appropriate to separate them from the matrix. Adisk of 3 pixels preserves fine structures and keeps the originalshape. Since the deletion of the areas of high local entropy alsoerodes grain boundaries, each image processing routine usingthis method finishes with a morphological dilation to restorethe eroded boundaries.

Appendix F: Workflow of the complete fly ash segmentationprocess

Figure F1 shows the detailed workflow of the complete fly ashsegmentation process including all operations and parame-ters. It is structured in different sections, which are describedin Section ‘Fly ash segmentation’.

C© 2013 The AuthorsJournal of Microscopy C© 2013 Royal Microscopical Society, 251, 188–204

Q U A N T I F I C A T I O N O F F L Y A S H I N H Y D R A T E D , B L E N D E D P O R T L A N D C E M E N T 2 0 3

Fig. F1. Schematic overview of the fly ash segmentation algorithm. Rectangular fields represent operations and ellipsoid fields represent grey scale imagesor binary masks.

C© 2013 The AuthorsJournal of Microscopy C© 2013 Royal Microscopical Society, 251, 188–204

2 0 4 F . D E S C H N E R E T A L .

Appendix G: Segmentation of round pores within fly ash

The segmentation of round pores within fly ash is shown foran example image of OPC-FA after 90 days of hydration inFigure G1. To segment porous areas, a grey level segmenta-tion with a slightly higher threshold (h2 + 0.4 × (h3 − h2))than the normal porosity threshold (h2) is carried out. Theresulting mask is shown in Figure G1(b). Afterwards, smallpores are eroded and objects with a higher roundness than0.9 are selected (Fig. G1c). The same procedure with differentparameters yields another mask, which is merged with themask in Figure G1(c). The resulting mask is dilated to obtainalso the surrounding pixels of the pores and multiplied by theoriginal image (Fig. G1d). The following grey level segmen-tation with a higher threshold than the porosity (h2 + 0.55× (h3 − h2)) serves the segmentation of relatively bright pix-els. The resulting mask (Fig. G1e) shows the segmentation ofclosed rings where pores are within fly ash, and open ringsor just fragments where porous areas are within the matrix.The selection of closed objects (objects that increase their areaby a hole filling operation) yields mainly fly ash pores in theresulting mask.

Appendix H: Segmentation of fly ash from the entropysegmented image – matrix of combined entropy and greylevel segmentations

The effect of the entropy segmentation is dependent on thesize of the environment and the threshold value used for thegrey level segmentation of the entropy image. Therefore, fourentropy segmentation procedures with different parametersare applied and combined with grey level segmentation. Dueto the variability of the grey levels of fly ash, four grey levelsegmentations with different thresholds are used. The combi-nation of these procedures results in 16 binary images whereof12 masks (Mi) are further processed (see Table H1 and Fig. F1).

Table H1. Different combinations (marked by an x) of entropy filters and thresholds for grey level segmentation used for the generation of the 12 differentmasks Mi in section III of the process (Fig. F1).

Entropy segmentation parameters

Grey level Thresholds [strel = d4][strel = d5]

Min Max Otsu’s level Otsu’s level + 0.02 Otsu’s level + 0.04 Otsu’s level

h3 − 30 × c h3 + 30 × c x xh3 − 25 × c h4 x xh3 − 10 × c h4 x x x xh3 − 7 × c h3 + 15 × c x x x x

Fig. G1. Scheme of the image processing routine to segment round poreswithin fly ash shown for the example of a BSE image of OPC-FA after 90days of hydration.

C© 2013 The AuthorsJournal of Microscopy C© 2013 Royal Microscopical Society, 251, 188–204