NANOLIME CONSOLIDATION OF

POROUS LIMESTONES: INFLUENCE

ON THE PORE NETWORK, DURABILITY

AND WEATHERING BEHAVIOUR.

Jasper Gryffroy Student number: 01100356

Promotor: Prof. Dr. Veerle Cnudde

Copromotor: Dr. Tim De Kock

Jury: Marleen De Ceukelaire, Dr. Jan Dewanckele

Master’s dissertation submitted in partial fulfilment of the requirements for the degree of master in geology

Academic year: 2016 - 2017

2

1. Acknowledgements

Despite the time spent alone with data tables and blanc pages a

master’s dissertation is more a collaborative work than a one-man

job.

So let me start off with expressing my gratitude for my promotors

Prof. Dr. Veerle Cnudde and Dr. Tim De Kock for their comments

and advise on my dissertation and making all this possible.

Especially the guidance of Dr. Tim De Kock through the numerous

procedures, methods, interpretations, etc. has been invaluable.

Another thanks goes to the people at ARQUIDE UPCT for

providing the nanolime suspension and the Tabaire stone samples.

Then there are Maxim Deprez, Jeroen Van Stappen and

Géraldine Fiers whose help with MIP, Brazilian testing and SEM was

as valuable as the methods themselves.

And finally a special thanks to Stefanie Van Offenwert for sharing

her office with me and Laurenz Schröer and a thank you to them both

for the quick exchanges of ideas and an enjoyable working

atmosphere.

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2. Table of contents

1.Acknowledgements 2

2.Table of contents 3

3.Introduction 5

3.1. Research goals 6

4.Materials and methods 7

4.1. Nanolime 7

4.2. Stone samples 7

4.2.1. Lede stone 7

4.2.2. Tabaire stone 8

4.3. General workflow 8

4.4. Nanolime Treatment 10

4.4.1. Application 10

4.4.2. Carbonatation 10

4.5. Phenolphthalein test 10

4.6. Capillarity 11

4.7. Porosity 11

4.8. Mercury Intrusion Porosimetry 12

4.9. Computed Tomography 13

4.9.1. Scanning 14

4.9.2. Pre-processing and reconstruction 14

4.9.3. Comparing images 15

4.9.4. Image analysis 15

4.9.5. Visualisation 15

4.10. Brazilian test 15

4.11. Petrophysical durability estimator 16

4.12. Scanning Electron Microscopy 17

4.13. Weathering treatment 17

5.Results 18

5.1. Phenolphthalein test 18

5.2. CT visualization 18

5.3. SEM 21

5.4. Capillarity 23

5.5. Porosity 24

5.6. Mercury Intrusion Porosimetry 26

5.7. Brazilian test 28

5.8. Petrophysical Durability estimator 29

5.9. Weathering treatment 29

6.Discussion 36

6.1. Sample Size 36

6.2. Phenolphthalein test 36

6.3. CT visualization 36

6.4. SEM 37

4

6.5. Porosity 37

6.6. Mercury intrusion porosimetry 37

6.7. Capillarity 38

6.8. Brazilian test 39

6.9. Weathering treatment 39

6.10. Petrophysical durability estimator 39

6.11. Nanolime deposition 40

7.Conclusions 41

7.1. Tabaire stone characteristics 41

7.2. Nanolime deposition 41

7.3. Effectivity of the nanolime treatments 41

7.4. Future research 42

8.Reference list 43

9.Appendix 46

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3. Introduction Natural building stones have been excavated from the

subsurface of Flanders since Roman times. However, it is

only a superficial layer of Flanders’ subsurface that has

produced a great variability in relative soft or weathering-

prone building materials (Dreesen and Dusar, 2004). Natural

building stones have become a long-lasting material in

architecture making them prevalent in historic monuments.

Therefore, natural stone is an important part of the cultural

heritage (De Kock et al., 2013).

Usage of natural stones in buildings also exposes them to

the environment allowing many weathering agents such as

rainwater to act upon the stones. Weathering agents may

cause a rapid change in the initial petrophysical properties of

rocks, and thus limit their durability. In fact, the durability of a

building stone can be defined as the measure of the ability of

natural building stone to endure and maintain its essential

and distinctive characteristics of strength, resistance to decay

and appearance in relation to a specific manner, purpose and

environment of use (Benavente et al., 2004).

One of the stones which represent a valuable part of the built cultural heritage of northern Belgium

and the Netherlands is the Lede stone (or Balegem stone) (De Kock et al., 2015). For this stone

sulphation is the main threat causing black crusts as a common degradation phenomenon (figure 1).

More to the south in Spain the Tabaire stone is a building stone which has been used in the building

of the Cartagena port’s quay. The stone is a locally used building stone excavated since Roman times

near the city of Cartagena (Francisco et al., 2013). Despite the fact that the stone was widely used in

historical buildings, little is known about its fundamental properties and furthermore the quarry shows

deterioration patterns such as alveolization, sanding and peeling (figure 2) (Lanzón et al., 2014).

Ideally deteriorated stones are replaced by the same stone type from the original quarry site, but

availability and economic factors may form problems (De Kock et al., 2013). A second option is to find

a replacement stone of which the optical and technical properties as well as ageing characteristics

should fit the original stone as close as possible. For the Lede stone however it remains difficult to find

a replacement stone that matches it’s visual and petrophysical properties (De Kock et al., 2013), while

for the Tabaire stone not a lot is known about its properties. A third option is a consolidation treatment

of the weathered stone.

A consolidation treatment should guarantee three

fundamental requirements: effectiveness (i.e. improvement of the

mechanical strength), compatibility (with the treated substrate)

and long term durability (resistance to different damage

mechanisms) (Hansen et al., 2003; van Hees et al., 2014;

Toniolo et al., 2011). Products used for conservation issues

should thus be compatible, from the mechanical, chemical

physical and aesthetical point of view, with the substrate on

which they are applied (Borsoi et al., 2015). Several

consolidation products for natural building stones exist such as

organic and lime based consolidants. Organic consolidants

however have no chemical compatibility with calcareous

materials such as the Lede stone, and therefore can be

potentially harmful for the treated substrate (Borsoi et al., 2016a;

Hansen et al., 2003; Rodriguez-Navarro et al., 2013). Lime-

based consolidants are possible alternatives for application on

Figure 1: Weathered Lede stone with visible black gypsum crust (Saint Bavo’s Cathedral, Ghent Belgium). (Dewanckele et al., 2012)

b

a

Figure 2: Weathering patterns of the Tabaire stone in the roman quarry. a: alveolization, b: sanding and peeling. (Lanzón et al., 2014)

6

limestone (Dei and Salvadori, 2006; Hansen et al.,

2003). Due to low effectiveness of limewater (a

traditionally-used Ca(OH)2 aqueous solution)

alternatives have been looked for in the last decade

(Borsoi et al., 2016a; Giorgi et al., 2010; Rodriguez-

Navarro et al., 2005).

Nanolime suspensions bring a new approach in

consolidation by means of Ca(OH)2. The suspensions

consist of very small Ca(OH)2 particles that are

dispersed in an alcoholic solvent, such as ethanol or

propanol. The alcoholic nature of the suspension

avoids carbonation by carbon dioxide. In addition, the

Ca(OH)2 nanoparticles are better dispersed compared

to traditional lime water treatments (Daniele and

Taglieri, 2010, 2012; Lanzón et al., 2014). Nanolimes

have proven to be effective in recovering superficial

loss of cohesion by multiple studies (Ambrosi et al.,

2001; D’Armada and Hirst, 2012; Daniele et al., 2008)

which is good for superficial structures such as frescos

and plaster (figure 3) or filling materials such as mortar

(Daehne and Herm, 2013).

For building stones superficial consolidation is not

enough and a more in-depth mass consolidation is

needed. Research into the applicability of nanolime suspension as in-depth mass consolidants is still

ongoing. So far back migration during drying of the nanolime suspension causing the nanolimes to

accumulate at or just beneath the drying surface (Borsoi et al., 2015) is the most major issue that

needs to be resolved.

3.1. Research goals This research focusses on the effects of two different nanolime treatments on weathered Lede

stone and fresh Tabaire stone samples and an evaluation with a durability estimator. One nanolime

treatment serves as a standard treatment while the second is an attempt to mitigate the back migration

problem of nanolime suspensions by receding the drying front into the sample. Pore structure and rock

strength characteristics of the stone samples are tested on reference samples and nanolime treated

samples as a first assessment of the effectiveness of the nanolime treatment. Test results allow the

calculation of the durability estimator and reference sample test results also serve as a basic

characterization of the relatively unknown Tabaire stone. A second assessment of the nanolime

treatment is done by weathering nanolime treated and reference samples and use CT data to evaluate

the effects. CT data of the weathering treatment are also used to evaluate the ability of the durability

estimator to predict weathering resistance of the stones.

Figure 3: Example of nanolime application during

the pre-consolidation in the restoration of the wall

paintings (Santa Maria del Fiore Cathedral,

Florence). Box: treatment location: (top) before

restoration (bottom) after restoration. (Ambrosi et

al., 2001)

7

4. Materials and methods

4.1. Nanolime The nanolime used in this research was produced by ARQUIDE (Departamento de Arquitectura y

Tecnología de la Edificación) UPCT (Universidad Politécnica de Cartagena, Spain). They used the

precipitation method: NaOH is added to CaCl after which CaOH particles are separated by a

centrifuge. The resulting CaOH particles of nanometre size are brought into a suspension in

isopropanol. The physical and chemical characteristics of the suspension have also been tested by

ARQUIDE UPCT. Their results are presented in table 1.

Table 1: Physical and chemical features of diluted nanolime suspensions (a)

Range estimated from the smallest and largest particles detected in HRTEM images. (ARQUIDE UPCT)

Physical characteristics Data

Sedimentation (after 7 days) negligible

Density [kg/m3] 791

Boiling point [ºC] 82

Viscosity (at 25 ºC) [cps] 2.08

Surface tension (at 25ºC) [mN/m] 20.17

Nanoparticles range size [nm] 5-600 (a)

Chemical characteristics Data

Ca(OH)2 concentration [g/L] 5

Amount of water in the mixture [%] <0.05

Mineral phases detected by XRD Portlandite

CaCO3 (carbonation) [%] <0.02

pH 8.35

Electrical conductivity [µS/cm] 0.03

4.2. Stone samples

4.2.1. Lede stone

The Lede stone is a historic building stone

from north-western Belgium which has been

used as a building stone since the Middle Ages

(De Kock et al., 2015). Now it is one of the most

important natural building stone in north-western

Belgium and is therefore prominently present in

the built heritage of the area (De Kock et al.,

2017). The stone is extracted from the calcite

cemented layers in the Lede Formation (Eocene,

Lutetian) (Fobe and Spiers, 1992) in the region

of Balegem (Belgium).

The Lede stone is, in general, an arenaceous,

bioclastic limestone. The bioclasts are diverse,

and represent a shallow-marine fauna with

planktonic and benthic organisms. Among the

macroscopic bioclasts are nummulites, serpulids,

gastropods, bivalves, Echinodermata and shark

teeth. Microbioclasts are mainly foraminifera. The quartz content can reach up to 40%, and has a

bimodal population with a minor fraction of well-rounded grains of 300–500 µm and a major fraction of

sub angular grains under 150 mm in size (De Kock et al., 2013). The glauconite content can range up

cm

Figure 4: Weathered surface of the Lede stone block from the Sint Martinus Basilica

8

to 5%. Accessory minerals are feldspars, zircon, tourmaline and pyrite. The intergranular cement is

granular microspar of ferroan calcite with local syntaxial cement associated with Echinodermata

overgrowth. The primary porosity is both intergranular and intragranular. The cryptocrystalline tests of

foraminifera provide a microporosity, and secondary moldic porosity occurs (De Kock et al., 2013,

2015)

The Lede stone used in this study, originates from a weathered building block from the Sint

Martinus Basilica in Halle (figure 4). Several characteristics of the stone may have changed due to

weathering. No treatments were done to remove the patina or gypsum crusts created by weathering.

Samples were drilled with an 8 mm diameter drill bit and were 1.5 cm ± 0.5 cm long. After drilling, the

samples were stored in a ventilated drying oven at 40°C until constant mass was obtained. This is

done because of the highly anisotropic heat expansion coefficient of calcite which can cause micro

cracking at higher temperatures (Siegesmund et al., 2000).The same storage or drying procedure is

used after all further tests and treatments.

4.2.2. Tabaire stone

The Tabaire stone is a calcarenite with

abundant fossil remains and numerous cavities

caused by dissolving calcium carbonate and is

quarried in Canteras a locality of Cartagena,

Spain (Francisco et al., 2013; Lanzón et al.,

2014). The quarries were exploited by

Carthaginians and Romans as can be seen in

the Punic wall (225–220 BC) or the Roman

theatre of the city (Lanzón et al., 2014). As many

other limestones, the stone is prone to

weathering (Camuffo, 1995; Siedel et al., 2010;

La Russa et al., 2011; Anania et al., 2012) so

that protective methods are needed to preserve

the blocks' surface in historical buildings and

constructions (Cultrone et al., 2007; Ventolà et

al., 2012). In mineralogical terms, the Tabaire

stone is mostly composed of calcite, and

containing low amounts of quartz. Dolomite and

muscovite are present in lower proportions, whereas gypsum and halite are identified in some

samples. The microstructure of the stone is composed of micritic and sparitic crystals, quartz grains

and fossils embedded in the stone matrix (Lanzón et al., 2014).

Samples were drilled with an 6 mm diameter drill bit and were 2 cm ± 0.5 cm long. The smaller drill

bit size was due to limitations in the dimensions stones delivered by ARQUIDE (UPCT) (figure 5). After

drilling, the samples were stored in a ventilated drying oven at 40°C until constant mass was obtained.

The same storage procedure is used after all tests and treatments.

4.3. General workflow An important differentiation between a test and a treatment is needed for this study. A test refers to

any procedure with the goal of characterizing a property (e.g., porosity) of the subjected sample. A

treatment refers to any procedure with the goal of changing one or more properties of the sample.

As there are a multitude of research goals, a considerable amount of tests were done on both

reference samples and samples treated with nanolime. In order to make the test results comparable

and unambiguous, a clear overview of all the tests and treatments is given (figure 6). As Tabaire and

Lede stone underwent the same test and treatment procedures, the overview is identical for both.

Tests on nanolime treaded samples were always done on paired samples. On the first, the nanolime

suspension is dried fast with pressurized air, whilst the second is dried slow under ambient conditions

without pressurized air. More details on this can be found in the ‘nanolime treatment’ section below. All

cm

Figure 5: Tabaire stone sample provided by ARQUIDE (UPCT).

9

samples start out as reference samples. Two samples designated for CT evaluation were immediately

scanned (1)

. These CT samples and half of the reference samples underwent nanolime treatment (a)

.

Before carbonatation, 12 nanolime treated samples were used for the phenolphthalein test (2)

, while

the 10 remaining samples underwent carbonatation (b)

. CT samples were scanned again after

carbonatation (3)

and capillary and porosity tests were done on both nanolime treated samples as well

as reference samples (4)

. Of both reference and nanolime treated samples, two samples were used for

mercury intrusion porosimetry (MIP) and two for Brazilian test (5)

. The sample halves remaining from

the Brazilian test on the nanolime treated samples were used for scanning electron microscopy (SEM) (6)

. SEM can be done after Brazilian testing even though Brazilian tests are destructive. They break the

sample in two sagittal sections, ideal for SEM. More on this can be found in their respective

paragraphs. A new reference sample was used for the CT scanner (7)

and subsequently all three CT

samples underwent acid weathering treatment simulating prolonged exposure to an urban

environment (c)

. Finally the reference CT sample and the two nanolime treated CT samples were

scanned after the weathering treatment (8)

.

Using the data of the CT scans and porosity, capillarity, MIP and Brazilian tests, the stones can be

characterized, a durability estimator (Benavente et al., 2004) can be calculated and the effect of

nanolime on the stones can be assessed before and after weathering. A detailed description of every

individual test and treatment is given in the following paragraphs.

Sample names (e.g. ‘L03r’) are also used to clarify if the associated test result pertains to a

reference sample or a nanolime treated sample. A sample name consists out of two parts. The first

part uniquely identifies the sample with an uppercase letter for the stone type (T = Tabaire, L = Lede)

and two digits. The second part is a lower case letter that signifies if the sample is a reference sample

or which nanolime treatment the sample has undergone before the test (r = reference sample , s =

slow dried nanolime treated and f = fast dried nanolime treated).

Figure 6: Workflow scheme Orange: reference, blue: nanolime treated, oval: test, rectangle: treatment Phen= phenolphthalein, Hg= mercury intrusion porosimetry, Braz= Brazilian test, P & C= porosimetry and capillarity

10

4.4. Nanolime Treatment

As the aim of this research is to assess the effects of nanolime on

building stones and their properties, the nanolime application method is

designed with upscaling and comparability of test results in mind. The

sample set up mimics conditions in a building wall, while the nanolime

application has similar conditions as a spray-on treatment for building

walls. As back migration towards the surface during drying is an issue

with nanolimes (Borsoi et al., 2015), pressurized air was used to blow dry

the samples after application on the surface where the nanolime was

applied. The aim of this is to recede the drying front into the sample and

to accelerate drying (Franzen and Mirwald, 2004; Shahidzadeh-Bonn et

al., 2007), thus fixating the nanolime below the stone surface. Control

samples were treated without blow drying to check the effectiveness of

this treatment.

All samples were sealed on their lateral and back side with parafilm

constraining evaporation to only the application side (figure 7a). CT

samples were sealed in a similar way using a shrink tubing and a CT

mount. To mimic the conditions of an exposed side of a building stone,

sealed samples were mounted horizontally in flower foam (figure 7b).

4.4.1. Application

Before usage, the nanolime was placed in an ultrasonic bath (45 kHz) for 15 minutes to minimize

aggregation phenomena and maintain penetration ability (Niedoba et al., 2017; Swanton, 1995).

Nanolime application was done for each sample in 5 cycles. For slow dried samples, one cycle

consists out of applying 10 drops of nanolime with a syringe to the exposed surface followed by a two

minute resting period. For fast dried samples, one cycle consists out of applying 10 drops of nanolime

with a syringe to the exposed surface, fast drying for 10 seconds with pressurized air (until the surface

is visibly dried) and a two minute resting period (figure 7b). 8 ml of nanolime was used for the 10

Tabaire and 10 Lede samples, hence 0.4 ml/sample.

After nanolime application, the phenolphthalein test was immediately executed on the designated

samples while the rest underwent carbonatation.

4.4.2. Carbonatation

The samples were brought into a climatic chamber in order to let carbonatation take place under

controlled conditions. They were kept there for 14 days at 60% RH and T = 23°C. Relative humidity

has a great effect on carbonatation (López-arce et al., 2014): higher RH (>75%) induces faster

carbonatation, larger particle sizes and higher crystallinity, while lower RH (<54%) gives rise to slower

carbonatation, smaller particle sizes with lower crystallinity. Relative humidity has also an effect on the

crystal structure created by carbonatation, but this is beyond the scope of this study (López-arce et al.,

2014). The conditions used here (60% RH) strike a balance between the two. Subsequent storage

was done in a ventilated drying oven at 40°C until constant mass is obtained.

4.5. Phenolphthalein test The penetration and retention of the nanolime was assessed by the phenolphthalein test. Samples

were broken longitudinally and a 1% phenolphthalein solution (60% ethanol, 40% water) was sprayed

on the cross section of the samples visualising the nanolime macroscopically. Phenolphthalein is a pH

indicator which is colourless for pH < 8.2 and transitions into purple from pH 8.2 to 9.8. Here a colour

change indicates the presence of nanolime (pHAnl = 8.35). The method has already been proven to be

successful in visualising nanolime in porous media (Borsoi et al., 2012, 2015, 2016a, 2016b).

The phenolphthalein test was performed after 0 hours, 24 hours and 96 hours after nanolime

application. At 0 hours to test the penetration of the nanolime before any significant drying can occur.

At 24 and 96 hours in order to assess the retention of nanolime after drying. To have results

representative of samples that underwent carbonatation, drying conditions (T = 20°C, 50% RH) are

a

b

Figure 7: a) example of sealed samples. Left: Lede, Right: Tabaire. b) nanolime application

11

kept as close as possible to carbonatation conditions with all but the application side remaining

sealed. At these conditions and a time span of 24 hours, carbonatation of nanolime remains negligible

(Borsoi et al., 2016b). At 96 hours complete drying is ensured, but some carbonatation may have

taken place. Carbonatation at 50% RH however is rather slow, so this effect is minimal (López-arce et

al., 2014).

4.6. Capillarity To measure capillarity, plastic meshes were glued onto sample

trays to allow for free water flow underneath the sample (figure 8a).

Capillarity testing was done according to EN 1925, with some

adaptions to account for the small sample size and a drying

temperature at 40°C instead of 70°C. Dry specimens were weighted

(md) on a precision balance (accuracy of 0.0001 g) and the area of the

top was calculated (A). In this case, the top is the side on which

nanolime was applied or in case of reference samples the equivalent

side. The top was immersed in water to a depth of 3 ± 1 mm water and

weight measurements were made every 60 seconds (mi).

Because of the small size and irregular surface of the samples,

holders were 3D printed to keep them upright during capillary water

uptake, but samples were weighted without holder (figure 8b). As The

weight difference between measurements was below 1% of the water

absorbed by the specimen before enough data could be gathered. This

end criterion of the test was changed to a weight difference between

two successive measurements below 0.001 g.

When every measurement i is shown in a graph according to

following formulas:

𝑦𝑖 =𝑚𝑖 − 𝑚𝑑

𝐴 𝑎𝑛𝑑 𝑥𝑖 = √𝑡𝑖 (1)

With ti = elapsed time at measurement i [s], md = sample dry mass [g], mi= sample weight at ti [g]

and A = area of the immersed side [m²]. 𝑦 is plotted as the ordinate and 𝑥 as the abscissa.

If the first part of the graph has a minimum of four or five data points and the correlation coefficient

between the first part of the graph and it’s regression straight line is greater than 0.95 or 0.9

respectively, the coefficient of water absorption by capillarity or capillarity coefficient (C [𝑔 𝑚²√𝑠⁄ ]) is

represented by the slope of that regression straight line and can be calculated from any point on it:

𝐶 =𝑚𝑖 − 𝑚𝑑

𝐴 ∗ √𝑡𝑖

(2)

Depending on the orientation of the planes of anisotropy during capillary tests a distinction is made

concerning water absorption coefficient. When capillarity is tested perpendicular to the planes of

anisotropy C is noted as C1, parallel to the planes of anisotropy C2 is used.

4.7. Porosity Porosity was calculated in accordance with EN 1936, with the exception of the used drying

temperature (40°C instead of 70°C) The samples were weighted dry (md) and then put under a

vacuum in order to remove all air out of the pores. Demineralized water was then introduced under

vacuum until the samples were completely immersed (figure 9a) and the immersed samples were

returned to atmospheric pressure to settle. Each sample was then weighed under water (mh) and after

removal of excess water with a dampened cloth they were weighed above water (ms) (figure 9b). Open

porosity (po [%]) was calculated with following formula:

𝑝𝑜 =𝑚𝑠 − 𝑚𝑑

𝑚𝑠 − 𝑚ℎ

∗ 100 (3)

With md = sample dry mass [g], mh = saturated sample mass under water [g], ms = saturated

sample mass above water [g].

a

b

Figure 8: Setup for capillarity test. a: sample tray with glued on mesh and examples of the Lede and Tabaire stone being tested,

b: close-up of Lede stone holder

12

4.8. Mercury Intrusion Porosimetry

MIP provides an indication of the pore throat distribution of natural stones from the nanometre and

micrometre scale, making it a powerful characterization tool for pore size distributions and porosity

(Abell et al., 1999; Gao and Hu, 2013; Giesche, 2006). The method for acquiring pore size distribution

is based on the principle that a pressure is needed to make a non-wetting fluid such as mercury

intrude into a porous medium. The magnitude of the pressure needed can be related to the pore size

with the Washburn equation:

𝛥𝑃 = 2 ∗ 𝛾 ∗ cos (𝜃)

𝑟𝑝𝑜𝑟𝑒

(5)

With ΔP = the pressure difference across the mercury

interface, γ = surface tension of mercury, θ = the contact

angle between the solid and mercury and rpore = the pore size

(Giesche, 2006). The mercury surface tension of 0.48 N/m

used here is commonly accepted by researchers and a

contact angle is often given a fixed value out of practical

considerations (here: 142°) (Giesche, 2006). With these two

assumptions a sample can be brought into a container filled

with mercury. While the pressure is increased, pressure and

intruded volume are measured. The Washburn equation uses

a model where all pores have a cylindrical shape and are

each entirely and equally accessible to the outer surface.

Several issues should be taken into account using this model

as very few materials fulfil these requirements (Diamond,

2000). The model does not differentiate between one long

cylinder or several shorter cylinders of the same diameter.

Pores closed off from the outside are not accounted for so

only the open porosity is characterized. Furthermore, pore

shapes of natural stones are seldom cylindrical which will influence MIP results. Finally the

accessibility assumption gives rise to the most important effect, often called the inkbottle effect

(Diamond, 2000). Larger pore volumes connected to the exterior of the sample through smaller pores

will be added to the pore volume of those smaller pores (figure 10). In the best case all pores will be

classified under the pore size of their largest pore throat and in general there will be an

underestimation of larger pores while there is an overestimation of smaller pores.

Using MIP it is also possible to get a measure of the open porosity (pMIP [%]) of the sample:

𝑝𝑀𝐼𝑃 = 𝑉𝑖

𝑉𝑠

∗ 100 (6)

With Vi = the volume of mercury intruded at maximum pressure and Vs = the bulk volume of the

unintruded sample (Cook and Hover, 1999).

Figure 9: a: vessel for introducing water under vacuum. The vacuum pump is attached at valve 1 and water is introduced through valve 2

b: setup to weigh submerged samples with the precision balance.

2

1 a b

Figure 10: Diagram of the inkbottle effect with the volumes coloured according to their intrusion pressure. Left MIP will provide a correct pore diameter vs volume relation. On the right the volume of the larger pore will be incorrectly added under the smaller pore diameter.

13

MIP measurements were done at the Magnel Laboratory for concrete research (UGent). Due to the

high range of pore sizes, a high range of pressures was needed for MIP. To this end two MIP

machines were used: a low pressure unit for up to 200 kPa and a high pressure unit for up to 200 MPa

(figure 11a and 11b). This allowed for a reliable pore size distribution from 50 nm to 100 µm. The

Intruded volume was measured with a dilatometer which can measure only a certain amount of

volume change (figure 11c). This limits the sample size depending on the porosity. Therefore, too

large samples were reduced to appropriate size using fine grit sandpaper, after which they were rinsed

with demineralized water and dried in a ventilated drying oven at 40°C until constant mass was

obtained. Samples were weighted before being put into the dilatometer and the dilatometer was

pumped vacuum by the low pressure MIP unit to remove air from the pore structure. Next mercury was

added and pressure was increased to 200 kPa while the intruded volume was measured. To switch

machines, pressure was released and the sample with mercury and dilatometer were weighted.

Finally, the high pressure units continued up to 200 MPa. Dedicated software then merges the

datasets of the two pressure units together. After MIP residual mercury remained in the samples due

to the inkbottle effect, making the samples unfit for any further tests and requiring safe disposal.

4.9. Computed Tomography Micro-CT (μCT) is a powerful non-destructive imaging and analysis technique. It can be used to

visualize and investigate the internal structures of objects in 3D (Cnudde and Boone, 2013). An X-ray

source sends X-rays through the sample to be investigated and they are picked up by a detector

(figure 12). The intensity of these X-rays is determined by Beer’s law:

𝐼 = 𝐼0 ∗ 𝑒− ∫ µ(𝑠)𝑑𝑠 (8)

Where I0 = the incident beam intensity, s = the path of the X-ray and μ(s) = the linear attenuation

coefficient along the ray path s (Cnudde and Boone, 2013). With this formula, µ(s) can be calculated

and is mainly dependent on the density and atomic number of the material along the ray path s. So far

the result is only a 2D image or radiography. To get a 3D image, the sample is rotated in respect to X-

ray source and detector while images are taken. Dedicated algorithms can calculate µ for each point

inside the subjected sample creating a 3D image of the samples in which structures depending on

density and atomic number of the materials are revealed. This procedure is called computerized

transverse axial tomography (CT) (Cnudde and Boone, 2013). The CT scanner used for this thesis,

makes use of a conical X-ray beam allowing for geometrical magnification (figure 12 and 13).

c a b

Figure 11: MIP instrumentation. a: low pressure unit, b: high pressure unit, c: dilatometer

14

4.9.1. Scanning

All scans were performed at the µCT scanner HECTOR from the centre for X-ray tomography of

the Ghent university (UGCT) (Masschaele et al., 2013, 2007) (figure 13). For all scans, the source was

set at 120 kV and 10 W while a 1 mm aluminium plate was used as beam hardening filter to harden

the X-rays which diminishes the cupping effect (Cnudde and Boone, 2013). For the X-ray detector, a

binning of two was used to minimize noise effects. All these settings, combined with a fixed source-

detector distance, resulted in a voxel size of 6.97 µm for Tabaire and 8.73 µm for Lede stone. The

smaller voxel size of Tabaire results from the difference in sample size (respectively 6 mm and 8 mm

diameter), influencing the used magnification. Before each scan 40 offset images were taken with the

X-ray source off to account for the background noise and 120 flat field images were taken with the X-

ray source on but without an object to account for X-ray source noise.

4.9.2. Pre-processing and reconstruction

The following processing procedures were applied on the 2D radiographs of the scanned samples

using the reconstruction module of the program XRE (appendix: figure A). The program allows for

easy repetition of identical pre-processing settings for different scans. For every scan, images were

normalized over the flat field and offset images. After this, a FDK noise lifter (level 0.4), ring filter (level

1) and spot filter (level 8) were applied. For each scan, individually the best centre of rotation (COR),

vertical centre (VC), source object distance (SOD), source detector distance (SDD) and tilt were

sought in order to optimize image quality. This process is partially automated, but remains operator

dependent. For every scan, a beam hardening correction off 0.2 was selected. The grey value range

was set to -0.2 to 2.5 for Lede stone and -0.2 to 2 for Tabaire stone in order to maximise the usage of

available grey values. These grey values were manually selected on the first scan of each stone, but

kept the same for the subsequent scans. This procedure was used to optimize image quality, while

keeping all images comparable for the following processing procedures. For each scan the result of

Figure 12: Diagram of a conical X-ray beam setup allowing geometrical magnification. (Cnudde and

Boone, 2013)

X-ray tube

sample detector

Figure 13: picture of HECTOR with a Lede sample mounted in the sample holder.

15

pre-processing is a set of horizontal reconstructed images which stacked together form a digital

volume representing the scanned sample.

4.9.3. Comparing images

Dataviewer is a software packet from Bruker microCT which allows to register two CT scans of a

same object (e.g., before and after nanolime application). Each reconstructed dataset was loaded into

the program where one dataset was selected as “reference dataset” while the other as the “target

dataset”. A differential image was made between the two and 3 cross sections were displayed along

the major planes (appendix: figure B). The differential image is a subtraction of the grey values of the

reference dataset from the grey values of the target dataset. The resulting values are rescaled so that

negative values become positive. The result is average grey values where little to no change took

place, high values where grey values of the target image were lower than those of the reference image

and low grey values for the reverse situation. First a manual positioning and rescaling was done after

which an automated positioning was performed along the three major planes and in 3D, resulting in an

identical geometric position. After performing the difference, the results are the two original datasets

now in the same geometric position and a new differential dataset also in this geometric position.

4.9.4. Image analysis

For quantitative results a dedicated software package is needed to process the 3D volume. Here

Octopus Analysis from Inside Matters, now XRE, was used to isolate components (e.g., pore structure

and deposited nanolime) from the sample and to quantify their characteristics. This was done by first

applying 6 neighbourhood median filter to the digital volume to reduce noise. Next selecting a circular

region of interest (ROI) was done. This allows to discard the parts of the digital volume outside this

vertical cylinder for further steps. Next the air above the irregular sample surface was discarded by

thresholding the digital volume based on grey values which is extremely operator dependent due to

effects such as partial volume and image noise (Baveye et al., 2010). A dual threshold was used in

which the first threshold withholds all voxels selected, while the second threshold only withholds those

voxels in contact with voxels from the first threshold. Threshold values were selected on best fit for

each sample. This results in a binary image with the withheld voxels as foreground and rejected voxels

as background. Binary operations (remove isolated foreground and background voxels , expand and

shrink mask and fill holes) were performed to include pores that were previously excluded by

tresholding. The resulting withheld voxels represent the sample volume including pores as close as

possible and are saved as a volume of interest (VOI). This VOI was used in all further calculations

such as porosity and slice volume. This was done by again applying the process of thresholding and

suiting binary operations within the VOI (appendix: figure C).

4.9.5. Visualisation

Visualisation of the CT images was done using VGStudio from Volume Graphics. Both complete

scans as well as thresholded segments can be rendered in 3D, allowing for visual examination of the

effect of the nanolime treatment.

4.10. Brazilian test The Brazilian test is a laboratory test to measure the indirect tensile strength of rocks. For this, a

compressive stage CT50000 from Deben with a motor speed of 0.1 mm/minute was used.

A cylindrical sample (of which all the dimensions are accurately known) was wrapped in Teflon and

placed in the compressive stage. Teflon wrapping was done to minimize friction between the

compressive stage and the sample. The sample was loaded by two opposing compressive loads on

the cylindrical side (figure 14a and 14b), with the load continuously increasing until failure of the

sample. At the moment of failure the tensile strength (σ [MPa]) can be calculated, using the following

equation:

𝜎 =𝐹𝑚𝑎𝑥

𝜋 ∗ 𝐷 ∗ 𝐿 (9)

16

With Fmax = the applied load at failure [N], D = sample diameter [mm], L = length of the sample

[mm]. Failure of the sample results in a longitudinal fracture, providing a cross section along the

nanolime penetration axis. The surface created by the fracture can be used for electron microscopy

(see SEM paragraph).

4.11. Petrophysical durability estimator Stone durability depends heavily on both strength and pore structure properties, although it has

mainly been estimated either from strength or from pore structure properties. So here an new

durability estimator, introduced by Benavente et al. (2004), is used taking both into account. The

petrophysical durability estimator (PDE) is focussed on weathering due to salt crystallization causing

stress over pore surfaces. The PDE represents a relationship between the materials accommodation

for ice crystallization stress and the materials resistance:

𝑃𝐷𝐸 = 𝑋

𝜎 (10)

With X being a parameter or estimator based on pore structure and σ = a parameter or estimator of

the strength of the material (Benavente et al., 2004). Here σ is always the tensile strength determined

by the Brazilian test while X can be several properties (e.g., capillarity coefficient).

One of the estimators based on pore structure is the durability dimensional estimator (DDE) [µm-1

],

which is calculated from MIP data and is defined by Ordóñez et al. (1994) as follows:

𝐷𝐷𝐸 = ∑𝐷𝑣(𝑟𝑖)

𝑟𝑖

∗ 𝑝𝑐𝑜𝑛 (7)

With Dv = the pore size distribution, ri = the pore size and pcon = the connected porosity. This

durability estimator contains full information about the pore structure and can estimate stone durability

given that several important decay mechanisms are inversely related to pore size, such as capillary

pressure during the wetting and drying cycles (Scherer et al., 2001; Winkler, 1997).

a b

Figure 14: a: CT50000 from Deben with a Bentheimer sample as example. The left stage is secured on the right and the load is measured while compressing the sample. b: schematic of the sample during the Brazilian test with the loading forces and resulting fracture.

17

4.12. Scanning Electron Microscopy

SEM works in a similar way as an optical

microscope, but using electrons instead of visible

light to create an image (figure 15). Interactions

of the electrons with the sample results in

different forms of radiation, amongst others: back

scattered electrons (BSE), the amount of which

measured by the detector mainly depend on

atomic number and topology of the sample. The

BSE signal represents thus the topology of the

sample and can give information on the local

atomic composition. (Sutton et al., 2007).

In this research a JEOL JSM-5310LV SEM

system was used to confirm deposition of

nanolime both at the surface of the samples as

well as more in depth, away from the surface

were nanolime was applied. Samples are coated

with a carbon layer to avoid charging the

samples during imaging. Carbonatated nanolime

has a similar composition as the carbonate stone

samples so it’s identification was done on

morphological characteristics. Carbonatated

nanolime particles have a hexagonal plate like

morphology and a tendency to agglomerate in clusters during deposition (Borsoi et al., 2016b; Daniele

et al., 2008). Together with the size (5-600 nm) these are the main characteristics that were used for

nanolime identification with SEM. When indications of nanolime presence are found imaging is done

with the photomicrography software LINK ISIS by oxford instruments, which is coupled to the SEM.

4.13. Weathering treatment The stones under investigation are both common building

stones and are commonly exposed to a city environment. In

such an environment, acidity due to urban activity and

moisture are two big players in the weathering of stones. In

order to simulate this kind of weathering, so-called ‘dry

deposition’, a determination of resistance to ageing by SO2

action in the presence of humidity is done following the old

EN 13919. The dry samples were immersed in demineralized

water for 24 hours. After this they were put upright into a

closed container with an acid solution for three weeks in order

to simulate a prolonged exposure to an acid atmospheric

environment (figure 16). The acid solution had a volume ratio

of 10 units demineralized water to 3 units 6% sulphurous acid

(H2SO3). In accordance with the norm both container volume

and acid solution were downscaled to 7 l and 91 ml respectively. Changes in the samples due to

weathering were assessed by means of comparing pre and post CT images.

Figure 15: schematic of a typical scanning electron microscope and imaging process. (Sutton et al., 2007)

Figure 16: Weathering treatment setup. Styrofoam is used to keep the samples from directly contacting the acid solution.

18

5. Results

5.1. Phenolphthalein test Figure 17 and 18 show the results of the phenolphthalein test for the Lede and Tabaire stone

samples. In both figures a, b and c are from samples that have been blow dried, while d, e and f are

from samples left to dry out in ambient air conditions. Splitting and photographing of these samples

was done at a time span of 0 hours for a and d, 24 hours for b and e and 96 hours for c and f. Upon

splitting at 0 hours, the wetting front of the nanolime suspension could be observed at around 1.5 to 2

cm depth in the samples. This is not visible in figures 17 and 18 due to the phenolphthalein solution

being sprayed onto the surface.

The Lede stone samples show no significant difference for both drying methods and for all time

spans: the phenolphthalein shows minimal to no penetration of the nanolime into the stone. The

nanolime is present on the exposed surface and up to 5 mm far on the sides of the sample. Note that

although the slanted top surface in figure 17c gives the impression of nanolime penetration, this is not

the case. The upper right corner coloured by the phenolphthalein in figure 17e is due to a crack in the

sample.

For the Tabaire stone the nanolime has penetrated the samples ranging from 0.5 to 2 cm depth. In

figure 18a and 18d The penetration depth of the nanolime itself is less deep than the wetting front of

the nanolime suspension. The penetration depth varies between the samples, but there is no clear

connection between penetration depth and drying method or drying time.

5.2. CT visualization

Volumes of the difference images between pre and post nanolime treatment that indicate an

addition of material are displayed in blue on the digital volumes of the samples before nanolime

treatment in figures 19, 20, 21 and 22. In the difference images no significant zones of material loss

could be observed, only the zones of material addition.

f

b c

e d

a

Figure 18: Tabaire stone treated with nanolime and sprayed with a phenolphthalein solution. A, b and c: fast dried, d, e and f: slow dried. A and d at 0h, b and e at 24h, c and f at 96h.

a b

d

c

e f

Figure 17: Lede stone treated with nanolime and sprayed with a phenolphthalein solution. A, b and c: fast dried, d, e and f: slow dried.

A and d at 0h, b and e at 24h, c and f at 96h.

19

Figure 19: Lede stone sample L22. Digital volume from before the nanolime treatment. Underwent nanolime treatment with slow drying process. Blue: volumes of added material between the pre and post nanolime treatment scans.

Figure 20: Lede stone sample L27. Digital volume from before the nanolime treatment. Underwent nanolime treatment with fast drying process. Blue: volumes of added material between the pre and post nanolime treatment scans.

20

Figure 22: Tabaire stone sample T06. Digital volume from before the nanolime treatment. Underwent nanolime treatment with fast drying process. Blue: volumes of added material between the pre and post nanolime treatment scans.

Figure 21: Tabaire stone sample T05. Digital volume from before the nanolime treatment. Underwent nanolime treatment with fast drying process. Blue: volumes of added material between the pre and post nanolime treatment scans.

21

5.3. SEM

Using SEM possible nanolime clusters were discovered near the surface of the Lede stone sample

(figure 23). Deeper into the Lede stone sample however no indications were found of nanolime

presence. For the Tabaire stone sample possible nanolime clusters were found both at the surface as

well as deeper into the sample (figure 24 and 25). Greater magnification than these pictures was not

possible with the SEM equipment as this caused distortions in the image as can already be seen on

the edges of figure 24.

Figure 23: SEM photograph at the top of nanolime treated Lede sample (L41). The top surface of the sample is to the left. A possible nanolime cluster is indicated with a red arrow.

22

Figure 24: SEM photograph at the top of nanolime treated Tabaire sample (T12). The top surface of the sample is to the right. A possible nanolime cluster is indicated with a red arrow.

Figure 25: SEM photograph deeper into nanolime treated Tabaire sample (T12). The top of the sample is to the right. A possible nanolime cluster is indicated with a red arrow.

23

5.4. Capillarity

Capillarity test results are represented graphically in figures 26 and 27. The exact capillarity

coefficient values of each sample and averages are given in tables 2 and 3. Capillarity curves of the

samples can be found in the appendix (figures D, E, F, G, H, I, J and K). Some capillarity coefficient

values are estimations based on two data points of the capillarity curve. This is done because these

samples reached saturation before the required minimum of four data points for the first part of the

capillarity curve was reached. These estimated values are indicated as smaller markings in figures 26

and 27 and a grey box in tables 2 and 3 and probably represent a minimum value of C.

Except for one sample (L35r) the Lede stone reference samples show a smaller capillarity

coefficient variability of 35.9 𝑔 𝑚²√𝑠⁄ with sample L35r the variability is 89.7 𝑔 𝑚²√𝑠⁄ . Nanolime

treated Lede samples show a variability of 81.0 𝑔 𝑚²√𝑠⁄ , with slow dried samples a variability of 63.5

𝑔 𝑚²√𝑠⁄ and fast dried samples a variability of 81.0 𝑔 𝑚²√𝑠⁄ . Averages show a 31.1 𝑔 𝑚²√𝑠⁄ (or 50%)

increase in capillarity coefficient following the slow dried nanolime treatment, while the fast dried

nanolime treatment results in a 13.4 𝑔 𝑚²√𝑠⁄ (or 21%) increase. Taking into account both treatments

together; there is an increase in capillarity coefficient of 22.3 𝑔 𝑚²√𝑠⁄ (or 35%) in comparison to the

reference samples.

For the Tabaire stone data points are limited because a lot of samples were saturated within one or

two minutes as can be seen in the capillarity curves in the appendix (figures F and G). This makes it

impossible to calculate a capillarity coefficient value for these samples. Reference samples of the

Tabaire tone have a variability of 110 𝑔 𝑚²√𝑠⁄ . Nanolime treated Tabaire samples show a variability of

67.4 𝑔 𝑚²√𝑠⁄ , with slow dried samples a variability of 64.6 𝑔 𝑚²√𝑠⁄ and fast dried samples a variability

of 67.4 𝑔 𝑚²√𝑠⁄ . Averages for the Tabaire stone samples show a 54.4 𝑔 𝑚²√𝑠⁄ (or 39%) decrease in

capillarity coefficient following the slow dried nanolime treatment, while the fast dried nanolime

treatment results in a 40.7 𝑔 𝑚²√𝑠⁄ (or 29%) decrease. Taking into account both treatments together;

there is a decrease in capillarity coefficient of 47.5 𝑔 𝑚²√𝑠⁄ (or 34%) in comparison to the reference

samples.

In comparison the Lede stone samples have lower variability in capillarity coefficient and a lower

average capillarity coefficient than the Tabaire stone samples.

Table 2: Results of the capillarity test on the Lede stone samples. Values in dark grey are an estimation based on the two first data points of the capillarity curves in the appendix.

Table 3: Results of the capillarity test on the Tabaire stone samples. “ / ” = sample was saturated too fast to be able to calculate the coefficient of water absorption capillarity. Values in dark grey are an estimation based on the two first data points of the capillarity curves in the appendix.

24

5.5. Porosity

Porosity test results are represented graphically in figures 28 and 29.The exact porosity values of

each sample and averages are given in tables 4 and 5.

Reference samples of the Lede stone show a variability of 5.62% porosity. Nanolime treated Lede

stone samples show a similar variability of 4.48% porosity. For the nanolime treated samples the slow

dried samples are the cause of the 4.48% variability, while the fast dried samples have a slightly

smaller variability of 3.01%. For the Lede stone nanolime treated samples show a slightly smaller

porosity variability and their porosity distribution is within that of the reference samples. Averages

show a 0.7% increase in porosity following the fast dried nanolime treatment. Following the slow dried

nanolime treatment and for both treatments together; there is an increase in porosity of 0.8% in

comparison to the reference samples.

0

20

40

60

80

100

120

140

160

Reference Nanolime slow fast

0

20

40

60

80

100

120

140

160

Ca

pil

lari

ty [

g∕𝑚

²√ s

]

Capillarity: Lede stone Reference

Nanolime slow

Nanolime fast

Figure 26: Results of the capillarity test on the Lede stone samples. Left: Individual samples values in order of table 2. Right: Boxplots representing the minimum, 1

st quartile, median, 3

rd quartile and maximum of the capillarity results.

40

60

80

100

120

140

160

180

200

Cap

illa

rity

[𝑔

∕𝑚²√

s]

Capillarity: Tabaire stone Reference

Nanolime slow

Nanolime fast

40

60

80

100

120

140

160

180

200

Reference Nanolime slow fast

Figure 27: Results of the capillarity test on the Tabaire stone samples. Left: Individual samples values in order of table 3. Right: Boxplots representing the minimum, 1

st quartile, median, 3

rd quartile and maximum of the capillarity results.

25

Reference samples of the Tabaire stone show a variability of 2.49% porosity. Nanolime treated

Tabaire stone samples show a lower variability of 1.96% porosity. For the nanolime treated samples

the slow dried samples are the cause of the 1.96% variability, while the fast dried samples have a

smaller variability of 0.60%. Averages for the Tabaire stone show a 0.9% decrease in porosity from the

reference samples following both the slow dried, fast dried and both nanolime treatments together.

In comparison the Lede stone samples show a porosity variability of over twice as large than the

Tabaire stone samples, while having a lower average porosity.

Table 4: Results of the porosity test on the Lede stone samples.

Table 5: Results of the porosity test on the Tabaire stone samples.

26

5.6. Mercury Intrusion Porosimetry The pore size distributions of the MIP measurements of

the Lede and Tabaire stone are presented in figures 30 and

31, where 𝑑𝑉 log(𝐷)⁄ is plotted in function of the equivalent

pore diameter. Note that the merging of the data from the two

MIP units happens at 0.2 MPa which corresponds to an

equivalent pore diameter of 7563 nm. Porosities and DDE

acquired with the MIP data are given in table 6. Porosities

and DDE were only calculated for samples with a meaningfull

MIP equivalent pore size distribution.

For the Lede stone reference sample L14r and both

nanolime treated samples have a sudden spike to extremely

high values around an equivalent diameter of 7500 nm. This means something went wrong with either

17

18

19

20

21

22

23

24

Reference Nanolime slow fast

17

18

19

20

21

22

23

24

Po

ros

ity [

%]

Porosity: Lede stone Reference

Nanolime slow

Nanolime fast

Figure 28: Results of the porosity test on the Lede stone samples. Left: Individual samples values in order of table 4. Right: Boxplots representing the minimum, 1

st quartile, median, 3

rd quartile and maximum of the porosity results.

27

28

29

30

31

32

33

34

Po

ros

ity [

%]

Porosity: Tabaire stone Reference

Nanolime slow

Nanolime fast

27

28

29

30

31

32

33

34

Reference Nanolime slow fast

Figure 29: Results of the porosity test on the Tabaire stone samples. Left: Individual samples values in order of table 5. Right: Boxplots representing the minimum, 1

st quartile, median, 3

rd quartile and maximum of the porosity results.

Table 6: Porosities and DDEs calculated from MIP data. ‘nanolime’ is the average of T14s and T17f.

27

merging of the data, 1 MIP measuring device or both measuring devices. The distribution and all MIP

data of these samples is meaningless. Reference sample L03r shows two major peaks at an

equivalent pore diameter of ~280 nm and ~1600 nm. The major peak at ~1600 nm has two minor

peaks at ~1200 nm and ~1800 nm on top of it.

For the Tabaire stone all three samples show a similar pore size distribution with some distinct

differences between the reference sample (T25r) and the nanolime treated samples (T14s and T17f).

Reference sample T25r shows one major peak at an equivalent pore diameter of ~1200 with two

minor peaks on top at ~1100 nm and ~1600 nm, while the nanolime treated samples do not have the

minor peak at ~1600 nm confining the major peak to ~1100 nm.

Some general trends can be noted for the meaningful equivalent pore diameter distributions. Both

reference samples L03r and T25r show a steep decline around ~22000 nm and for these reference

samples and samples T14s and T17f the shape of the subsequent part of the distribution curve is

almost identical.

Figure 30: Equivalent pore diameter size distribution of the Lede stone samples. All graphs use the same x-axis, thick lined graphs use the left thick y-axis and the thin lined graph uses the right y-axis.

0

5

10

15

20

25

30

0

50

100

150

200

250

300

10 100 1000 10000 100000

dV

/dlo

g(D

) [

cm³/

g]

equivalent pore diameter [nm]

MIP: Lede stone L14r

L01s

L32f

L03r

L03r

Figure 31: Equivalent pore diameter size distribution of the Tabaire stone samples.

0

5

10

15

20

25

30

10 100 1000 10000 100000

dV

/dlo

g(D

) [

cm³/

g]

equivalent pore diameter [nm]

MIP: Tabaire stone T25r

T14s

T17f

28

5.7. Brazilian test

The results of the Brazilian test

are given in table 7. Length values

are the average of four

measurements on the sample at 90°

from each other. Diameter values

are the average of three

measurements (at the top middle

and bottom of the sample). For

further use in the Durability

estimator a general value for the

nanolime treatment is calculated as

the average of the fast and the slow

dried treatments. To give further

insight on the failure behaviour of

the stones the force versus compression graphs of the samples are given in figures 32 and 33.

0

50

100

150

200

250

300

350

400

0 100 200 300 400 500 600

Forc

e [

N]

compression [µm]

Brazilian test: Lede stone

L26r

L13s

L41f

Figure 32: Compression force versus compression graph of the Brazilian tests on the Lede stone samples.

0

50

100

150

200

250

300

350

400

0 100 200 300 400 500 600

Forc

e [

N]

compression [µm]

Brazilian test: Tabaire stone

T21r

T12s

T13f

Figure 33: Compression force versus compression graph of the Brazilian tests on the Tabaire stone samples.

Table 7: Results of the Brazilian test. Measured average length (L) and diameter (D) of the sample and compressional force at failure (Fmax) with the resultant calculated tensile strength (σ).

29

5.8. Petrophysical Durability estimator

Four different petrophysical durability estimators are calculated based on the results of previous tests.

All PDEs use the tensile strength gotten from the Brazilian test and use either porosity values from the

porosity test, porosity values or the DDE from the MIP test or capillarity coefficient values from the

capillarity test (table 8). The resulting PDEs are given in table 9.

5.9. Weathering treatment Depth logs calculated from the X-ray CT digital volumes of the samples from before and after the

weathering treatment are given in figures 34, 35, 38, 39, 42 and 43. Porosity, average grey value and

volume of the sample (slice volume) are calculated for every horizontal slice of 1 voxel in thick within a

VOI closely encompassing the sample and plotted in the depth logs. Absolute differences between the

pre and post weathering logs are a result of the weathering and nanolime treatment effects, but they

are also a consequence of operator dependent tresholding of the digital volumes and edge effects.

Edge effects become prominent at the top of the surface, where a smaller digital volume is considered

within the VOI, influencing the average grey values and porosity values. As a consequence it is not

possible to isolate which cause(s) are responsible for the absolute differences and only relative

differences should be taken into account when interpreting the depth logs.

Vertical cross sections of the digital volumes of the samples before and after weathering are given in

figures 36, 37, 40, 41, 44 and 45. Additional vertical cross sections perpendicular to those of figures

36, 37, 40, 41, 44 and 45 can be found in the appendix (figures L, M, N, O, P and Q respectively).

Table 8: Data from previous tests used to calculate the PDEs.

Table 9: PDE values calculated from the test results given in table 8.

30

Fig

ure

35:

Lede s

tone L

31r:

Poro

sity,

avera

ge g

rey v

olu

me a

nd s

lice v

olu

me d

epth

lo

g.

O

range:

pre

weath

erin

g t

reatm

ent. B

lack:

post w

eath

erin

g t

rea

tment.

Fig

ure

34:

Lede

sto

ne L

22s: P

oro

sity,

avera

ge

gre

y v

olu

me a

nd s

lice v

olu

me d

epth

lo

g.

Blu

e: pre

weath

erin

g t

reatm

ent. B

lack:

post w

eath

erin

g t

reatm

ent.

31

Fig

ure

36:

Lede s

tone L

31r:

C

ross s

ectio

n a

long t

he

z-a

xis

of th

e d

igital

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d b

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Left

: pre

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Rig

ht:

post w

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Fig

ure

37:

Lede s

tone L

22s:

Cro

ss s

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he

z-a

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of th

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onstr

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d b

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Left

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Rig

ht:

post w

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32

Fig

ure

39:

Lede s

tone L

27f: P

oro

sity,

avera

ge g

rey v

olu

me a

nd s

lice v

olu

me d

epth

lo

g.

Blu

e: pre

weath

erin

g t

reatm

ent. B

lack:

post w

eath

erin

g t

reatm

ent.

Fig

ure

38: T

abaire s

tone T

06f: P

oro

sity, avera

ge g

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nd s

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Blu

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weath

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lack:

post w

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ent.

33

Fig

ure

40:

Lede s

tone L

27f:

Cro

ss s

ectio

n a

long t

he

z-a

xis

of th

e d

igital

volu

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onstr

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d b

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Left

: pre

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Rig

ht:

post w

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Fig

ure

41:

Ta

baire s

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06f:

Cro

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Left

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Rig

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34

Fig

ure

42: T

abaire s

tone T

05s: P

oro

sity, avera

ge g

rey v

olu

me a

nd s

lice v

olu

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epth

lo

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Blu

e: pre

weath

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g t

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ent. B

lack:

post w

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g t

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Fig

ure

43: T

abaire s

tone T

24r:

Poro

sity, avera

ge g

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olu

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epth

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g.

Ora

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pre

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ent. B

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post w

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35

Fig

ure

45:

Ta

baire s

tone T

05s:

Cro

ss s

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of th

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d b

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Left

: pre

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Rig

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post w

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Fig

ure

44:

Ta

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24r:

C

ross s

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n a

long t

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z-a

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Left

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Rig

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post w

eath

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36

6. Discussion

6.1. Sample Size To keep the results of the different tests comparable a uniform samples size was needed. Several

tests however have conflicting size requirements and as such samples size of the stone samples was

chosen to strike a balance between the size requirements these tests. To be able to detect the

nanolime particles with X-ray CT the resolution of the scans needs to be high. For the X-ray CT setup

used (HECTOR), voxel size (and thus resolution) can go down to 4 µm and is dependent on the size

of the sample to be scanned (Cnudde and Boone, 2013; Masschaele et al., 2013). As the nanolime

particles are only 5-600 nm the smallest possible sample size is required. The pore measuring

capacity of the dilatometer used in MIP also imposes a size restriction of 2 cm³ and the Deben

compressive stage CT50000 limits sample size to a few centimetres. On the other hand the European

require sample sizes of minimum 7 cm diameter and 5 cm height for capillarity and a volume of 60 cm³

for porosity. As a further complication Tabaire stone samples delivered only allowed for a sample

diameter of 6 mm. As a result for cylindrical samples with a diameter of 8 mm for Lede stone and 6

mm for Tabaire stone were chosen.

6.2. Phenolphthalein test In the case of the Lede stone the phenolphthalein test shows little to no penetration of the nanolime

suspension into any of the stone samples. Along the sides some nanolime seems to have gotten

through but this as well remains superficial and it is thought that this is just a small amount of nanolime

suspension that managed to go between the sample and the parafilm enclosing the sample.

For the Tabaire stone nanolime penetration does take place but is limited to only one or two

centimetres. Two explanations for this are the nanolime particles not being transported as deep as the

wetting front of the alcohol and the limited amount of nanolime suspension applied per sample (0.4

ml). There is no clear difference in nanolime penetration and retention after drying between the fast

and slow dry method. The phenolphthalein test does indicate that nanolime remains present in depth

after drying for both methods. How much of the nanolime remains in depth cannot be known with this

test.

6.3. CT visualization Nanolime particles are at maximum 600 nm large (table 1), while the voxel size of the Lede stone

and Tabaire stone scan are 8.73 µm and 6.97 µm respectively. As features smaller than the voxel size

can’t be distinguished, individual nanolime particles cannot be distinguished in the reconstructed CT

dataset, but they do contribute to the reconstructed digital volume by means of the partial volume

effect (Cnudde and Boone, 2013). In this instance the partial volume effect can be ignored, as the

signal created by this effect is lost in the noise of the two scans being compared and the comparison

process itself. Nanolime particles however do have the tendency to accumulate in micrometre sized

clusters which can be distinguished in the CT digital datasets (Rodriguez-Navarro et al., 2013). The

signal of the difference image is thus that of locations where nanolime particles have clustered and

possible noise or artefacts from the two compared scans or difference process.

For the Lede stone the locations of the volumes of material addition are primarily at the surface of

the sample and show no significant difference between the two nanolime treatments. This indicates

that in both cases nanolime clusters are at the surface of the sample, while deeper into the sample

only small volumes of material addition are found, which are most likely due to noise or artefacts.

The Tabaire stone has a more pronounced difference between the nanolime treatments. For the

slow dried treatment numerous volumes of material addition are located at the drying surface while for

the fast dried treatment the amount and volume of nanolime clusters is much less. This could indicate

that the fast drying method has diminished accumulation and subsequent clustering of nanolime at the

drying surface and the nanolime particles are more spread through the sample up so that they are not

distinguished by the CT digital dataset.

37

6.4. SEM

SEM results indicate the same as the phenolphthalein test and CT visualisation: the nanolime did

not significantly penetrate into the Lede samples but did penetrate into the Tabaire sample. In both

Lede as well as well as Tabaire sample, indications of nanolime clusters were found at the top of the

samples but only in the case of the Tabaire sample indications of nanolime clusters were found more

in depth of the sample.

It must however be noted that for the SEM images this was the largest magnification possible

before the images became too distorted to be usable and even at this magnification focus of the image

was not optimal.

6.5. Porosity The small sample sizes have the same effect of exaggerating variability as with the capillarity test.

Variabilities, averages and distribution of Lede stone reference samples and nanolime treated

samples (both slow dried and fast dried) are not significantly different. Any differences that do exist

such as the slightly smaller variability and slightly higher average porosity of the nanolime treated

samples compared to the reference samples can be explained by the high variability of the samples in

combination with the relative small number of nanolime treated samples. These results are consistent

with the previous results indicating an ineffective nanolime treatment.

For the Tabaire stone variability of the porosity values of reference samples and nanolime treated

samples are similar. Fast dried nanolime treated samples do show a smaller variability (less than a

third) than the other samples. This could however be due to the small number of samples. Distribution

and averages of the nanolime treated samples show a small drop in porosity of nanolime treated

samples compared to reference samples (figure 29). Although the drop in average porosity is similar in

magnitude to that of the Lede stone samples, it is more significant for the Tabaire stone samples as

the natural variability of the samples is much smaller. The porosity values of the nanolime treated

samples do remain within the range of natural variability of the reference stone samples. This drop

could be the effect of nanolimes particles filling or blocking pores, but can also be due to the limited

number of nanolime treated samples enhancing the effect of natural vairability. Further studies would

be needed to clarify these results, preferentially using a larger number of samples, a sample size

conform with the European norm and a nanolime suspension of a higher concentration in order to get

a more distinct nanolime treatment signal versus natural variability of the stone samples.

6.6. Mercury intrusion porosimetry Something went wrong during the testing procedure of the nanolime treated Lede samples and a

reference sample (L14r) causing all data form these tests to be invalid and unusable. On a second

reference sample (L03r) the test was successful and chows a bimodal pore diameter distribution of the

pores between 50 nm and 100 µm with peaks around 280 nm and 1600 nm.

The Tabaire stone shows a unimodal pore diameter distribution between 50 nm and 100 µm

centred around 1200 nm. One major difference between reference samples and nanolime treated

samples is the minor peak at 1600 nm, possibly a result of the filling of these pores by clustered

nanolimes. Individual nanolime particles are about three times as small as the pore throat diameter of

1600 nm and can easily enter, but when clustering occurs a cluster of a mere dozen nanolime

particles is enough to block a pore of this size while back migration occurs during drying. Further

nanoparticles are then blocked and trapped inside these pores. The filling of these pores by

nanoparticles thus causes the loss of the peak in the pore diameter size distribution. If this is the

process for the disappearance of the peak at 1600 nm it would be expected that smaller pores would

be filled up by the same process. This is however not the case as the peak at 1100 nm is not

diminished in the nanolime treated samples compared to the reference sample. A possible explanation

for this is that due to this smaller pore size not enough nanolime particles pass through during drying,

preventing the formation of a cluster blocking the pore. As for why there is no effect on the distribution

of larger pores, two explanations are possible. A first possibility is that cluster formation cannot

physically reach the required size to block these pores. A second possibility is that at a nanolime

38

concentration of 5 g/L and a volume of 0.4 ml/sample, there was not enough nanolime present to form

clusters of the size needed to block larger pores. In order to test these hypotheses, further research is

needed. Between the two different nanolime treatments no significant difference can be noted.

The peculiar drop in the pore diameter size distribution at 22 µm of the reference samples and

subsequent similar trajectories of all pore diameter size distributions cannot be explained by possible

effects of the nanolime treatment and could possibly be invalid data. No problems could however be

detected during the testing procedures of these datasets so this invalid data cannot be confirmed or

excluded as a cause for this pattern. In any case this opens a discussion towards the validity of the

data acquired for which more testing is needed to answer conclusively.

Porosity measurements based on the MIP data are an underestimation of the actual porosities of

the samples. MIP total porosity measurements are known to underestimate porosities where pores are

too small or too isolated to be intruded by mercury (Cook and Hover, 1999). The difference between

MIP porosity measurements and EN porosity measurements is too high (± 10% and ± 20% porosity

difference for Lede and Tabaire respectively) to be only explained by this effect. Some other unknown

reason must be at play. This may be an indication that there is an unknown error in the MIP

measurement data.

6.7. Capillarity Due to the use of the stone holders water was pulled up ± 1 mm between them and the sample as

a consequence of capillarity forces. This larger area of contact between water and sample causes an

initial faster uptake of water until the capillary front within the sample reaches that height. As this

happens well within one minute (the first measuring point) this does not affect the capillarity coefficient

values, but does cause a higher initial mass change than without the sample holders. The small

sample sizes do have the effect of exaggerating variability. Estimated values of capillarity coefficient

are generally an underestimation as these are based on weight difference measurements when the

sample reaches full saturation and thus water uptake slows down.

For the Lede stone capillarity coefficient values of fast dried and slow dried nanolime treated

samples show similar variability to each other and the reference samples. Capillarity coefficient values

of the nanolime treated samples show an increase compared to the reference samples as shown by

their averages and in figure 26. The capillarity coefficient values of the nanolime treated samples do

remain within the range of variability of the reference samples, especially when keeping in mind that

four of the reference samples capillarity coefficient values are underestimations. If these results of the

capillarity test are solely due to natural variability. Then they point to an ineffective nanolime treatment

for the Lede stone samples, which is consistent with the nanolime remaining on the surface of the

sample and not penetrating. It is also possible that the nanolime treatment had an adverse effect on

the capillary water uptake of the weathered Lede stone samples by (partially) reconnecting the pore

structure that was broken up during weathering enhancing capillary flow (Franzen and Mirwald, 2004;

Washburn, 1921). Intruded nanolime particles however must have been with so few to not trigger a pH

change in the phenolphthalein test, or be registered by CT or SEM that this effect would be minimal to

insignificant. Further tests would be needed with weathered limestones in which nanolimes do clearly

penetrate to test this. To this goal a higher concentration could be used or another suspension agent

than isopropanol with a lower boiling point making the nanolime suspension more kinetically stable

(Borsoi et al., 2016a).

For the Tabaire stone capillarity coefficient values of nanolime treated samples are significantly

lower than those of the reference samples, but most do stay in range of the natural variability of the

reference samples. It should also be noted that the four reference capillary values are

underestimations and that eight of the reference samples reach saturation so fast that a capillarity

coefficient calculation was impossible. These eight samples have an even higher capillarity coefficient

value than the four who could be estimated. The nanolime thus has a significant effect in lowering the

capillarity coefficient of the Tabaire stone samples. On average the nanolime treatment reduces

capillary sorption by a third. This change in capillarity coefficient is due to the superficial deposition of

the nanolime particles which can be seen in the CT visualisation (figures 21 and 22), (partially) closing

of entryways into the pore system of the stone sample and thus impeding the capillary imbibition of

39

water into the stones. Between the fast dried and slow dried nanolime treated samples there is no

significant difference in capillarity coefficient (figure 27). Differences in averages and variability are due

to a limited number of samples and the small sample size, which enhance the effect of natural

variability in these statistics.

6.8. Brazilian test Brazilian tests on the Lede stone samples show no improvement in tensile strength in fact they

seem to show a lower strength after treatment. No process involving the nanolime treatment and

weathered limestone could be found explaining the deterioration in tensile strength. Considering other

results such as Brazilian test on the Tabaire stone, porosity and capillarity indicating no significant

effects of the nanolime on the Lede stone samples. Deterioration of the samples due to nanolime

treatment is implausible. The drop could be explained by the natural variability of the Lede stone (De

Kock et al., 2015) and differences in weathering between the different samples. In any case, more

data is needed to draw a definitive conclusion. The failure behaviour of the Lede stone samples can

be seen in the force, compression graph of the samples. The small drops in force before the maximum

force breaking the sample is reached indicate small cracking before complete failure of the sample

structure.

Tabaire stone samples show a slight increase in tensile strength after nanolime treatment, but

there is no significant difference between the two nanolime treatments. Nanolime treatment might

have had a slight effect on sample tensile strength, but the difference could also be due to natural

variations between the samples. Brazilian tests on more samples are needed to know definitive

results. Failure behaviour of the Tabaire stone is more sudden and no signs of cracking before

structure failure takes place can be found in the force, compression graph.

6.9. Weathering treatment As previously stated Absolute values are not used for interpretation, only relative differences

between the depth logs are used.

For all samples except sample T06 there is a clear increase in volume at the top, coinciding with a

lower average grey value of the post weathering log compared to the pre weathering log. These

changes are explained by the formation of a gypsum crust, as it is formed by the reaction of the

sulphurous acid with calcite, has a lower grey value than calcite in X-ray CT (which can be seen in

figures 36, 37, 40, 41, 44 and 45 as a darker layer at the top surface) and accumulates near the

surface of the stone (De Kock et al., 2017). The porosity data are less easily interpretable at the

surface as they are more affected by edge effects. Except for sample L31r, all samples show a drop in

porosity values near the surface post weathering due to the formation of a gypsum crust.

From the data and images of the samples no clear difference can be discerned between the two

nanolime treatments and between the nanolime treated samples and the reference samples. The

treatments were not effective enough to be distinguishable from the effects of natural variability within

the stones.

6.10. Petrophysical durability estimator A high PDE predicts the sample to weather more easily. Comparing PDE predictions of the

samples versus their weathering behaviour cannot be done as the weathering treatment is

inconclusive. As the original PDE based on the DDE is focussed on frost weathering, while the

weathering process under investigation is atmospheric acid weathering, the PDEDDE is not optimal for

assessing susceptibility urban environment weathering. A more appropriate PDE is the one based on

capillary absorption, as acid weathering if porous stone is mainly governed by their interaction with

fluids for which capillarity is the main rock characteristic (Franzen and Mirwald, 2004). A PDE based

on porosity will be less accurate as the influence of porosity is already partially accounted for in the

tensile strength. For example: for stones with the same skeletal material, a high porosity stone will

have a lower tensile strength and vice versa. The most pronounced effects expected from the

nanolimes are on the pore structure characteristics and as discussed in previous paragraphs these

effects are minimal. The effects of nanolimes on tensile strength on the stone depend on the

40

deposition of the nanolime particles. The current superficial deposition has a limited effect as only e

few millimetres of the stone are consolidated.

For the Lede stone the higher PDE values of the PDEpo and PDEC2 for the nanolime treated

samples in comparison to the reference samples are mainly owed to the lower tensile strength of the

nanolime treated samples as both capillarity tests and porosity test showed no significant difference,

so the same discussion points can be made as in the Brazilian test paragraph.

For the Tabaire samples all PDEs indicate a lower susceptibility to weathering for the nanolime

treated in comparison to samples the reference samples. Between the two nanolime treatments there

is no significant difference in PDE values.

6.11. Nanolime deposition All tests indicate that few to no nanoparticles intruded into the weathered Lede stone sample, while

they were able to intrude into the les porous Tabaire stone. This is contradictory to what would be

expected from a nanolime treatment on a porous limestone such as the Lede stone: nanolime should

be able to easily penetrate into the Lede stone (Borsoi et al., 2016a). The Lede stone samples used in

this research were exposed to an urban climate and consequent weathering does alter the stones

properties. One of these alterations is common for the Lede stone is the formation of a gypsum crust,

which leads to both a denser layer near the surface and a more porous layer behind it (Hendrickx,

2012; Maravelaki-Kalaitzaki and Biscontin, 1999). Evidence for a gypsum crust in the used samples

can be found as black superficial crusts created by the incorporation of airborne dust and particulate

matter in the gypsum (De Kock et al., 2017). CT cross sections of the Lede stone before the

weathering test also show a less porous zone at the top of the samples indicating the presence of a

gypsum crust (figures 36, 37 and 40). This denser, less porous gypsum crust layer could have acted

as a fine-porous barrier impeding the intrusion of the nanolime particles, as it is known that nanolimes

have a very limited penetration in fine-porous materials (Borsoi et al., 2016a). The equivalent pore

diameter size distribution from MIP shows a higher fraction of smaller pores for the Lede stone

compared to the Tabaire stone samples, which could have further impeded nanolime intrusion. In

future research weathering effects such as gypsum crusts should be taken into account. A sample

treatment removing these weathering crusts, altering the nanolime treatment method and/or altering

the nanolime suspension (by using a higher concentration or another suspension agent) could provide

a solution in further research.

As the nanolime did not penetrate the Lede stone samples, only the test results of the Tabaire

stone samples can be used to assess the nanolime treatments effectivity. CT visualization shows a

superficial deposition of the nanolime particles due to back migration as the phenolphthalein test

proves the penetration of the nanolime into the samples. The effect of the nanolime treatment was

only minimal on the Tabaire samples: there was no effect on porosity and weathering and little to no

effect on capillarity, tensile strength and MIP results. This low effectivity is partially due to the

superficial deposition of the nanolime, which explains the tensile strength, MIP, porosity and

consequent PDE results. Capillarity results and the weathering behaviour should still be affected by a

superficial nanolime consolidation, yet the weathering treatment shows no difference between

reference and nanolime treated samples and while the capillarity results show the largest difference it

remains minimal. A second cause for the lack of effect of the nanolime treatment is the amount of

nanolime used: the concentration of the suspension is only 5 g/L and the amount of nanolime

suspension used for each sample is 0.4 ml (sample surface: ± 0.3 cm², sample volume : ± 0.5 cm³). In

future research a higher concentration and larger volumes of nanolime should be used in order to get

a more pronounced difference between nanolime treated samples and untreated samples.

For comparing the fast dried with the slow dried nanolime treated, again only the results of test on

the Tabaire stone samples can be used. The CT visualization indicates that compared to the slow

dried nanolime treatment, the fast dried treatment caused less clustering and favoured a more spread

out deposition of the nanolime near the surface. All subsequent test however do not show any

difference in effects between the two nanolime treatments. Capillarity coefficients, porosity values,

equivalent pore diameter distributions from MIP, tensile strength and failure behaviour of the Brazilian

test, PDE values and weathering behaviour are all equivalent for the two nanolime treatments. This

41

indicates that the fast drying of the nanolime had no significant effect in distributing the nanolime

deposition. As with the difference between reference and nanolime treated samples, the cause for the

lack of difference between the two nanolime treatments could also be attributed to the low amount of

nanolime suspension applied on the samples. To test which is the cause for the lack of difference a

higher nanolime concentration should be used in future research.

7. Conclusions

7.1. Tabaire stone characteristics The physical characteristics that were determined are given in table 10. The Capillary absorption

coefficient (C1) is an average of four estimations and the actual value will be higher, porosity (p0) is an

average of 27 measurements and tensile strength (σ) is based on one measurement.

Table 10: physical characteristics of the Tabaire stone acquired from the tests as averages. C1: the capillary absorption coefficient parallel to the planes of anisotropy, p0: open porosity, σ: tensile strength.

Physical characteristics Data

C1 [g⁄(m²√s)] 141.1

p0 [%] 30.0

σ [Mpa] 0.99920

MIP data also shows a unimodal pore size distribution for the pores between 50 nm and 100 µm

centred around 1200 nm. Failure behaviour of the Tabaire stone is sudden without signs of micro

cracking.

7.2. Nanolime deposition Deposition of the nanolime in both the Tabaire stone and The weathered Lede stone remains for

both the fast dried and slow dried nanolime treatment superficial. In the case of the Lede stone this

was due to an inability of the nanolime particles to penetrate through the gypsum crust present at the

surface of the weathered samples. For the Tabaire stone it was a consequence of back migration

towards the drying surface, leaving only minimal amounts of nanolime in-depth in the sample. For the

Tabaire stone the fast dried nanolime treatment had the effect of diminishing cluster formation in

comparison to the slow dried nanolime treatment but the majority of the nanolime remained at or near

the surface.

7.3. Effectivity of the nanolime treatments For the Lede stone no significant difference could be discerned in any test (porosity, capillarity,

MIP, Brazilian, PDE and weathering behaviour) between nanolime treated samples and reference

samples. Consequently there was no difference between the two nanolime treatment applied on the

Lede stone samples.

While the weathering treatment showed no difference between treated and untreated Tabaire stone

samples other test did show differences. There is a slight drop in porosity and a slight increase in

tensile strength after nanolime treatment and MIP shows a decrease in pore volume of pores with an

equivalent pore diameter of around 1200 nm. While the CT visualisation shows a more dispersed

distribution of the nanolime particles for the fast dried nanolime treatment, no difference is found in any

other test between the two nanolime treatments. The fast dried nanolime treatment was thus

ineffective in improving in-depth consolidation of the Tabaire stone.

42

7.4. Future research

This research is only a first step into using a combination of CT and other tests to investigate the

possibilities of nanolimes for in-depth consolidation and as such there is still a lot that can be

investigated in future research.

First a lot of variables in the nanolime treatment method can be changes in order to acquire desired

effects such as in-depth deposition. As the effects of nanolimes were only minimal in this research, a

higher nanolime concentration should be used. Different suspension agents can change the stability of

the suspension allowing for better penetration, changing the application method to e.g. capillary

uptake can allow for more nanolime to be applied in a consistent manner. Drying and carbonatation

mechanics can also be fine-tuned by changing environmental factors such as temperature and relative

humidity or using a second application of nanolime suspension or applying water as was done in

Niedoba et al., (2017).

To acquire more precise data non-destructive test such as porosity and capillarity should be done

on the same samples before and after nanolime treatment as was done here with CT. More samples

should also be used in each test to account for natural variability. Sample sizes used should also be

conform with the European Norm for the test they are to be used, while for CT a smaller sample size

can improve resolution in order to be able to better visualize the small nanolime particles. Results

between CT and other tests requiring larger samples would then be harder to compare. However, if

CT images are able to visualise depositional behaviour of the nanolimes this can in turn be used to

better explain the results gotten from tests such as porosity an Capillarity. Alternatively, as nanolime

deposition is mainly variable along the axis of application and drying, the CT images can be summed

along a perpendicular axis allowing to be able to assess how deep nanolimes deposit without the need

for a high enough resolution to distinguish individual nanoparticles. This has been done by Niedoba et

al., (2017).

For the PDE further research should be done to fine-tune for a certain weathering environment.

Here the PDEDDE pertains mainly to frost weathering while the PDEC would be more appropriate for

acid weathering in an urban environment.

43

8. Reference list

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46

9. Appendix

Fig

ure

A: S

cre

enshot

of th

e p

rogra

m X

RE

durin

g t

he r

econstr

uctio

n o

f a T

abaire s

tone s

am

ple

47

Fig

ure

B:

Scre

enshot

of

the p

rogra

m D

ata

vie

wer

durin

g g

eom

etr

ic p

ositio

nin

g o

f a L

ede s

tone s

am

ple

. T

hre

e p

erp

endic

ula

r cro

ss-s

ections

of

the d

iffe

rence

im

age c

an b

e s

een.

48

Fig

ure

C: S

cre

enshot

of th

e p

rogra

m O

cto

pus A

naly

sis

durin

g T

hre

shold

ing o

f th

e p

ore

volu

mes o

f a L

ede s

tone s

am

ple

49

1000

1200

1400

1600

1800

2000

2200

2400

2600

2800

5 15 25 35 45

Δm

/A [

g/m

²]

√t [s]

Capillarity curves: Lede stone

L03r

L05r

L14r

L16r

L42r

L47r

L49r

Figure D: Capillarity curves of the Lede stone reference samples part 1. Mass of water absorbed divided by the area of the immersed base as a function of the square root of time.

1000

1200

1400

1600

1800

2000

2200

2400

2600

2800

5 15 25 35 45

Δm

/A [

g/m

²]

√t [s]

Capillarity curves: Lede stone

L15r

L24r

L26r

L31r

L33r

L35r

L37r

Figure E: Capillarity curves of the Lede stone reference samples part 2. Mass of water absorbed divided by the area of the immersed base as a function of the square root of time.

2000

2500

3000

3500

4000

4500

5 10 15 20 25 30 35

Δm

/A [

g/m

²]

√t [s]

Capillarity curves: Tabaire stone

T18r

T21r

T22r

T38r

T40r

T41r

Figure F: Capillarity curves of the Tabaire stone reference samples part 1. Mass of water absorbed divided by the area of the immersed base as a function of the square root of time.

50

Figure G: Capillarity curves of the Tabaire stone reference samples part 2. Mass of water absorbed divided by the area of the immersed base as a function of the square root of time.

2000

2500

3000

3500

4000

4500

5 10 15 20 25 30 35

Δm

/A [

g/m

²]

√t [s]

Capillarity curves: Tabaire stone

T23r

T24r

T25r

T39r

T42r

T43r

Figure H: Capillarity curves of the Lede stone slow dried nanolime treated samples. Mass of water absorbed divided by the area of the immersed base as a function of the square root of time.

0

500

1000

1500

2000

2500

3000

5 10 15 20 25 30 35

Δm

/A [

g/m

²]

√t [s]

Capillarity curves: Lede stone

L01s

L07s

L13s

L39s

0

500

1000

1500

2000

2500

3000

5 10 15 20 25 30 35

Δm

/A [

g/m

²]

√t [s]

Capillarity curves: Lede stone

L04f

L08f

L32f

L41f

Figure I: Capillarity curves of the Lede stone fast dried nanolime treated samples. Mass of water absorbed divided by the area of the immersed base as a function of the square root of time.

51

0

500

1000

1500

2000

2500

3000

3500

4000

5 10 15 20 25 30 35

Δm

/A [

g/m

²]

√t [s]

Capillarity curves: Tabaire stone

T10s

T12s

T14s

T16s

Figure J: Capillarity curves of the Tabaire stone slow dried nanolime treated samples. Mass of water absorbed divided by the area of the immersed base as a function of the square root of time.

1000

1500

2000

2500

3000

3500

4000

4500

5000

5 10 15 20 25 30 35

Δm

/A [

g/m

²]

√t [s]

Capillarity curves: Tabaire stone

T11f

T13f

T15f

T17f

Figure K: Capillarity curves of the Tabaire stone fast dried nanolime treated samples. Mass of water absorbed divided by the area of the immersed base as a function of the square root of time.

52

Fig

ure

L:

Lede s

tone L

31r:

C

ross s

ectio

n a

long t

he z

-axis

of

the d

igital volu

me

constr

ucte

d b

y X

-ray C

T.

Perp

endic

ula

r on the c

ross

sectio

n o

f fig

ure

36.

Left

: pre

weath

erin

g.

Rig

ht:

post

weath

erin

g.

Fig

ure

M:

Lede s

tone L

22s:

Cro

ss s

ectio

n a

long t

he

z-a

xis

of th

e d

igital

volu

me c

onstr

ucte

d b

y

X-r

ay C

T.

Perp

endic

ula

r on t

he c

ross s

ectio

n o

f fig

ure

37.

Left

: pre

weath

erin

g.

Rig

ht:

post w

eath

erin

g.

53

Fig

ure

N:

Lede s

tone L

27f:

Cro

ss s

ectio

n a

long t

he z

-axis

of

the d

igital volu

me

constr

ucte

d b

y X

-ray C

T.

Perp

endic

ula

r on the c

ross

sectio

n o

f fig

ure

40.

Left

: pre

weath

erin

g.

Rig

ht:

post

weath

erin

g.

Fig

ure

O:

Ta

baire s

tone T

06f:

Cro

ss s

ection a

long t

he z

-axis

of

the d

igital volu

me

constr

ucte

d b

y X

-ray C

T.

Perp

endic

ula

r on the c

ross

sectio

n o

f fig

ure

41.

Left

: pre

weath

erin

g.

Rig

ht:

post

weath

erin

g.

54

Fig

ure

P:

Ta

baire s

tone

T2

4r:

C

ross s

ection a

long t

he z

-axis

of

the d

igital volu

me

constr

ucte

d b

y X

-ray C

T.

Perp

endic

ula

r on the

cro

ss s

ectio

n o

f fig

ure

44.

Left

: pre

weath

erin

g.

Rig

ht:

post w

eath

erin

g.

Fig

ure

Q:

Ta

baire s

tone T

05s:

Cro

ss s

ectio

n a

long t

he z

-axis

of

the d

igital volu

me

constr

ucte

d b

y X

-ray C

T.

Perp

endic

ula

r on the

cro

ss s

ectio

n o

f fig

ure

45.

Left

: pre

weath

erin

g.

Rig

ht:

post w

eath

erin

g.