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Journal of Cleaner Production 185 (2018) 168e175

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Journal of Cleaner Production

journal homepage: www.elsevier .com/locate/ jc lepro

One-part geopolymer cement from slag and pretreated paper sludge

Elijah Adesanya a, Katja Ohenoja a, Tero Luukkonen a, Paivo Kinnunen a, b,Mirja Illikainen a, *

a Faculty of Technology, Fiber and Particle Engineering Unit, PO Box 4300, 90014, University of Oulu, Finlandb Department of Civil and Environmental Engineering, Imperial College London, London, SW7 2BU, United Kingdom

a r t i c l e i n f o

Article history:Available online 3 March 2018

* Corresponding author.E-mail address: Mirja.illikainen@oulu.fi (M. Illikain

https://doi.org/10.1016/j.jclepro.2018.03.0070959-6526/© 2018 Published by Elsevier Ltd.

a b s t r a c t

The aim of this study was the valorization of paper sludge waste from paper industry in designing one-part geopolymer cement using ground granulated blast furnace slag as main precursor. The effects ofincreasing the amount of paper sludge pretreated with a constant amount of sodium hydroxide (2% ofslag) on mechanical strength, heat evolution, setting time and durability were analyzed. The reactionproducts were characterized using X-ray diffraction and thermogravimetric analysis. The findingsshowed that paper sludge can successfully be used as a secondary source of calcium carbonate in theone-part (“just add water”) geopolymer. The compressive strength of the cured geopolymer increasedwith increasing the amount of paper sludge, and the non-reacted sludge acted as a filler. The maximumstrength (42MPa, 28 d) was reached at 18wt-% paper sludge addition. The main reaction products werecalcium aluminate silicate hydrate and hydrotalcite-like cementitious gels. From the perspective of itsblast-furnace slag content, strength properties, water absorption and setting time, the one-part geo-polymer cement could be classified as a CEM IIIC and Type 32.5N class cement under the Europeanstandard EN-197.

© 2018 Published by Elsevier Ltd.

1. Introduction

The increasing demand for environmentally friendly and sus-tainable construction materials has necessitated finding alterna-tives or substitutes for ordinary Portland cement (OPC).Environmental impacts and emissions of greenhouse gases fromOPC production has been widely documented, and with theincreasing infrastructure development worldwide, the demand andproduction of OPC cement as binder in concretes structures willcontinue to increase (Cement Industry Energy an, 2017; Ali et al.,2011; Schneider et al., 2011; Damtoft et al., 2008; Flatt et al.,2012). Different alternatives for OPC in construction materials,measures to reduce the reliance on its use and curbing the detri-mental environmental impacts have been suggested by severalauthors. Partial substitution of OPC with different supplementarycementitious materials has been suggested and is already the normin many countries (Islam et al.,; Alex et al., 2016; Rissanen et al.,2017; Vegas et al., 2009; Evaluation of paper industry,). In addi-tion to this, the design and development of alternative binders has

en).

been active: such as alkali-activated cements based on alumino-silicate materials (including industrial by-products or wastes, likefly ash, slags and tailings (Adesanya et al., 2016; Kiventer€a et al.,2016; Yliniemi et al., 2015)) activated with alkaline solution (typi-cally sodium silicate). These materials are called (two-part) geo-polymers or alkali-activated materials, and the background,chemical and elemental principles, reaction behavior and engi-neering properties of them have been reviewed at length (Proviset al., 2015; Garcia-Lodeiro et al., 2014; Puertas et al., 2011; CaijunShi, 2033; Wang et al., 1995; Davidovits, 2011; Duxson et al., 2007).Geopolymers designed from two-part mix are comprised of alka-line solutions and solid powdered precursors. However, the large-scale production of geopolymers using this method makes itscalability difficult due to handling issues and viscosity of thealkaline solutions and are presently restricted to relatively small-scale applications (Nematollahi et al., 2015).

One-part geopolymers are suggested to solve some of the short-comings of conventional two-part geopolymer binder designs. One-part geopolymers are made with blends of solid alkaline activatorsand solid aluminosilicates precursors. Different synthesis routes indesigning one-part geopolymer cements have been proposed orreviewed (Duxson and Provis, 2008; Abdollahnejad et al., 2015; Kimet al., 2013; Luukkonen et al., 2017). In order to enable large-scale

E. Adesanya et al. / Journal of Cleaner Production 185 (2018) 168e175 169

application of geopolymer products and also to reduce the economiccost in handling aqueous alkaline solution, developing one-partgeopolymer mix “just-add water” similar to OPC is necessary. Thatbeing said, two-part geopolymers might be more useful in precastwork whereas one-part geopolymers could find applications espe-cially in in situ casting (Provis, 1016). Since the introduction of one-part geopolymers, several efforts in designs have been reported bydifferent authors. Ke et al. (2015) designed one-part geopolymersblended with thermally treated red mud and sodium hydroxide. Themaximum compressive strength was 9.8MPa after 7 days.Nematollahi et al. (2015) made one-part geopolymer cement usingclass F fly ash, slag and hydrated lime as aluminosilicates sourceblendedwith anhydrous sodium silicate and sodiumhydroxide. Theyreported highest strength of 37MPa for the hydrated one-part geo-polymer cement, which was 5MPa weaker than two-part geo-polymermixwhen compared. Peng et al. (2014) calcined low-qualitykaolin at temperatures between 650 and 1050 �C with sodium hy-droxide or sodium carbonate to make one-part geopolymer cement.In comparison to two-part geopolymer mix with a compressivestrength of 16MPa, they reported compressive strength of 63 and48MPa when the one-part mix was made with NaOH and Na2CO3

respectively. The energyconsumptionof someof themethodsused inmaking one-part geopolymer may limit its commercial viability andalso increase impact of production on environment. Also, alkali sili-cate usage could be expensive and also has impacts on environmentfrom its production (Torres-Carrasco et al., 2015): sodium silicate isestimated to account for about 75% of the carbon footprint of geo-polymer concrete (Mellado et al., 2014). Therefore, there is a need forsodium silicate-free one-part geopolymer, which leads to lowerenvironmental impacts and lowers the cost of raw materials. To thisend in some one-part geopolymer studies, synthetic sodium silicatehas been replaced with rice husk ash, maize cob ash, silica residuefrom chlorosilane production, or geothermal silica (Hajimohammadiet al., 2008; Peys et al., 2017; Sturm et al., 2015, 2016; Gluth et al.,2013).

Meanwhile, in the pulp and paper industry a number of differenttypes of solid wastes and sludge are produced from pulping,deinking processes and wastewater treatment at paper mills. In2005, about 11 million tons of solid wastes were generated inEurope alone (Mill SludgeManagement,). According to the statisticsfrom CEPI (CEPI), a yearly production of paper mill sludge is morethan 4.7 million tons in Europe, and a global production has beenpredicted to rise in the future (Mabee and Roy, 2003). Additionally,

Table 1Chemical composition of paper sludge and GGBFS (weight-%).

CaO Al2O3 SiO2 MgO

GGBFS 38.5 9.58 32.33 10.24Paper sludge 95.1 1.0 0.7 0.8

Fig. 1. Starting materials. a) As-received image of non-treated paper slud

about 300 kg of sludge is generated per 1 ton of recycled paper(Utilization ofWaste Pape, 2665). Sludgewastes have a high varietyof properties depending on the factory and process parameters, buttypically they have a chemical composition suggesting that theycould be utilized in concrete materials (Lightweight mortars,): theycontain different ratios of calcium, aluminum, sodium and silicon.Paper sludge with and without calcination has been previouslyused in concrete as a cement replacement material (Lightweightmortars, ; Martínez-Lage et al., 2016; Valorization of paper mil,2868), and it has been found that it possible to replace 20% ofOPC without significant loss in durability.

In earlier studies, chemical grade CaCO3 has been used in orderto achieve one and two-part geopolymer cements (Kim et al., 2013;Abdel-GawwadA, 1016) (Mathur and Sharma, 2008). Here, we uti-lized paper sludge as the CaCO3 source (i.e., as an activator) incombinationwith a small amount of NaOH in the mix design of theone-part geopolymermortars. This paper sludge contains 58.4% drymatter and 41.6% of water. We measured strength development,durability, drying shrinkage and water absorption of the mortarsamples. Isothermal conduction calorimetry was used to analyzethe heat of hydration of the fresh-state samples, and X-raydiffraction used in analyzing the mineralogy of starting materialsand reaction products.

2. Experimental program

2.1. Materials

Paper sludge obtained from Stora Enso Paper mill (Oulu,Finland) was used in this study as a CaCO3 source. The dried papersludge contains 87.8% of CaCO3 and 12.2% of cellulose fibers and it isa primary sludge originating from primary clarification at waste-water treatment plant. Ground granulated blast furnace slag(GGBFS) was supplied by Finnsementti (Finland). The median par-ticle size (d50) of the slag was 10.21 mm as analyzed using a Beck-mann Coulter LS 13320 laser diffraction particle size analyzer.Oxide compositions of the materials as determined by X-ray fluo-rescence (PANalytical Omnian Axiosmax) together with loss onignition (LOI, determined with PrepAsh, Precisa) results are shownin Table 1. For the paper sludge, LOI at 525 �C is due to fibersdegradation and at 950 �C due to the decarbonation of calcite[CaCO3 (s) / CaO (s) þ CO2 (g)]. X-ray diffractograms of the papersludge and GGBFS are shown in Fig. 1. In the XRD pattern of the raw

Fe2O3 Others LOI (525 �C) LOI (950 �C)

1.23 8.11 N/A �1.30.1 2.3 12.2 50

ge. b) X-ray diffractograms of paper sludge and GGBFS (C e CaCO3).

Table 3Mix composition of mortar samples.

Dry activator (g) GGBFS (g) w/b s/b

GP1a 29 421 0.31 0.94GP2 29 421 0.31 0.94GP3 56 393 0.31 0.94GP4 85 363 0.31 0.94GP5 113 338 0.31 0.94

a Contains only paper sludge.

Fig. 3. SEM images of (a) non-treated milled paper sludge (GP1) and (b) pretreatedmilled paper sludge (GP5).

E. Adesanya et al. / Journal of Cleaner Production 185 (2018) 168e175170

materials, the raw paper sludge shows peaks which consists ofmainly calcite. The broad peak at 25e35 �2q reflects the amorphousstructure of the slag. Alkaline solution was prepared by dissolvingsodium hydroxide (NaOH) pellets with 99% purity (Merck KGaA) indistilled water. Standard sand in accordance with EN 196-1 (Nor-mensand GmbH) was used as aggregates in the mortar sampleswith a sand-to-binder weight ratio of 0.94.

2.2. Paper sludge pretreatment

The paper sludge-based dry activators were prepared by mixing6.3M NaOH with increasing amount of paper sludge (see Table 2)using a dispersing instrument (IKA T25 Ultra Turrax) for approxi-mately 4min. The mixed sludge were then dried in the oven at95 �C for two days. The resultant flakes as seen in Fig. 2, were thenmilled using a tumbling ball mill (Germatec, Germany) to achievean average d50 of 15 mm.

The mineralogy of the samples after treatment with NaOH inFig. 2 shows that Calcite (CaCO3) and Pirssonite (Na2Ca(CO3)2.2H2O)are the major phases and small quantities of Portlandite Ca(OH)2and Natrite (Na2CO3) were also identifiable. The postulated chem-ical reactions during the pretreatment are described below inequations (1) and (2). The use of paper sludge as a source of calciumcarbonate serves a dual purpose: first reacting with NaOH to formsodium carbonate and pirssonite and also as fillers.

2NaOH þ CaCO3 / Na2CO3 þ Ca(OH)2 (1)

Na2CO3 þ CaCO3 þ 2H2O / Na2Ca(CO3)2.2H2O (2)

The microstructures of paper sludge show cellulosic fibers(Fig. 3) in both non-treated and pretreated paper sludge. Theobserved fibers (arrowed) seem shorter after activating with NaOHand milling.

2.3. Sludge-cement and mortar preparation

In order to prepare one-part geopolymer cement, each dryactivator was then blended with GGBFS according to Table 3. The

Table 2Mix composition of the dry activator preparation.

NaOH (6.3M) Paper sludge (g) % content

GP1a 0 246 0:100GP2 90 246 27:73GP3 90 486 16:84GP4 90 726 11:89GP5 90 960 9:91

a Contains only paper sludge.

Fig. 2. (L-to-R) Pretreated paper sludge after drying at 95 �C, and XRD diffractograms oN¼Natrite).

powder mix was blended using a mechanical mixer for 3min toobtain a homogenized dry powder. Five mortar mixes (GP1eGP5)were then fabricated using the blended geopolymer cement andmixed according to EN 196-1 (SFS-EN 196-1, 2011). The differentmix compositions have the same water-to-binder mass wight ratio(w/b) and sand-to-binder mass weight ratio (s/b).

After mixing, the freshmortar samples for strength test were castinto rectangular molds with dimension of 20� 20� 80mm, whilethe drying shrinkage samples were cast into 40� 40� 160mmmolds. All samples were cured at 35 ± 3 �C and demolded after 24 h.Sample for shrinkage determination were placed in constant hu-midity chamber (22 �C and 50%± 5 relative humidity), while the restof the samples were cured at 22 �C and at 75% relative humidity untiltesting time.

2.4. Sample analysis

The heat evolution during activation was observed withisothermal conduction calorimetry (TAM Air Instruments) at 20 �C.The blended cement wasmixed ex-situ before the heat of hydrationwas recorded. Setting time was determined using an automatedVicat apparatus (Matest E044N, Italy) at room temperature ac-cording to EN196-3 standard for paste sample. The unconfinedcompressive strength (UCS) and flexural strength was performedon four replicate samples at 7, 28 and 50 days using a Zwick testingmachine with a maximum load of 100 kN and a loading force of2.4 kN/s for UCS and 0.05 kN/s for flexural strength. The average

f the pretreated paper sludge after drying (P¼Pirssonite, C¼Calcite, D¼ Portlandite,

Table 4Setting time of blended cement pastes at room temperature.

Initial set (Ti) Final set (Tf)

GP2 2 h 20min 3 h 25minGP3 10 h 50min 12 h 30minGP4 11 h 10min 12 hGP5 12 h 14 h 10min

E. Adesanya et al. / Journal of Cleaner Production 185 (2018) 168e175 171

value was then reported as the UCS and flexural strength. Waterabsorption of mortars was determined according to EN 5513 (SFS,2009) after 28 days of curing.

Drying shrinkage measurements were carried out on duplicateprism samples with dimensions of 40� 40� 160mm, adapted ac-cording to ASTM C596 (ASTM-C596,). The lengthmeasurement wascarried out at the age of 1, 3, 7, 14, 28 and 90 days. The lengthchange (LC) over time was then calculated using equation (3).

LCð%Þ ¼ Li � LxG

� 100; (3)

where Li is the difference between the comparator reading and thereference bar at 3 days, Lx is the length at each curing age of thespecimen and G is the nominal effective length.

X-ray diffraction (Rigaku SmartLab 9 kW) fitted with Co anodewas used to analyze the mineral phase of the paste samples using ascanning rate of 0.02�2q/step from 10 to 60�2q, the mineral phaseidentifications were then performed using PDXL2 software suitewith integrated access to the PDF-4 2015 database. Scanning elec-tron microscope (SEM) analysis was conducted using a Zeiss UltraPlus microscope to analyze the surface morphology of the pre-treated paper sludge. For thermogravimetric analysis, crushedsamples of similar mass were transferred to crucibles and heated40e1000 �C at 10 �C/min in a nitrogen atmosphere.

3. Results and discussion

3.1. Heat release and setting time

Heat evolution curves in Fig. 4 from the conduction calorimetersshows the effect of increasing paper sludge on the induction andacceleration stages of the blended cements until 95 h of reaction.The calorimetric curves for all samples except GP1 are similar to theOPC heat of hydration stages (Jansen et al., 2012). Curves are sub-divided into four stages including initial dissolution period, theinduction period, acceleration/deceleration period and the retar-dation period (Shi and Day, 1995). For GP1, which was used as areference sample, alkali activation produced only an initial peakdue to wetting and dissolution of the blended cement. No otherpeaks were logged until the end of the test at 96 h. This is com-parable to previous study on GGBFS hydration (Shi and Day, 1995).All other samples showed similar initial peaks followed by the in-duction period, which was prolonged according to the amount of

Fig. 4. Heat evolution curve of blended cements.

pretreated paper sludge (from GP2 to GP5). This is trailed by anacceleration peak whose magnitude is lower than the initial peak.The acceleration peak for GP2 appeared after 12 h of mixing whileGP5 acceleration peaked appeared after 80 h of mixing. This is dueto the higher alkalinity of the activator powder in GP2 than GP3,GP4 and GP5. Presumably the higher alkalinity in GP2 aided in thedissolution of the calcium alumino-silicate glass in the slag duringearly stages of activation, contributing to the nucleation, growthand precipitation of reaction products (i.e. C-(A)-S-H), which takesplace earlier and is seen as the heat release peak. The mortarsamples were cured at a higher temperature (35 �C vs. 22 �C), whichexplains the faster setting time in the mortar samples compared tothe peaks in the heat release analyses.

The cumulative heat evolution of the different blended cementsis shown inset of Fig. 4. The increasing amount of pretreated papersludge from GP2 to GP5 delays the reaction as can be seen form thisfigure. The initial increase is associated with the heat evolvedduring the wetting and dissolution of the blended cement withwater. The subsequent period correlates to the induction while thefinal increase is assigned to the acceleration period. Althoughthe reaction for GP5 was not completed before 96 h, it seems thatthe increasing content of paper sludge in the sample correlates tohigher heat release from GP2 to GP5. Also, no other increment wasnoticed with GP1 sample after the initial peak.

In addition, initial setting (Ti) and final setting (Tf) time testswere done for each blended cement paste (see Table 4). Theretardation of setting time with increasing paper sludge in the mixis partly due to lower alkalinity of formed carbonates (and sodiumcarbonate during activation of the paper sludge), which are neededto dissolve Ca2þ from the slag. When hydrated, the CO3

2� and Ca2þ

released from the blended cements react to form carbonate saltssuch as calcite and pirssonite, which are proposed to lengthen theinduction period together with the lower pH of the pretreatedpaper sludge. This interaction is proposed to take place before theprecipitation of the Ce(A)eSeH gel (Bernal et al., 2015; Garcia-Lodeiro et al., 2015; Ke et al., 2016). Over a period of time, theexcess CO3

2� in the blended cement slowly reacts in the systemwhich may have influenced the later high strength achieved withGP5 matrix (see Fig. 5) (Ke et al., 2016; Fern�andez-Jim�enez andPuertas, 2001). Also, it should be noted that all samples set be-tween 3 and 8 h when they were cured at slightly elevated tem-perature (i.e. at 30e35 �C). For GP1, the setting time was notrecorded because the sample did not harden after three days.

3.2. Mechanical properties and water absorption

The effect of increasing the amount of dry activators in theblended geopolymer cement onmechanical strength at 7, 28 and 50days can be seen in Fig. 5. At 7 days, the compressive strength(Fig. 5a) was highest with GP2 and lowest with GP5. The alkalinecarbonates/bicarbonates (sodium carbonate and pirssonite) fromthe dry activator are proposed to be concentrated in GP2 than inGP5. And might have ensured increase dissolution of slag particlesat early age of the mortars and directly influencing the 7 daysstrength of GP2. This is also consistent with the setting time andheat release of the samples previously discussed. Xu et al.

(a) (b)

Fig. 5. Compressive (a) and flexural strength (b) of the mortars at 7, 28 and 50 days.

E. Adesanya et al. / Journal of Cleaner Production 185 (2018) 168e175172

(Characterization of Aged Slag Concretes,) also explained that inNa2CO3-activated slag binders, the long term activation reaction is arecurring process in which the Na2CO3 creates buffered alkalineenvironment. The amount of CO3

2� available in the systemwould besustained by the gradual dissolution of CaCO3 slowly increasing thepH. The Ca released during this period would react with the dis-solved silicate from the BFS forming new reaction products pro-posed as a C-A-S-H type product thereby increasing the strengthlater on. This assertion correlates with the delayed strengthdevelopment of the samples in this study with increasing amountof pretreated paper sludge. On the other hand, the flexural strength(Fig. 5b) development of the mortars during the curing period hassimilar trend with minor difference observed with GP3. Overall,other samples (except GP3) had a similar flexural strength of11MPa at 50 days.

Total water absorption for the blended geopolymer mortars at 3,7 and 14 days of water immersion are shown in Fig. 6. The waterabsorption values increase slightly with time for each mortar afterprolonged immersion until 14 days. The increase in the amount ofpaper sludge in mortar resulted in reduction of water absorptionvalues. All samples had water absorption below 5%, GP5 mortarspresent the lowest absorption at 4.1%, while GP2 had the highestwith 4.8% at 14 days. This may be partly due to the filler effect of thepaper sludge in the mortar resulting in the reduction of the poresizes. Since absorption is ameasure of the total water required to fillopen voids within the net volume of a mortar, it can be concluded

Fig. 6. Water absorption variation with time for GP-mortars.

that the porosity of the GP-mortars also reduces from GP2 to GP5.This is similar to previous findings using limestone dusts (Ghosh,)and calcium carbonate in geopolymerization (Abdel-GawwadA,1016). Based on its blast-furnace content, strength properties, wa-ter absorption and setting time recorded, the one-part geopolymercements could be classified as a CEM IIIC and Type 32.5N classcement under European standard EN-197 (E. Standards,).

3.3. Drying shrinkage

Drying shrinkage of all four blended geopolymer cement mor-tars with different pretreated paper sludge incorporated wereobserved up to 90 days (Fig. 7). A higher content of pretreated papersludge in the mortar resulted in higher drying shrinkage, withlowest shrinkage of 0.14% (GP2), and highest of 0.39% (GP5) at 90days. The increase in drying shrinkage at higher paper sludge levelsis consistent with the literature data on carbonate mineral additionto metakaolin (Yip et al., 2008). Contrary to the strength propertiesof the GP-mortars, there is no correlation between dryingshrinkage and strength gain. According to Law et al. (2012), nocorrelation between durability properties and strength exists andthe former are dependent on factors such as resistance to chlorideingress and carbonation, both of which can initiate undesirableeffects on the mortars. However, none of the specimens showednoticeable shrinkage cracks when observed visually.

Fig. 7. Drying shrinkage of the mortar samples.

E. Adesanya et al. / Journal of Cleaner Production 185 (2018) 168e175 173

3.4. Reaction product analysis

X-ray diffraction (XRD) and derivative thermal analysis (DTG/TGA) analysis were undertaken for all paste samples to determinethe degree of activation and the phase composition of the main re-action products. XRD curve for GP1 shows an amorphous hump andcrystalline peaks corresponding to similar diffraction in GGBFS andnon-treated paper sludge XRD (see Fig. 8 and Fig. 1). In other sam-ples, the introduction of the dry activator in the mix modifies thediffraction pattern: a new phasewas identified at 13.5�2q as the peakof hydrotalcite (Mg6Al2CO3(OH)16.4H20). Hydrotalcite is commonlyobserved as a part of the main reaction products of alkali-activatedGGBFS when the slag has MgO content higher than 5 weight-%together with C-(A)-S-H (Ben Haha et al., 2011; van Jaarsveld et al.,1997; Wang and Scrivener, 1995; Provis and Bernal, 2014). Themain crystalline peak was that of calcite, and some remnants ofPortlandite and pirssonite could also be found. Amorphous phasewas also present, and the hump indicating amorphous phases re-duces from GP1 to GP5. This could be due to the increasing crystal-line calcite content from GP1 to GP5 which are undissolved in eachmatrix decreasing the relative amorphous content of themix. Similarobservations was reported when calcite content was increased inmetakaolin geopolymer (Yip et al., 2008).

Thermogravimetric analysis of the hydrated slag and the de-rivatives of the analysis corroborated the XRD findings for themost part. With all the samples, a distinct peak is noticeable at105 �C (peak a), which indicates the removal of free waterand the decomposition of C-A-S-H type reaction products(Wang and Scrivener, 1995; Sha and Pereira, 2001; Ismail et al.,2013) and subsequently at 200 �C (see Fig. 9) (Ben Haha et al.,

Fig. 8. XRD of paste samples at 28 days curing age (P¼Pirssonite, C¼Calcite, D¼ Por-tlandite, H¼Hydrotalcite). For comparison, XRD graph of GP1 is also shown in thefigure. Horizontal dashed lines are used to indicate the area of the amorphous hump.

Fig. 9. TGA/DTG of blended cements after 28 days of activation.

2011; Taylor, 1997). The broad peak at 300e400 �C (peak b) canbe attributed to hydrotalcite-type phases, which corresponds torelease of structural water and CO2 (Wang and Scrivener, 1995).The beginning of calcium carbonate decomposition (peak c) isobserved between 550 and 620 �C and continues subsequently atthe temperature range of 680e760 �C (peak d). This indicatesthe decomposition of carbonate-containing phases includingcalcite and sodium carbonate originating from the pretreatedpaper sludge (Jin et al., 2014; Sha, 2002; Ismail et al., 2014).Portlandite was not identified or does it correlate with any of thepeaks.

3.5. Further discussion

This study shows that within the current mix-design one-partgeopolymer concrete can be designed from NaOH pre-treated pa-per sludge and GGBFS conforming with CEM IIIC and Type 32.5Nclass cement under the European standard EN-197 (E. Standards,).This study also highlights the possibility of reducing the environ-mental footprints of geopolymer concrete that comes from the useof sodium silicate. Furthermore, in order to replace OPC, thepackaging and marketability of geopolymer cement could beimproved using the methods proffered in this study. Thirdly, usingthe described method of pretreating and utilizing paper sludge inthe blended geopolymer design, reduces the disposals and theassociated environmental and economic impacts of paper sludgewaste streams. On the contrary, the energy consumption fromdrying and milling of the treated activator can potentially have aneconomic impact, though it is estimated to be lower than sodiumsilicate activated geopolymer or OPC. In industrial scale, energyconsumption can be further reduced bymilling GGBFS with treatedpaper sludge instead of separate milling.

Although the sample without NaOH (GP1) contains calcite andcompletely amorphous slag, the 7 days strength was not possible tomeasure and that mortar sample was therefore not analyzedfurther. Also, from the calorimetry study in Fig. 4, no heat peak wasrecorded throughout the duration of the analysis. GGBFS cemen-titious properties is known to be latent, except when it is blendedwith chemical activator such as OPC, gypsum, alkali activators, etc.(Shi and Day, 1995; Kumar et al., 2005; Richardson, 2000). There-fore, under the NaOH pre-treated paper sludge environment, thecementitious properties of the blended cement can however beimproved.

The content of the paper sludge in each mortar has an effect onthe amount of produced sodium carbonate (Na2CO3) when its pre-treated with NaOH. This pretreatment reaction involves release ofcarbonates ions on contact with Naþ from the activator that readilyform pirssonite (Na2Ca(CO3)2.2H2O) and sodium carbonate (seereaction 1 and 2). This could explain the increase in the amount ofcarbonates seen in the XRD and TGA of the samples and thestrength performance as paper sludge amount increased. Further-more, the strength gain is probably due to better packing as papersludge content increased in the samples: during pre-treatment theunreacted calcite forms crystalline precipitates like Ca(OH)2 (Yipet al., 2008) and possibly act as fillers in the geopolymer system.As would be expected, the unreacted calcite content increases fromGP2 to GP5 (Fig. 2). Additionally, previous work done by Yip et al.(2008) have demonstrated that in a metakaolin-based geo-polymer optimum content of calcite, without an adverse effect onthe strength, is achieved at 20% content of calcite. They alsopostulated that the increased long-term strength may be due tosurface binding of the geopolymer gel phase on the carbonatemineral. In the current study, the maximum amount of papersludge quantify in GP5 is 18% and minimum of 4% in GP2.

Drying shrinkage increment with increasing paper sludge

E. Adesanya et al. / Journal of Cleaner Production 185 (2018) 168e175174

content is likely due to the evaporation of water absorbed on theorganic matter of the paper sludge. Free water in the matrix isprobably absorbed into structure of the dry activator and whenexposed to air it evaporates contributing to the drying shrinkage asseen in Fig. 7.

TG/DTG of the activated geopolymer cement indicated thepresence of C-A-S-H-like reaction product, hydrotalcite whichaccrued as a result of the magnesium content of the slag and alsocarbonated matrix. Considering the increase in strength at laterages, it is possible that carbonation may have contributed to thestrength gain of the mortars at later curing ages. The observedformation of C-A-S-H like products is characteristic of alkali-activated geopolymer.

4. Conclusion

We have investigated the use of paper sludge as a waste-basedsource of calcite in one-part geopolymer cement, and found it tobe suitable for that purpose. After pretreatment with sodium hy-droxide, paper sludge can act as a slag activator and also as a filler.The increasing amount of the pretreated paper sludge had a signif-icant effect on the mechanical and durability properties of the pre-pared mortars. It was found to increase the compressive strength upto 48MPa at 50 days. Additionally, the sample with the lowestamount of paper sludge had considerable strength gain (40MPa) at50 days and lowest drying shrinkage value. It is considered thatrheology studies of these cement should be made to define how toimprove the workability. In conclusions, the final material is >98%side-stream based, and CO2 emissions can be avoided per ton ofpaper sludge used assuming that it would have been incinerated.

Acknowledgements

The research was funded by the European Regional Develop-ment Fund (A70189) under the auspices of the MINSI project andthe following companies: SSAB Europe Oy, Ekokem Oy, Stora EnsoOy, Pohjolan Voima Oy and Oulun Energia. The authors would liketo thank Jarno Karvonen and Jani €Osterlund for their contribution tothe laboratory work. Lubica Kriskova and Antigoni Katsiki of KULeuven and VUB Brussels respectively for helping with isothermalcalorimetry.

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