Construction and Building Materials 49 (2013) 281–287

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Construction and Building Materials

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Synthesis and thermal behavior of geopolymer-type material from wasteceramic

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.08.063

⇑ Corresponding author. Tel./fax: +86 25 8359 2903.E-mail address: qinli_nju@126.com (Q. Li).

Zengqing Sun, Hao Cui, Hao An, Dejing Tao, Yan Xu, Jianping Zhai, Qin Li ⇑State Key Laboratory of Pollution Control and Resource Reuse, and School of the Environment, Nanjing University, Nanjing 210023, PR China

h i g h l i g h t s

�Waste ceramic was collected, grinded and screened to synthesize geopolymer.� The synthesized geopolymer showed a maximum 28 d compressive strength of 71.1 MPa.� The geopolymer showed a higher compressive strength after heat treatment of 1000 �C.

a r t i c l e i n f o

Article history:Received 19 May 2013Received in revised form 6 August 2013Accepted 29 August 2013

Keywords:GeopolymerWaste ceramicSynthesisThermal behaviorCompressive strength

a b s t r a c t

Waste ceramic was activated by alkali hydroxides and/or sodium/potassium silicate solutions to synthe-size geopolymer-type material in this study. The synthesized geopolymer pastes were characterized bymechanical test, TG-DSC, SEM, XRD, as well as FT-IR analyses. And the thermal behavior of synthesizedgeopolymer was determined in terms of compressive strength evolution by exposure to 100, 200, 400,600, 800, and 1000 �C. The synthesized geopolymer pastes exhibited a maximum 28-day compressivestrength of 71.1 MPa and favorable anti-thermal properties by showing a higher compressive strengthof 75.6 MPa after heat treatment of 1000 �C. The results indicate that waste ceramic could serve as a sat-isfying source material for thermostable geopolymer.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

As the world’s largest ceramics producer and consumer, Chinaproduces over one million tons of all kinds of waste ceramics (tiles,pan forms, blocks, and so on) which are just land filled or stackedevery year. With increasingly stricter restrictions on landfills inChina, ways for comprehensive utilization of waste ceramics haveto be explored. Reutilization of waste ceramic as part of the feed-stock for the production of ceramics has been practiced, but theamount of wastes reused in that way is still negligible [1]. Effortshave also been made in the cement industry to use recycled cera-mic and calcined clays as alternative cementitious materials [2–5].However, according to Sanchez De Rojas [6] and Goncalves et al.[4], the increase of the cement replacement by calcined claymaterials would result in the increase in total porosity and thereduction in strength. Nowadays, there are several papers [6–10]that study the possibility of using ceramic wastes in substitutionof natural aggregate (sand or gravel). Though a good workabilitycan be obtained using waste ceramic as aggregates, the studies in

this field showed that some problems arise. Senthamarai andManoharan [11] studied the properties of concrete with electricalceramic waste aggregate where he found that the compressive,splitting tensile and flexural strengths of ceramic waste coarseaggregate concrete were lower than conventional concrete. Resultsobserved by Cachim [12] indicated that brick residuals could beused as partial replacement of natural aggregates without reduc-tion of properties for 15% replacement and with reductions up to20% for 30% replacement. Binici [13] reported a decrease in bothabrasion and chloride resistances when using crushed ceramic asfine aggregate substitution. Hence ways to compressively reutilizewaste ceramic should be expanded.

Geopolymers are a class of inorganic polymers synthesized bypolycondensation of [SiO4] and [AlO4] tetrahedral in alkali acti-vated aqueous solutions [14,15], which was firstly introduced byJoseph Davidovits in the late 1970s [16]. According to Davidovits[16,17], geopolymers are polymeric silicon–oxygen–aluminumthree-dimensional materials containing a variety of amorphousto semi-crystalline phases. Based on such a unique structure, geo-polymers exhibit higher mechanical strength, much more excellentchemical, fire and heat resistances, lower thermal conductivity andshrinkage than ordinary Portland cement (OPC) [18]. Moreover, as

Fig. 2. Scanning electron microscope image of waste ceramic powder.

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Komnitsas and Zaharaki [19] studied, geopolymers can generallydeliver a great reduction in CO2 emission and require less energycompared with OPC. Thus geopolymers can be regarded as a ‘greenconcrete’ [19,20]. Due to the sound properties as well as significantenvironmental benefits of geopolymer products, geopolymer tech-nology has been drawing great interest [21–23].

On one hand, large number of scholars have developed a varietyof raw materials such as metakaolin [24,25], fly ash [18,26–28],and slag [29,30] to synthesize geopolymer matrixes of excellentperformances. All these materials could supply the polymerizationreaction with sufficient silica and aluminum [20,31]. On the otherhand, waste ceramic is a kind of typical silicon rich material, there-fore using waste ceramic as source material to manufacture geo-polymer should be an efficient and environmental friendlyintegrated utilization of it. However, no dedicated study has beenreported about systematic characterization of the synthesis andhigh-temperature properties of geopolymer pastes using wasteceramic as source material.

In this study, waste ceramics were creatively introduced to dis-cuss its probability for geopolymer synthesis. Focus would be onthe synthesizing process concerning selection of best activatingsolution. An optimal geopolymer product will be selected for fur-ther high temperature tests in terms of compressive strength gainsor losses after exposure to high temperatures (100, 200, 400, 600,800, and 1000 �C). The geopolymer products were characterizedby mechanical testing, Thermogravimetric Analysis-DifferentialScanning Calorimetry (TG-DSC), Scanning electron microscopy(SEM), X-ray diffraction (XRD), as well as Fourier Transform Infra-red Spectroscopy (FT-IR) analyses. Results obtained in this studywill enrich the studies of geopolymer and provide a promisingalternative way to reutilize waste ceramic economically andenvironmentally.

2. Experimental procedure

2.1. Materials

The ceramic used in this study was derived from municipal waste collection toenhance the social value of this research, it’s a mixture of titles, pan forms, blocksand so on. The ceramics were first ultrasonic washed to remove contaminants suchas paper scraps, metal, plastic, or organic matters. And the dried ceramics werecrushed, pulverized in a ball mill for 45 min and then screened and measured ona Mastersizer 2000 laser analyzer (Malvern, UK). Fig. 1 shows the particle size dis-tribution of ceramic powder, the average particle size (d50) of milled ceramic is30.17 lm. Fig. 2 shows the SEM image of the grounded ceramic powder, which con-sists exclusively of irregular, coarse and angular particles. 9800XP+ X-ray fluores-cence spectrometer (XRF) (ARL, Switzerland) and ARL X’TRA X-ray diffractometer(Thermo, Switzerland) were employed for analyzing the chemical compositionsand crystal phases of waste ceramic powder, and the results are listed in Table 1

Fig. 1. Particle size distribution of ground waste ceramic.

and Fig. 3, respectively. The main chemical compositions of waste ceramic areSiO2 and Al2O3, with the major crystalline phases of being quartz (SiO2) and albite(NaAlSi3O8).

An industrial grade sodium silicate solution (SiO2 = 26.5%, Na2O = 8.5%, andH2O = 64.8; molar ratio of SiO2/Na2O, 3.2), sodium and potassium hydroxides (ana-lytical grade), as well as deionized water, were employed in this investigation toprepare activating solutions. The activating solutions can be divided into threegroups, i.e. groups A, B, and C. Each group consists of three solutions, making a totalof nine activators. Activators A1–A3 are mixtures of sodium silicate and sodiumhydroxide; B1–B3 are blendings of sodium silicate and potassium hydroxide solu-tions; and C1–C3 are sodium and/or potassium hydroxide solutions. Compositionsof all the activating solutions are detailed in Table 2. All alkaline solutions werestored for at least 24 h before use.

2.2. Geopolymer synthesis and analysis

Geopolymers were synthesized by mixing ground waste ceramic with each acti-vating solution with a liquid/solid ratio of 0.4, respectively, as summarized in Ta-ble 3. The 5 min mixing process was followed by casting the slurries in tripletmoulds of 20 mm cubes and another 5 min vibration to remove entrained air bub-bles. The moulds were then sealed with polyethylene film and set in a standard cur-ing box at 60 �C with 100% humidity under ambient pressure. After 24 h of setting,the samples were demoulded and subjected to curing at 60 �C standard curing boxin sealed polypropylene boxes for further 27 d.

The compressive strength values of synthesized geopolymers were measuredusing a NYL-300 compressive strength testing apparatus (Wuxi Jianyi, China), withthe force applied at a rate of 1.0 kN/s. Geopolymer of the highest compressivestrength was selected and subjected to high-temperature performance test. The se-lected specimens were calcined in a muffle furnace at 100, 200, 400, 600, 800, and1000 �C for 2 h, respectively. Compressive strength gains or losses of the calcinedspecimens were then measured. The results reported were the average of threereplicates.

Simultaneous TG-DSC was carried out on a STA 449C (Netzsch, Germany) ther-mal analyzer to determine the mass loss history of selected geopolymer at elevatedtemperatures. The sample was heated from 40 to 1100 �C in an inert nitrogen envi-ronment with the heating and nitrogen purging rates kept constant at 10 �C and25 ml min�1, respectively. The ground waste ceramic as well as selected geopoly-mers before and after thermal exposures was characterized via SEM, XRD, and

Table 1Chemical composition of waste ceramic.

Chemical composition Content (wt%)

SiO2 65.52Al2O3 21.00CaO 6.00K2O 3.31MgO 1.95Fe2O3 1.11Na2O 0.36TiO2 0.20SO3 0.17BaO 0.15LOIa 0.14

a LOI, loss on ignition at 960 �C.

Fig. 3. X-ray diffraction data of waste ceramic.

Table 2Composition of activating solutions (M, Na and K; R, molar ratio of SiO2/Na2O).

Activating solution Geopolymer ID Content (wt.%)

KOH NaOH SiO2 H2O

A1 GA1 – 24.84 22.34 52.82A2 GA2 – 22.0 23.18 54.81A3 GA3 – 19.87 23.82 56.31B1 GB1 16.55 11.85 21.28 50.32B2 GB2 14.73 10.55 22.21 52.51B3 GB3 13.15 9.70 22.92 54.19C1 GC1 – 30.10 – 69.90C2 GC2 40.38 – – 59.62C3 GC3 23.15 16.88 – 59.97

Table 3Mix design and calculated molar ratios for waste glass-based geopolymers (M, Naand/or K).

Sample ID Liquid/solid (mass ratio) Si/Al Si/M Al/M

G1 0.4 3.13 3.94 1.26G2 0.4 3.15 4.28 1.36G3 0.4 3.16 4.57 1.45G4 0.4 3.12 3.30 1.06G5 0.4 3.13 3.60 1.15G6 0.4 3.14 3.87 1.23G7 0.4 2.75 3.03 1.10G8 0.4 2.75 2.11 0.77G9 0.4 2.75 2.25 0.82

Fig. 4. Compressive strength of geopolymer pastes.

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FT-IR. Microstructural images were observed by a S-340N scanning electron micro-scope (Hitachi, Japan). X-ray diffractometry was conducted by an ARL X’TRA high-performance powder X-ray diffractometer using Cu Ka radiation at 40 mA and40 kV over the range (2h) of 5–65�. The FT-IR spectra were recorded by a Nexus870 FTIR Spectrometer (Nicolet, US). The KBr pellet method was used to preparethe samples, which was scanned at a range of from 4000 to 400 cm�1.

Fig. 5. Compressive strength evolution of selected geopolymers (GB2) after thermalexposures.

3. Results and discussion

3.1. Compressive strength

As illustrated in Fig. 4, the 28 d compressive strength values ofgeopolymers of groups A and B both gave a much higher compres-sive strength than those activated by group C. The specimen acti-vated by activating solution B2 achieved the highest 28 dcompressive strength of 71.1 MPa, and was chosen for furtherstudy of high-temperature property of waste ceramic based geo-polymer. As Xu et al. [27] discussed, the presence of a properamount of soluble Si in the activation solution contributes thedevelopment of the compressive strength, thus geopolymers acti-vated by silicate solutions exhibited much higher compressive

strength than those activated by hydroxide solutions. For the caseof geopolymer blocks activated by the sodium silicate solution con-taining K+ (activating solutions of group B), they exhibit a highercompressive strength, Phair and van Deventer owed this to thestronger basicity of K+, which allowed higher rates of silicate disso-lution [32,33]. In alkali hydroxide system, geopolymer activated byNaOH showed higher compressive strength than those activated byKOH, this result is consistent with Davidovits’s study [17].

3.2. High-temperature property

The high-temperature property of geopolymer was investigatedin terms of compressive strength evolution after exposure to differ-ent temperatures. Fig. 5 shows the compressive strength gains orlosses after heating procedure, with the sample without high-temperature treatment as a contrast. The compressive strengthgradually increased after a sharp decline at first (the lowestcompressive strength appeared at 100 �C), with the compressivestrength of specimen heated at 1000 �C even slightly higher thanthe unheated geopolymer. Pan et al. [34] suggested that thestrength evolution of geopolymers after exposure to elevatedtemperatures depends on the dominant process of the followingtwo factors: damages caused by thermal incompatibility leadingto strength decrease; further geopolymerization and/or sinteringleading to strength increase.

Fig. 6. Photograph of geopolymer samples before and after thermal exposures.

Fig. 7. Thermogravimetric-differential thermal analysis of the optimal geopolymer(GB2).

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The sharp reduction of compressive strength upon the initialheating is probably caused by the loss of structural water as wellas the possible concomitant development of micro cracks, whichwill weaken the structure, as Lyon et al. [35] and Lemougna et al.[36] suggested. The further geopolymerization and/or viscous sin-tering at higher temperature densified the internal structure of

Fig. 8. Scanning electron microscope images of selected geopolymers (GB2) of (a) 28 d cuto 1000 �C.

geopolymer matrix, resulting in the increase of compressivestrength. The compressive strength of the geopolymer samplesdid drop at different degrees during the thermal exposures. Andthe reduction in strength at temperatures 100–800 �C may implythat there is some collapse before it shows high strength again athigher temperatures. The photograph of geopolymer samples (asshown in Fig. 6) shows that there’s no distortion, no peeling andeven no cracks on the surface of the geopolymers after thermalexposures. Comparing with the cement materials, which is proneto crack and collapse when heated, we thought that the wasteceramic-based geopolymer is a potential candidate for fireproofmaterial.

3.3. TG-DSC analysis

The TG-DSC curves of the selected geopolymer are presented inFig. 7. Approximately 9% mass loss occurred over the temperaturerange of 40–1100 �C, and about 88% of which is lost when temper-ature is below 250 �C, which corresponds to the endothermic peakon the DSC curve. It should be emphasized that the TG curve inFig. 7 is of a smooth nature, thus supporting the absence of hy-drates in the crystalline form, e.g. Ca(OH)2, CaCO3, or ettringite[37] . From 250 to 1100 �C, the DSC curve shows an endothermicreaction, possibly due to the onset of solid state reactions duringsintering of geopolymer matrix, which is presumably responsiblefor the densified structure (Section 4) and compressive strength in-crease developed at higher temperatures.

red; (b) after exposure to 100 �C; (c) after exposure to 600 �C; and (d) after exposure

Fig. 10. Infrared spectra of waste ceramic and selected geopolymer (GB2) beforeand after thermal exposures.

Z. Sun et al. / Construction and Building Materials 49 (2013) 281–287 285

3.4. Scanning electron microscopy analysis

The Scanning Electron Microscopy images of the selected geo-polymers before and after thermal exposures are shown in Fig. 8.Fig. 8(a) presents a well-formed geopolymer of specimen GB2 after28 d standard curing. The largely homogeneous microstructuresuggests a well geopolymerization behavior, demonstrating thatwaste ceramic could serve as alternative source materials for geo-polymerization. After heating to 100 �C, as shown in Fig. 8(b),numerous small cracks can be observed across the cross-sectionof the microstructure, with lengths ranging from several hundrednanometers to several microns. Fig. 8(c) shows the microstructureof selected geopolymer after heating to 600 �C, a reduced numberof cracks can be observed on the polished surface of the selectedspecimen, though the cracks are larger. Duxson et al. [38] haspointed out that the reduction in number of cracks of the micro-structure implies that the geopolymer undergoes a significant levelof thermal relaxation and healing of small cracks, which is consis-tent with the increase of compressive strength after thermal expo-sure. The specimen after heating to 1000 �C shows a muchsmoother and more compact surface (Fig. 8(d)), implying the mate-rial has been softened and the surface roughness has been reduceddriven by surface tension, thus resulting in a good compressivestrength.

3.5. XRD analysis

Fig. 9 shows the XRD diffractograms of waste ceramic as well asthe selected geopolymer samples before and after exposure tohigh-temperatures. It can be observed that the XRD patterns se-lected geopolymers unheated and heated at 100, 600 �C are simpleand similar, and the major crystalline phases are quartz (SiO2) andalbite (NaAlSi3O8). Although conventional geopolymers are X-rayamorphous, [36,39] the presence of crystalline phases in geopoly-mers prepared from circulating fluidized bed combustion (CFBC)bottom ashes [27] , and volcanic ash has previously been reported

Fig. 9. X-ray diffraction data of geopolyme

[36,40]. Verdolotti et al. [40] owned these crystalline diffractionpeaks to the unreacted feedstock crystal phases, or zeolites pro-duced in geopolymerisation process, which overlapped the amor-phous baseline. After exposure to a higher temperature of1000 �C, while the quartz and albite phases in the geopolymer ma-trix remain almost unchanged, the characteristic peak of geopoly-mer at range from 20� to 40� strengthened, and there are traces ofthe formation of new crystalline phases of zeolite (Na6[AlSiO4]6)and sodium calcium silicate (Na2Ca3Si6O16). Similar results werefound by Xu et al. [27], Lemougna et al. [36], and Bakharev [41]by studying the thermal properties of various materials based geo-polymers, respectively.

3.6. Infrared spectroscopy

Fig. 10 shows the FT-IR spectra of the waste ceramic powderand selected geopolymers before and after thermal exposures.

rs before and after thermal exposures.

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For all samples, there are two broad absorbance bands at 450–730 cm�1 and 820–1250 cm�1, respectively. As Verdolotti et al.[40] discussed that the absorbance at 450–730 cm�1 reflects thestretching vibration of Al–O–Si bonds, and the absorbance bandsat 820–1250 cm�1 arise from the variability of the bond anglesand bond lengths of the tetrahedral structures around the siliconatoms. However, compared with the ceramic powder, the selectedgeopolymer undergoes a very small shift of its Si–O–Si position tolower frequency with alternating Si–O and Al–O bonds, thus thevibration at 1080 cm�1 corresponding to Si–O and Al–O in theraw waste ceramic is shifted below 1000 cm�1. Such a shift is as-cribed to the penetration of Al4+ atoms into the original arrange-ment of Si-O-Si skeletal structure occurred during thepolycondensation process [21,22,37]. The bands at 3500 cm�1

and 1700 cm�1 are for O–H stretching and O–H bending, reflectingthe presence of structural water [22]. While, only minute differ-ences are shown within 450–1250 cm�1 among the FT-IR tracesof unheated and heated geopolymers, suggesting that most vibrantforms of the molecular chains were unaffected after high temper-ature calcination.

4. Conclusion

This work has achieved a successful geopolymerization of wasteceramic with high compressive strength and good high-tempera-ture properties. The compressive strength of waste ceramic-basedgeopolymers depends on the initial reacting system, and the alka-line activating solution plays an important role in geopolymeriza-tion process. Geopolymer of the optimal mix design gave thehighest compressive strength of 71.1 MPa. The waste ceramic-based geopolymer exhibits favorable thermal stability in terms ofcompressive strength evolution after thermal exposures. A highercompressive strength was even acquired after 2 h calcination at1000 �C, which may be due to the viscous sintering and the com-pletion of further geopolymerization reaction at high temperature.Both the SEM photomicrographs and FT-IR spectra have indicatedthe convention of waste ceramic to geopolymers. The results of thisstudy may provide an approach to in situ recycling waste ceramicfor production of value-added geopolymer composites, when thewaste ceramic is silica and alumina rich (content higher than70% is preferable) and easy to grind.

Acknowledgements

The authors gracefully acknowledge financial supports from theFoundation of State Key Laboratory of Pollution Control and Re-source Reuse of China, the Natural Science Foundation of China(No. 51008154), the Science and Technology Support Program (So-cial Development) of Jiangsu Province (BE2013703), the ResearchProjects on Environmental Protection of Jiangsu Province(2012030).

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