Optimization of geopolymer synthesis by calcination and polycondensation of a kaolinitic residue

Resources, Conservation and Recycling 40 (2003) 27–38

Optimization of geopolymer synthesis by calcinationand polycondensation of a kaolinitic residue

R. Cioffia, L. Maffuccib, L. Santorob,∗a Dipartimento di Ingegneria e Fisica dell’Ambiente, Università della Basilicata,

Contrada Macchia Romana, 85100 Potenza, Italyb Dipartimento di Chimica, Università di Napoli ‘Federico II’,

Complesso Universitario di Monte S. Angelo, Via Cintia, 80126 Naples, Italy

Received 5 August 2002; accepted 24 January 2003

Abstract

In this work a kaolinitic residue has been used for the synthesis of geopolymer-based building ma-terials. The synthesis takes place through a calcination step followed by a polycondensation step. Thecalcination step was carried out at temperatures ranging from 500 to 750◦C and times ranging between2 and 6 h. Optimum calcination conditions were found by evaluating the reactivity of the calcinedproducts by differential scanning calorimetry (DSC). The polycondensation step was carried out attemperatures ranging from 25 to 85◦C by reaction with sodium or potassium silicate. The compositionsof polycondensation systems stoichiometric for the synthesis of polysialate (PS), polysialatesiloxo(PSS) and polysialatedisiloxo (PSDS) geopolymers were tested, but only the first two were successful.An original quantitative analytical method was employed to determine the amounts of reacted silicateand water after polycondensation. The products obtained were tested for unconfined compressivestrength, apparent density, porosity, surface area and pore size distribution. The results show that it ispossible to produce good quality building materials starting from a kaolinitic residue.© 2003 Elsevier B.V. All rights reserved.

Keywords:Kaolinitic residue; Recycling; Geopolymers; Polysialate; Polysialatesiloxo

1. Introduction

The term ‘geopolymers’ was first introduced byDavidovits (1979)to designate a newclass of three dimensional silico-aluminate materials. Since then, interest in these new mate-

∗ Corresponding author. Tel.:+39-081-67-4028; fax:+39-081-67-4090.E-mail address:[email protected] (L. Santoro).

0921-3449/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0921-3449(03)00023-5

28 R. Cioffi et al. / Resources, Conservation and Recycling 40 (2003) 27–38

rials has steadily grown as both the ceramic and binder industries can draw great advantagefrom the implementation of new technologies for the manufacture of geopolymer-basedproducts.

Geopolymers comprise three classes of inorganic polymers that, depending on the silica/alumina ratio, are based on the following three different monomeric units: (–Si–O–Al–O–),polysialate (PS), SiO2/Al2O3 = 2; (–Si–O–Al–O–Si–O–), polysialatesiloxo (PSS), SiO2/Al2O3 = 4; (–Si–O–Al–O–Si–O–Si–O–), polysialatedisiloxo (PSDS), SiO2/Al2O3 = 6.

The synthesis of geopolymers takes place by polycondensation and can start from a varietyof silico-aluminates.Davidovits (1989, 1991, 1993), Palomo et al. (1992)andBarbosa et al.(2000)used metakaolinite to obtain geopolymers by reaction with alkali metal (Na or K)silicate. Alternatively,Ikeda et al. (1998), van Jaarsveld and van Deventer (1999), Phair et al.(2000), Xu and van Deventer (2000)andXu et al. (2001)have shown that geopolymerscan be obtained starting from many raw silicoaluminates; polycondensation takes place byreaction with alkali metal silicate.

The synthesis of geopolymers relies on the following reactions (Xu and van Deventer,2000):

The structure of geopolymers can be either amorphous or crystalline, depending onthe condensation temperature. Amorphous polymers are obtained at temperatures rangingfrom 20 to 90◦C, while crystalline polymers are obtained at 150–200◦C. The structure ofcrystalline geopolymers resembles that of zeolite A (Davidovits, 1991).

Geopolymeric materials are attractive because they have excellent mechanical properties,fire resistance and durability (Davidovits and Davidovics, 1988; Palomo et al., 1992). Fur-thermore, the reaction pathway requires either metakaolinite, obtained by calcining kaoliniteat temperatures of the order of 600–700◦C, or raw silico-aluminates. There is thus greatinterest in the reduced energy requirement for the manufacture of new materials based ongeopolymers.

The applications of geopolymer-based materials cover many fields. New ceramics,cements, matrices for hazardous waste stabilization, fire-resistant materials, asbes-tos-freematerials and high-tech materials are some of the potential uses of geopolymers (Davidovits,1991; van Jaarsveld et al., 1997, 1999).

All aspects of research on geopolymers were dealt with in the International Conference‘GEOPOLYMERE ‘99’ held in Saint-Quentine, France on June 30–July 2 1999, whichclaimed to be the first conference in 20 years to focus on the science behind new materialsfor the third millennium.

R. Cioffi et al. / Resources, Conservation and Recycling 40 (2003) 27–38 29

The literature pertinent to geopolymers deals mainly with product characterization, whileless effort has been made to understand the qualitative and quantitative features of thepolycondensation process.

In this work, a calcined kaolinitic residue has been used for the synthesis of geopoly-mers. Both the calcination step and the polycondensation step (reaction of metakaolinitewith sodium or potassium silicate) have been studied from a quantitative point of view.The reactivity of calcination products obtained under different experimental conditions hasbeen ranked by measuring the heat evolved during the initial polycondensation phase, thusoptimizing the calcination process. Following this, an original quantitative method has beendeveloped for the measurement of the amount of silicate and water reacted during the poly-condensation reaction. This has been carried out under different experimental conditions:temperature, polycondensation time and composition of the starting mixture.

The kaolinitic residue used in this work comes from a process of silica sand extractionfor glass manufacture. Its use enables optimal exploitation of natural resources, especiallybecause its aim is to produce good quality materials starting from a residue. Furthermore, thisprocess can potentially be extended to other silicoaluminate natural materials and industrialwastes such as clays, pozzolana, fly ash and so on.

2. Materials and methods

The kaolinitic residue used for geopolymer synthesis was characterized by means ofchemical analysis, X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) andthermal analysis (Chianese, 1998). The chemical composition was: SiO2, 39.41%; Al2O3,35.38%; K2O, 5.69%; CaO, 5.55%; Fe2O3, 1.15%; Na2O, 1.14%; MgO, 0.11%; loss onignition at 1050◦C, 11.35%. X-ray diffraction analysis showed that the material is mainlycomposed of kaolinite with impurities�-quartz, calcite and K-feldspar. The FTIR analysiswas essentially employed to check the Al coordination number, which proved to be VI.Thermal analysis (heating rate 10◦C min−1) showed that the loss of crystallization watertakes place within the temperature range 450–650◦C. This causes the crystalline structureto collapse and changes the Al coordination number from VI to IV (checked by FTIRanalysis).

The calcination step was carried out at 500, 550, 650 and 750◦C for 2, 4, and 6 h.The polycondensation step was carried out at 25, 40, 60 and 85◦C for times ranging

from 15 min to 28 days, depending on the reaction temperature. The polycondensationreaction was monitored by measuring the heat evolved by differential scanning calorimetry(DSC). This technique proved to be a useful tool for evaluating the reactivity of the calcinedkaolinitic residue as a function of calcination temperature and time.

The polycondensation reaction was carried out with three different mixtures of the cal-cined residue with alkali metal silicate solution to get PS, PSS and PSDS geopolymers. Thealkali metal silicate solution was prepared by dissolving appropriate quantities of reagentgrade silica (cristobalite) in water with sodium or potassium hydroxide. The amount ofwater used for the preparation of the alkali metal silicate solution was such as to ensureconstant workability of the mixes. Taking into account that the molar composition of thekaolinitic residue is 0.23(K2O + Na2O)·Al2O3·1.89SiO2, the stoichiometric amounts of


Table 1Grams of alkali metal hydroxide, silica and water per gram of calcined residue

Geopolymer NaOH KOH SiO2 H2O

K-PS – 0.5 0.023 0.4K-PSS – 0.5 0.44 0.5K-PSDS – 0.5 0.86 1Na-PS 0.36 – 0.023 0.5Na-PSS 0.36 – 0.44 1Na-PSDS 0.36 – 0.86 2

alkali metal hydroxide and silica to be added to get the three different types of geopolymerswere calculated as shown inTable 1.

The mixtures formed into small cylindrical samples (d × h = 2 × 3 cm2) which werecured at the four different temperatures for various times. These samples were used forthe quantitative determination of water and silica consumed during the polycondensationreaction. The amounts of reacted alkali metal silicate and water at any polycondensationcondition (time and temperature) were determined as follows. Each sample was groundand washed with a 0.1 M KOH solution to leach all the unreacted alkali metal silicate.This procedure is similar to the one usually used to stop hydration in cements and wascarried out at room temperature and for the time required to completely disgregate thesample. Checks were made to ensure that the low alkalinity of the washing solution avoidedunreacted metakaolinite dissolution (Maffucci, 1999). Then, the leached solid was rinsedwith acetone and with diethyl ether and oven dried at 40◦C. The cumulative amount ofreacted alkali metal silicate and water was obtained by weight difference between the solidrecovered after the above treatments and the residue initially employed. The amount ofreacted water was determined by weight loss of the recovered solid on ignition at the sametemperature as that of the residue calcination step.

Specific surface area, porosity and pore size distribution were measured by means of aCarlo Erba 1990 Sorptomatic apparatus employing the BET method of analysis.

Cubic samples 4 cm in size were also prepared for the measurement of unconfinedcompressive strength, density, specific surface area, porosity and pore size distribution.

3. Results and discussion

Of the six systems listed inTable 1, those labeled K-PSDS and Na-PSDS (SiO2/Al2O3ratio equal to six) did not harden at any of the temperatures tested. Thus, only the other foursystems were tested.

3.1. Optimization of the calcination step

The calcination step was optimized in terms of time and temperature by evaluating thereactivity of the products obtained in the different conditions tested through the measurementof the heat evolved during the polycondensation step.


Fig. 1. DSC traces of polycondensation systems with kaolinitic residue calcined at 750◦C for different times.

Fig. 1 shows the DSC traces obtained with the system of type K-PSS at 85◦C reactiontemperature. The mixes were made with the kaolinitic residue calcined 2, 4 and 6 h at 750◦C.It can be seen that the curves are very similar to each other, indicating that the calcinationtime has little effect on the reactivity of the product obtained at 750◦C. Heat is evolvedvery rapidly up to about 300 s; then, the rate of heat evolution lowers without very muchvariation up to about 800 s. After this time, the rate decreases rapidly approaching zero.

From the quantitative point of view and for the sake of comparison, the reactivity ofthe products obtained in the different calcination conditions can be evaluated taking intoaccount the heat of reaction as given by the area below each curve. These values are 532,572 and 574 J g−1 for 2, 4 and 6 h calcination time, respectively. It is clear that at 85◦C,for residue calcined at 750◦C, the maximum reactivity is reached just after 4 h calcinationbut that an almost equally high reactivity is reached at 2 h. In fact, the heat evolved in thiscondition is about 93% of that evolved when the calcination time is longer.

Fig. 2 shows the effect of calcination temperature for a uniform 2 h calcination. Again,the DSC traces are very similar, giving values of heat evolved of 528 J g−1 in the case of theresidue calcined 2 h at 650◦C and 487 J g−1 when the calcination is carried out at 550◦Cfor the same time. From these data it can be inferred that the reactivity of the product of a 2 hcalcination at 650◦C is almost the same as that of the residue calcined at 750◦C for the sametime and very close to the maximum reactivity attainable at the higher temperature for longercalcination times. In fact, the heat evolved after reaction of the residue calcined 2 h at 650◦Cis about 92% of that evolved when the residue undergoes the calcination step at 750◦C for4–6 h. On the other hand, the reactivity of the residue calcined 2 h at 550◦C is significantly


Fig. 2. DSC traces of polycondensation systems with kaolinitic residue calcined 2 h at different temperatures.

lower, as the heat evolved is only about 85% of the maximum. This is in agreement with theresults of thermal analysis carried out on the raw kaolinitic residue which showed that cal-cination is not complete at 550◦C (Chianese, 1998). DSC measurements carried out at thepolycondensation temperatures of 60 and 40◦C confirmed that the reactivity of the calcinedresidue increases as both temperature and time of calcination increase. No DSC measure-ment was possible at 25◦C due to the extremely low rate of heat evolution. Then, it was con-cluded that the optimum calcination time and temperature are, respectively, 2 h and 650◦C.These conditions were ensured when calcining the residue for the remaining part of the work.

3.2. Chemical characterization of the polycondensation step

The quantitative data of the amounts of reacted water and reacted water plus alkali metalsilicate were used to obtain the amount of reacted alkali metal silicate at the differentpolycondensation conditions. These results are reported inFigs. 3 and 4for the systems(Na,K)-PS and (Na,K)-PSS, respectively. WhileFigs. 1 and 2show no measurable heatevolution after about 1000 s (0.27 h),Figs. 3 and 4show that polycondensation continuesbeyond that time. This implies that the reaction has an initial heat producing phase followedby a different, scarcely heat producing phase.

Fig. 3refers to PS type geopolymers and shows that in all cases the amount of reacted al-kali metal silicate increases with reaction time and that ultimate values greater than 0.2 g g−1

of calcined residue are attained at any temperature (with the exception of Na-PS at 40◦C).Also, no great difference is observed at a given temperature between Na-PS and K-PSgeopolymers.


Fig. 3. Amount of reacted silicate of systems of type PS.

Fig. 4. Amount of reacted silicate of systems of type PSS.


The values of reacted alkali metal silicate can be easily converted in terms of percent-ages of the initial quantity added (seeTable 1). The maximum percentages of reacted alkalimetal silicate are about 57% for Na-PS geopolymers and about 44% for K-PS geopolymers,indicating that the fractional conversion is significantly higher when NaOH is used for thepreparation of the initial mixtures.

Fig. 4 shows that even for PSS type geopolymers no significant effect of the alkalimetal is observed on the absolute amount of reacted alkali metal silicate. In this case,the values are higher than those found for PS type systems, proving that a geopolymerof different composition is really formed. It is also seen that the amount of reacted alkalimetal silicate beyond the shortest times investigated increases slightly with reaction time.The maximum values are observed at 85◦C (about 0.6 g g−1 for both Na-PSS and K-PSS).As the temperature decreases, the values of reacted alkali metal silicate decrease steadily.However, even at 25◦C they are higher than those found for PS type geopolymers. In thecase of PSS geopolymers, the maximum percentages of reacted alkali metal silicate (at85◦C) are about 75% for Na-PSS and about 64% for K-PSS, confirming the higher alkalimetal silicate conversion degree for the systems with added NaOH.

3.3. Physico-mechanical characterization of geopolymers

Fig. 5shows the apparent density of the products obtained at the longest reaction times ateach temperature, that is 28 days at 25◦C, 3 days at 40◦C, 16 h at 60◦C and 5 h at 85◦C. Itis seen that the apparent density decreases with polycondensation temperature and is higher

Fig. 5. Apparent density of systems of types PS and PSS polycondensed at different temperatures/times.


Table 2Porosity and specific surface area of geopolymers obtained at 25◦C

Geopolymer Porosity (m3 g-1) Specific surface area (m2 g-1)

K-PS 0.0139 38.23K-PSS 0.0825 55.45Na-PS 0.0176 29.19Na-PSS 0.0811 44.32

for K-PS and K-PSS on one side in comparison to Na-PS and Na-PSS on the other side.A higher density of KOH-based products would be expected as potassium is heavier thansodium. No evidence is found of the effect of the type of polymer, as K-PSS products havea higher density than that of K-PS products, while Na-PS products show higher density thatNa-PSS products.

All density values lie within the range 1200–1600 kg m−3, so that lightweight productscould be manufactured starting from these inorganic polymers; for normal cementitiousbinders the density ranges between 1900 and 2300 kg m−3.

The products obtained after hardening at 25◦C were also submitted to porosity charac-terization. The data regarding specific surface area and specific pore volume are reportedin Table 2. These results show that a clear difference exists between systems of types PSand PSS. Both specific pore volume and surface area are greater for PSS type systems. Theeffect of added alkali metal is of lesser significance.

Fig. 6. Distribution of pores of systems polycondensed 28 days at 25◦C.


Fig. 7. Mechanical strength of systems of types PS and PSS polycondensed at different temperature/times.

Fig. 6shows the pore distribution with reference to three classes of size which, accord-ing to Anderson and Pratt (1985), are as follows: macropores (above 50 nm), mesopores(between 50 and 2 nm) and micropores (below 2 nm). Again, it is seen that a clear-cut dif-ference exists between PS and PSS type systems. In PS type systems micropores prevail,while mesopores prevail in the case of PSS type systems. As before, the effect of addedalkali metal is not significant.

Fig. 7shows the values of unconfined compressive strength measured on the same samplesin Fig. 5. It is seen that the effect of the type of polymer and alkali metal are correlated toapparent density; the products obtained with KOH show better strength and higher density.The effect of polymer type is not significant except for the products obtained at 60 and 85◦Cwith added NaOH. It is very remarkable that at 25 and 40◦C, K-PS and K-PSS products givehigher values than that required for cementitious pastes (32.5 MPa). Furthermore, the valuesof mechanical strength obtained at 25 and 40◦C with added NaOH are equally interestingfor the manufacture of pre-formed building blocks.

The remarkable decrease in strength observed at 60 and 85◦C (as well as the decreasein density) may be due to the heating in consequence of the exothermic polycondensationwhich can cause partial water evaporation with formation of micro cavities.

The observed favorable effect of KOH on strength is in agreement with literatureresults (van Jaarsveld et al., 1999; van Jaarsveld and van Deventer, 1999; Xu and vanDeventer, 2000; Xu et al., 2001). However, this effect can be only partially explained on aphysico-structural basis. In fact, while the density data are in agreement with the mechanicalstrength data, porosity and pore size distribution data are not. These are the same for Na-PSand K-PS products on one side and for Na-PSS and K-PSS products on the other. Equally,


differences in microstructural properties cannot be invoked, asPalomo et al. (1992)provedthat geopolymer-based materials are structurally featureless and substantially amorphous.

A possible explanation of the favorable effect of potassium on mechanical strength couldstem from an observation byMcCornick and Bell (1989). These authors consider that, dueto its larger size, potassium may favor a greater degree of polycondensation. However,our quantitative data on the amounts of reacted alkali metal silicate and water are notin agreement with this consideration. Therefore, it is more likely that the larger size ofpotassium has a favorable effect on crosslinking even if the polycondensation degree is leftunchanged.

4. Conclusions

The experiments have proved that it is possible to produce geopolymer-based good qualitybuilding materials through calcination and polycondensation of a kaolinitic residue.

As far as the calcination step is concerned, the use of DSC proved to be very usefulfor optimization purposes, due to the possibility to evaluate the reactivity of the productsobtained in the different calcination conditions tested.

The polycondensation step was carried out with potassium and sodium silicate and ad-dressed towards the formation of PS, PSS and PSDS products, but it was successful only forthe first two geopolymers. An original quantitative method was developed for the determina-tion of the quantities of metal alkali silicate and water reacted during the polycondensationstep.

The results of physico-mechanical characterization are highly appealing for the devel-opment of processes for the manufacture of both hydraulic binders and preformed buildingblocks. In this regard, the already known favorable effect of potassium was confirmed andfurther understanding was obtained from the polycondensation quantitative results.

The work is a contribution to the development of feasible pathways for the optimumexploitation of natural resources.

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Optimization of geopolymer synthesis by calcination and polycondensation of a kaolinitic residue

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