Monitoring and Characterization of Creation of

Geopolymers Prepared From Fly Ash and Metakaolin

by X-Ray Photoelectron Spectroscopy MethodM. Kanuchova,a L. Kozakova,a M. Drabova,a M. Sisol,a A. Estokova,b J. Kanuch,c and J. Skvarlaa

aThe Technical University of Kosice, Faculty of Mining, Ecology, Process Control and Geotechnology, Institute of MontaneousSciences and Environmental Protection, Park Komenskeho 19, 043 84 Kosice, Slovakia; maria.kanuchova@tuke.sk(for correspondence)bThe Technical University of Kosice, Faculty of Civil Engineering, Vysokoskolska 4, 042 00 Kosice, SlovakiacThe Technical University of Kosice, Faculty of Electrical Engineering and Informatics, Letna 9, 042 00 Kosice, Slovakia

Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12068

The formation of geopolymers from fly ash (FA) andmetakaolin (MK) was investigated. The effect of differentreactivity of FA and MK was studied by X-ray photoelectronspectroscopy method. Geopolymer materials were obtained byan alkaline activation of aluminosilicate sources. The aim ofthis work is to develop methods for working properties predic-tion of geopolymer materials based on the starting materials.Selection of a geopolymerization scheme is found to be a keyfactor to realize a beneficial effect of improved reactivity.Finally, a descriptive model of the mechanism of geopolymerformation, taking into account the quality of starting materi-als was proposed. VC 2014 American Institute of Chemical Engineers

Environ Prog, 00: 000–000, 2014

Keywords: XPS measurements, fly ash, geopolymers,ecology

INTRODUCTION

In the last years, several experimental researches havebeen focused on an efficient low cost valorization of flyashes in large quantities, released by burning coal in powerstations and/or from storage with negative implications onthe environment.

There are two main constituents of geopolymers, namelythe source materials and the alkaline liquids. The sourcematerials for geopolymers based on alumina–silicate shouldbe rich in silicon (Si) and aluminum (Al). These could benatural minerals such as kaolinite, clays, etc. Alternatively,by-product materials such as fly ash (FA), silica fume, slag,rice-husk ash, red mud, etc. could be used as an additive oran admixture material. The choice of the source materials formaking geopolymers depends on factors such as availability,cost, type of application, and specific demand of the endusers.

The alkaline liquids are soluble alkali metals that are usu-ally sodium or potassium based. The most common alkalineliquid used in geopolymerization is a combination of sodiumhydroxide (NaOH) or potassium hydroxide (KOH) andsodium silicate or potassium silicate. During the process of

forming geopolymers, i.e. geopolymerization, aluminosilicatematerials are dissolved into alkali solution to form free SiO4

and AlO4 tetrahedral units.Several studies have investigated the use of different clays

for the formulation of geopolymer materials and it has beenshown that metakaolin (MK) is the ideal one because of itshigh reactivity and purity compared to other materials [1]. MKis used to replace a part of cement or as a source of newcementless materials. MK reacts chemically with hydratingcement to form modified paste microstructure. In addition toits positive environmental impact, it improves the concreteworkability, mechanical properties, and durability [2].

Alkali activation of MK is a way of making new cementi-tious materials (hydroceramic–ceramic-like materials synthe-sized from a solid aluminosilicate and an alkali-rich solutionat low temperatures, <100�C). MK is essentially an anhy-drous aluminosilicate that is produced by thermal decompo-sition of kaolin, a naturally occurring clay basicallycontaining kaolinite [Al2Si2O5(OH)4] and trace amounts ofsilica and other minerals. The hydroxyl ions are stronglybonded to the aluminosilicate framework structure so thatonly temperatures in excess of 550�C are capable of eliminat-ing them. During the dehydroxylation process, a consider-able atomic readjustment occurs. The final result is a partiallyordered structure that cannot rehydrate in the presence ofwater (or does so very slowly). Due to its disorder and X-rayamorphous nature, MK possesses a huge reactive potentialwhen in the presence of an alkali/alkaline earth containingsolution [3].

Davidovits [4] described the alkali activation of MK usinga polymerization model similar to that proposed to describethe formation of zeolites or zeolite precursors from alkali alu-minosilicate solutions. More recently, Palomo et al. [3] havefocused on the identification of the fundamental factors thatcontrol the synthesis of MK-based hydroceramics (alkali acti-vation at temperatures between 35 and 85�C), their charac-terization, and the evaluation of their usefulness in acommercial setting.

Geopolymers are members of family of inorganic poly-mers. The chemical composition of the geopolymer materialis similar to natural zeolitic materials, but the microstructureVC 2014 American Institute of Chemical Engineers

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is amorphous. The polymerization process involves a quitefast chemical reaction under alkaline conditions in Si–Al min-erals that results in a three-dimensional polymeric chain andring structure consisting of SiAOAAlAO bonds [5].

Although the exact process of geopolymerization is notsufficiently understood, often it can be identified with theproposed mechanism contained in four phases, namely [6]:1. Dissolution: release of Si and Al aluminosilicate from

solid materials in a strongly alkaline aqueous solution(e.g., NaOH),

2. Nucleation: formation of Si or Si/Al oligomers in theaqueous phase,

3. Gelation: condensation of oligomers and creation of athree-dimensional aluminosilicate structure,

4. Polymerization: particles joining to form the geopolymerstructure and to set the overall strengthening of the solidin the final polymer structure.Chemical mechanisms of a geopolymer can then be

described by the following chemical reactions [6]:1. Dissolution:

a. Alkalination and creation of tetravalent aluminum inthe next sialate-group:

Si-O-Al (OH)32Na 1

b. Alkaline dissolution by connecting OH2 anion to thesilicon atom, which then changes its coordination ofvalence to the pentavalent:

2. Nucleation:Bond cleavage of SiAOASi group to form anintermediate silanol SiAOH on one hand, and the pro-duction of basic siloxo SiAOA group on the other hand:

3. Gelation:a. Further formation of silanol SiAOH groups and isola-

tion of ortho-sialate molecules as the basic unit in theformation of geopolymer:

b. Reaction of the base unit with a cation Na1 and theformation of SiAOANa terminal group

4. Polymerization:a. Condensation of ortho-sialate molecules via reactive

groups of SiAONa and aluminum hydroxyl OHAAl,with production of NaOH and creating cyclo-tri-sialatestructure, releasing NaOH, which again reacts and thussupports the new polycondensation and formation ofthe poly-sialate chain:

b. Making ortho-sialato-disiloxo cyclic structure in thepresence of water glass (soluble Na-polysiloxonate),releasing NaOH which reacts again:

c. Polycondensation into Na-poly(sialate-disiloxo) frame-work

During the chemical reaction occurring in the formationof geopolymers (reaction 4a), the released of water can beseen. This water, expelled from the geopolymer matrix dur-ing the curing and further drying periods, leaves behind

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nanopores in the matrix, which provide benefits to the per-formance of geopolymers. The water in a geopolymer mix-ture, therefore, plays no role in the chemical reaction; itmerely provides the water in a Portland cement concretemixture during the hydration process.

X-ray photoelectron spectroscopy (XPS) is a highly sensi-tive technique well suited for examining the compositionand chemical states of a surface. When a material is bom-barded with X-rays, photoelectrons may be emitted from thetop-most surfaces (typically 1–10 nm). All elements yieldphotoelectrons of a specific binding energy, enabling ele-mental analysis. Furthermore, small changes in the chemicalbonding environments result in small shifts in the photoelec-tron energy, thus allowing chemical information to beobtained. Among the chemical properties which may beevaluated from the binding energies are the oxidation state,the nearest neighbor atoms, and a type of bonding [7].

This article shows that XPS study is capable of providingkey information regarding initial setting and later transforma-tions taking place in geopolymer system. Recent works[8–12] describe process of geopolymerization by FTIR andNMR spectroscopy. However, they allow differentiation ofvarious types of bonds in a material only on a molecularlevel. In the present article, the XPS spectra of the inorganicpolymers confirmed previous works and showed the excel-lence of this method.

In this study, the geopolymerization phenomenon wasinvestigated using XPS method to compare geopolymers pre-pared from two different starting materials, namely MK andFA. The reported experimental work indicated that XPS anal-ysis may be used to obtain detailed information on thehydrolysis and condensation reactions occurring during syn-thesis of the inorganic polymers.

EXPERIMENTAL

Materials and ChemicalsTwo different materials were used in this study. MK is a

known standard material for formation of geopolymers. It is

an excellent source of aluminosilicate that is very reactive inthe presence of alkali solution. In contrast to MK, FA is anindustrial waste that is very inhomogeneous. Therefore, MKgeopolymers are often used as “model systems” by which FA-based geopolymers may be better understood. We chose thesematerials for better understanding of the processes of geopol-ymerization. MK was purchased from �CLUZ (Czech Republic)under the brand name of Mefisto K05. It was made by calcina-tion of kaolin at 750�C in rotary kiln. The calcination periodcannot be disclosed because it is the intellectual property ofthe �CLUZ (Czech Republic) company. FA was derived fromblack coal combusted in melting boilers of a District HeatingPlant in Ko�sice (Slovakia) at the temperature 1400–1550�C.Chemical composition of MK and FA determined by X-ray flu-orescence (XRF) spectrometer is shown in Table 1. Due to therelatively low calcium content, this FA should be classified asClass F according to the ASTM C618 definitions. The BET sur-face area of MK and FA, as determined by nitrogen adsorptionin Gemini 2360 (Micrometrics, USA) instrument, is 10.65 31023 m2/kg and 3.84 3 1023 m2/kg, respectively. The meanparticle size (d50) is equal to 3.87 mm and 19.29 mm, respec-tively, measured by a Diffraction Spectrometer Helos 12 LA(Sympatec GmbH, Germany).

Sodium hydroxide in a spherical form (Kittfort Praha Co.,97–99.5% purity) and sodium water glass (LARO, v.o.s.,molar ratio SiO2-to-Na2O 5 3.37, 36–38% of Na2SiO3,q 5 1336 kg/m3) were used for the synthesis of the geopoly-meric materials in this study.

Geopolymer PreparationAlkaline silicate solution with silicate modulus (SiO2/Na2O)

of 1.39 was prepared by dissolving the solid sodium hydrox-ide in sodium water glass until clear. MK or FA was mixedwith the alkaline silicate solution in a mechanical mixer for 7and 10 min respectively—this time was enough for homoge-neous paste formatting. In order to obtain a good workabilityfor the next working and manipulation, the liquid/solid bulkratios were 0.23 (MK product) and 0.26. Small samples wereprepared from fresh geopolymer pastes for XPS tests.

The mixtures were molded into prismatic molds withdimensions 40 mm 3 40 mm 3 160 mm (Figure 1), vibratedfor 5 min (Figure 2) to remove entrapped air bubbles and

Figure 1. Prismatic molds of geopolymers from FA (dark)and MK (light). [Color figure can be viewed in the onlineissue, which is available at wileyonlinelibrary.com.]

Figure 2. Vibrating table. [Color figure can be viewed in theonline issue, which is available at wileyonlinelibrary.com.]

Table 1. Chemical composition of metakaolin (MK) and fly ash (FA).

Wt % SiO2 Al2O3 Fe2O3 TiO2 MnO CaO MgO K2O Na2O P2O5 SO3 LOI*

Metakaolin 60.91 38.77 0.68 0.57 — 0.27 0.56 1.00 — 0.01 0.10 2.29Fly ash 44.27 24.01 9.67 2.91 0.13 2.36 0.97 1.18 0.90 0.46 0.40 19.81

*LOI 5 Loss on ignition.

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cured in a hot-air drying chamber at temperature of 22�Cfor 20 h, under atmospheric pressure and uncontrolledhumidity conditions. The FA paste, unlike the MK paste, didnot get hard under the aforementioned curing conditions.Therefore, it was cured at 80�C for next 6 h, duration ofcured was given by the experimental practice. Hardenedsamples were removed from the forms, marked and storedat ambient temperature until the flexural and compressivestrength test were applied. The flexural and compressivestrength of the hardened samples was measured after 1, 7,and 28 days using the hydraulic machine Form1Test MEGA100–200-10D (Table 2, Figure 3). The experimental processof preparation is listed in Figure 4.

Sample Characterization by XPSThe surfaces of geopolymers from the FA and MK were

analyzed by XPS. XPS measurements were performed usingXPS instrument (SPECS) equipped with PHOIBOS 100 SCDand non-monochromatic X-ray source. The survey surfacespectrum was measured at 40 eV transition energy and corespectra at 50 eV at room temperature. All spectra wereacquired at a basic pressure of 2 3 1028 mbar with MgKa

excitation at 10 kV (150 W). The data were analyzed bySpecsLab2 CasaXPS software (Casa Software). A Shirley andTougaard type baseline was used for all peak-fits. The sam-ples of geopolymers were first kept in a vacuum chamberfor about one day to remove water and then put into thedetector chamber under high vacuum to start the XPS mea-surement. The spectrometer was calibrated against silver (Ag3d). All samples showed variable degrees of charging due totheir insulating nature. The problem was resolved by the cal-ibration on carbon. The XPS of Si 2p, O 1s and Al spectra ofthe inorganic polymers were recorded in order to obtaininformation on the elemental and chemical composition.

Sample Characterization by ThermogravimetricAnalysis (TGA)/Differential ThermogravimetricAnalysis (DTA)

Thermal properties of prepared samples were studied bySTA 449F3 Thermo-analyser (Netzsch, Germany) in the tem-perature range from 26 to 800�C with the heating rate of10 K/min under nitrogen atmosphere using DSC/TG mode.The samples of about 18.0 mg in weight were heated inAl2O3 crucibles.

Measurement ProcedureThe geopolymer prepared from the MK and FA was inves-

tigated by XPS method daily for 28 days. We determined thedependence of amount of released electrons from the bind-ing energy measurements. The amount of released waterwas studied by TGA analysis in the second day. The flexuraland compressive strength of the hardened samples preparedfrom FA and MK was measured after 1, 7, and 28 days usingthe hydraulic machine.

RESULTS

The surface analysis carried out to characterize the C, O,Si, and Al is shown in Figures 5 and 6 for MK and FA,respectively. The binding energies for each material reflectexactly the chemical nature of the subunits of which it iscomposed and the position of peaks of Si, Al, C, and Oatoms is very important. The analyzed material contains O1s, C 1s, and two states of atoms Si—Si 2s and Si 2p. Alumi-num is detected as Al 2s and Al 2p.

From Figures 5 and 6, we can see that the MK has higherproportions of Si and Al than the FA. In addition, the peaklines of O 1s, C 1s, Si, and Al atoms in MK are sharper thanthose of the FA, indicating a greater purity of MK. Peak posi-tion of Si 2p in MK at 103 eV indicates the presence of SiO2

with good agreement with Barr and Seal [13] where the posi-tion of Si 2p is at 102.4 eV. Presence of Al2O3 is indicated bythe peak at 75 eV (74.3 eV for MK in Ref. 13).

The geopolymer prepared from the MK and FA was inves-tigated daily for 28 days. As mentioned earlier, the mecha-nism of the geopolymerization takes place in four steps:dissolution, nucleation, gelation, and polymerization. Whendissolving the hydroxide ions, they react with the surface ofthe aluminosilicates and disrupt covalent bonds SiAO andAlAO in SiO2 and Al2O3. There is a gradual release of Si andAl into the solution. At the beginning of the creation of geo-polymer, aluminum dissolves faster than silicon, resulting inthe formation of a silica layer on the particles of the material.Aluminum dissolved in water is adsorbed on the surfaceareas of these layers causing passivity of the surface. Alsoions of Al and Si dissolved in a solution formSiAOAAl(OH)3

2 anions, which subsequently react with theNa1 cations. This process equilibrates the charge balance onthe surface, eliminating negative surface charges and ena-bling formation of the monomers M1SiAOAAl(OH)3

2.The coordination of aluminum atoms changes from octa-

hedral to tetrahedral. Decomposition of SiO2 and Al2O3 wasalready observed in the first day of the creation of geopoly-mer, because the position of peaks of atoms changed (Table3). It is in agreement with the point 1 in process of geopoly-merization—dissolution (Eqs. 1a and 1b).

Table 2. Values of compressive and flexural strength of MK and FA based geopolymers.

Alkali activated material

Compressive strength (MPa)Flexural strength

(MPa)

1 day 7 days 28 days 1 day 7 days 28 days

Metakaolin* 54.3 69.1 70.2 7.3 8.4 5.5Fly ash** 43.0 44.6 50.2 7.3 7.2 6.6

*Cured at temperature of 22�C for 20 h.**Cured at temperature of 22�C for 20 h and next 6 h at temperature 80�C.

Figure 3. Measurement of the flexural and compressivestrength. [Color figure can be viewed in the online issue,which is available at wileyonlinelibrary.com.]

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In the process of geopolymerization, the amount and ratioof Si and Al plays an important role. As a source of solubleSi we used sodium water glass. The amount and ratio of Siand Al optimizes the speed of response, and can alsoincrease the level of geopolymerization. The required ratioof Si/Al has a positive effect on the strength of aluminosili-cate gel and also it helps to dissipate unreacted ash particles.In our work, we used different ratios of alkaline silicate solu-tion/solid bulk—0.23 (MK product) and 0.26 (FA product)for optimizing the preparation of both products. The alumi-nosilicate gel contains Al atoms with different coordinationnumbers. The coordination number of Al can cause differentreactivity of materials. Aluminum, which has the tetrahedralstructure, has a lower binding energy than that in the octahe-dral structure. The lower the binding energy, the smaller theenergy required to break the binding which causes an easierrelease of atoms [8]. In Table 4 we can see a shift in thebinding energy toward the lower values. It confirms changeof coordination of atoms of aluminum. The shift in the bind-ing energy toward the higher values was seen on the lastday of measurement.

From the literature it is known that in terms of geopoly-mer formation, the 7th day is characteristic for creation ofnew structures. In this phase, the amount of silanol groupsincreases—we can observe forming oligomers in the

aqueous phase and new chains. This is in agreement withthe point 2 in the process of geopolymerization—nucleationand with Eq. 2.

Oxygen plays an important role in the formation of geo-polymer. Four types of bonds exist in the geopolymer:SiAOASi, SiAOH, SiAOANa (only electrostatic interactionforces), and SiAOAAl. The type of bonds changes in theprocess of polymerization and the length of SiAOAAl chainsincreases, in accordance with the results published in Refs.[14,15]. Miyaji et al. [16] have investigated the surface of sili-cone substrates treated with 2, 5, and 10M NaOH using XPS.In their work, the obtained O1s spectra of the silicone sam-ples showed a dominant peak located at 532 eV, which wasassigned to siloxane (SiAOASi) bonds in the siliconeskeleton.

Figure 5. Energy spectrum of MK (magnesium anode).[Color figure can be viewed in the online issue, which isavailable at wileyonlinelibrary.com.]

Figure 6. Energy spectrum of FA (magnesium anode). [Colorfigure can be viewed in the online issue, which is availableat wileyonlinelibrary.com.]

Table 3. Core level binding energies (eV) in MK, in geopoly-mer prepared from MK, in FA, and in geopolymer preparedfrom the FA on the 1st day.

XPS line MK GP from MK FA GP from FA

Si 2p 103.00 101.5 105.5 101.5Al 2p 75.00 74.00 77.5 74.5

Figure 4. The experimental process of geopolymers preparation.

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The samples treated with 5 and 10M NaOH for more than3 h showed two additional peaks located at 533 and 531 eV,which were assigned to SiAOH and SiAONa, respectively[15]. They observed the formed amount of SiAOH andSiAONa increases with increasing time of NaOH treatment[15]. In our case for geopolymer from MK in the first day, thepeak position of O 1s is at 531 eV. However, additionalpeaks at 529.461 eV and 533.228 eV for SiAOH andSiAOANa, respectively, as well as a peak at 531.336 eV forO 1s in SiAOASi were also observed. These results areshown in Figure 7. For geopolymer from FA positions at529.462 eV for SiAOANa, at 533.899 eV for SiAOH, and at531.390 eV for SiAOASi were located. The differencesbetween the geopolymers from MK and FA can be seen bycomparison of Figures 7 and 8. These differences are caused

by impurities in FA and the peak positions are in goodagreement with the results by Miyaji et al. [16]. The differen-ces in binding energies of SiAOH, SiAOASi, and SiAOANacan be understood in terms of simple electrostatic interac-tions. There are, on average, fewer electrons at the oxygenatom in SiAOH than in SiAOASi, due to the fact that thehydrogen atom has a larger electronegativity than the siliconatom. Thus, the electron–electron repulsion at the oxygenatom decreases, hence the increase of the binding energy ofthe electrons [11].

The changes between geopolymer from MK and from FAon the 7th day are shown in Figures 9 and 10. For geopoly-mer from MK, we can see higher content of SiAOH groupsthan in geopolymer from FA. It means that geopolymeriza-tion in geopolymer from MK is faster and more active than

Table 4. Positions of peaks of Al 2p (eV) in geopolymer from metakaolin (GPM) and in geopolymer from fly ash (GPF) moni-tored from the 1st to the 28th day.

XPS line Binding energy (eV) XPS line Binding energy (eV)

Al 2p for MK 75 Al 2p for FA 77.5Al 2p for GPM in the 1st day 74 Al 2p for GPF in the 1st day 74.5Al 2p for GPM in the 7th day 74 Al 2p GPF in the 7th day 74Al 2p for GPM in the 23rd day 74 Al 2p GPF in the 23rd day 73.5Al 2p for GPM in the 28th day 74.5 Al 2p GPF in the 28th day 75

Figure 7. O 1s of geopolymer from MK (1st day).

Figure 8. O 1s of geopolymer from FA (1st day).

Figure 9. O 1s of geopolymer from MK (7th day).

Figure 10. O 1s of geopolymer from FA (7th day).

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Figure 11. TGA of geopolymer from MK (the 2nd day).

Figure 12. TGA of geopolymer from FA (the 2nd day).

Figure 13. Changes of Na 1s for geopolymer from MK.

Figure 15. Changes of O 1s of geopolymer from MK in the 1st and 23rd day of geopolymerization. [Color figure can beviewed in the online issue, which is available at wileyonlinelibrary.com.]

Figure 14. Changes of Na 1s for geopolymer from FA.

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in geopolymer from FA. This is confirmed also by TGA anal-ysis in the second day. The amount of released water ishigher for geopolymer from MK (Figures 11 and 12). Weightloss for geopolymer from MK is 16.36% and for geopolymerfrom FA it is only 11.78%.

The 15th day is also important. We observe a decrease inthe content of Na (Figures 13 and 14). There are no changes inpeak positions of Al atoms but in both geopolymers we see asignificant decline in size of the peak of aluminum atoms—Al2p. This indicates a beginning of oligomer condensates andcreation of a three-dimensional aluminosilicate structure(point 3 in the section of polymerization, Eqs. 3a and 3b.

This may be caused by surface passivity due to dissolu-tion of aluminum in alkaline solution. On the 23rd day wecan see a significant decrease of oxygen, which is related towater evaporation and to solidification of the geopolymer.The changes in content of oxygen in both geopolymers canbe seen in Figures 15 and 16. On the last day of observation(the 28th day), we see a significant change in the peak posi-

tions of aluminum atoms, which can be attributed to the sta-bilization of geopolymer structure. Barr et al. [17] reportedthat aluminum binding energies depend on coordinationnumber. Tetrahedrally coordinated aluminum generally has alower binding energy than octahedrally coordinated alumi-num which is in good agreement with our measurements. Ingeopolymer made from MK, we measured position for Al 2pat 75.00 eV and it changed to 74.5 eV on the 28th day. Forgeopolymer made from FA, the position for Al 2p was at77.5 eV and it changed to 75 eV on the 28th day.

DISCUSSION

In our X-ray photoelectron microscopy spectra, weobserved differences in formation of geopolymers from twodifferent sources—MK and FA from boilers. Progressive mon-itoring from the 1st to the 28th day showed that the forma-tion of the polymer chain is faster in geopolymer preparedfrom MK. From the obtained spectra of the two samples, we

Figure 16. Changes of O 1s of geopolymer from FA in the 1st and 23rd day of geopolymerization. [Color figure can be viewedin the online issue, which is available at wileyonlinelibrary.com.]

Figure 17. Amount of Al 2p and Al 2s (%) for geopolymerfrom MK. [Color figure can be viewed in the online issue,which is available at wileyonlinelibrary.com.]

Figure 18. Amount of Al 2p and Al 2s (%) for geopolymerfrom FA. [Color figure can be viewed in the online issue,which is available at wileyonlinelibrary.com.]

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can say that an important change in reaction mechanismoccurs on the 1st, 7th, 15th, 23th, and 28th day. Already onthe first day of formation of geopolymer we can see changesin the peak positions of Si and Al atoms, which indicate thedissolution of the elements in alkaline solution (in accord-ance with Eqs. 1a and 1b). On the seventh day, the peakpositions of atoms do not change too much, but the spec-trum changes its shape, suggesting formation of intermediatesilanol SiAOH group and production of basic siloxoSiAOASi group (in accordance with Eq. 2). The significantdecrease of sodium on the fifteenth day indicates formationof polymer chains in oligomers (in accordance with Eqs. 3aand 3b).

Solidification of the sample indicates a significantdecrease of oxygen on the 23rd day of geopolymerization(Figures 15 and 16). Peak positions of atoms of C, O, Si, andAl confirm condensation of ortho-sialat molecules via reactivegroups of Si and SiAONaAOAAlAOH, with formation ofNaOH and creating cyclo-tri-sialat structure by releasingNaOH. This NaOH again reacts and thus supports the newpolycondensation and the formation of the poly-sialat chain[6] (in accordance with Eqs. 4a–4c).

The differences between the two geopolymers are visiblefrom differences in their spectra. The biggest difference inmaking the geopolymers is the decline of the size of the peakof Al 2p and the growth of Al 2s peak (Figures 17 and 18).

CONCLUSION

Geopolymers can be considered as promising materialssatisfying demanding technical criteria. Also they allowachieving significant environmental benefits; therefore it isnecessary to know the exact composition and properties ofall their raw materials. Using XPS it is possible to determinethe suitability of fly ashes in the process ofgeopolymerization.

In this work, chemical properties of two different geopol-ymers—made from FA, and from MK—were studied. Sam-ples were analyzed for a period of 28 days. During this time,changes in peak positions of atoms of oxygen, carbon,sodium, and aluminum were observed. These changes werevisible already from the first day. Changes in peak positionsof Si and Al atoms clearly demonstrate the dissolution ofSiO2 and Al2O3 in alkaline solution. It is known from chemi-cal analysis of materials that FA contains much more impur-ities than MK. These impurities slow down significantlychemical reactions in the process of geopolymerization andcause insufficient dissolving of aluminum and silicon oxides.In terms of forming the geopolymer chains this phenomenonis undesirable. It is because aluminum and silicon atomsreleased during activation from alkaline oxides are indispen-sable building blocks in the polymer structure. The greaterpurity of MK suggests sharper peaks in the XPS spectra. Thisinvestigation proved that MK is better than FA in geopoly-mers, but kaolin needs more energy to convert it to MK,whilst FA is a waste and does not need any energy. So FA ismore cost-effective than MK. Thus, FA may be recommendedfor preparation of the geopolymers. It will save energyrequired for calcination and at the same time it will eliminatethe waste and pollution.

ACKNOWLEDGMENTS

The research has been supported by the Slovak Agencyfor Research and Development (APPV) (Grant No. 0423-11)and by the Scientific Grant Agency (VEGA) of the Ministry ofEducation of the Slovak Republic and the Slovak Academy ofSciences, Grant No. 1/1222/12 and Research excellencecentre on earth sources, extraction and treatment - 2nd

phase supported by the Research & Development Opera-tional Programme funded by the ERDF.

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Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep Month 2014 9