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Processing and characterization of calcined kaolin cement powder

Y.M. Liew a,⇑, H. Kamarudin a, A.M. Mustafa Al Bakri a, M. Luqman a, I. Khairul Nizar b, C.M. Ruzaidi a,C.Y. Heah a

a Center of Excellence Geopolymer System Research @ UniMAP, School of Materials Engineering, University Malaysia Perlis (UniMAP), 02600 Jejawi, Perlis, Malaysiab School of Environmental Engineering, University Malaysia Perlis (UniMAP), 02600 Jejawi, Perlis, Malaysia

a r t i c l e i n f o

Article history:Received 25 September 2011Received in revised form 14 December 2011Accepted 23 December 2011

Keywords:Cement powderCalcined kaolinGeopolymerizationNaOHSodium silicateAlkali activator

a b s t r a c t

This paper aimed at investigating the possibility of calcined kaolin to produce cement powder that couldbe an alternative to Portland cement by applying geopolymerization process. Cement paste was firstlymade by alkaline activation of calcined kaolin with alkali activator (mixture of 6–10 M NaOH and Na2SiO3

solution), heated in oven at temperature of 80 �C forming a solidified product, followed by pulverizationto fixed particle size powder. The parameters involved in this processing route (alkali concentration, cal-cined kaolin to activator ratio, alkali activator ratio and heating conditions) were investigated. For com-pressive testing, cement powder was added with water and then cured to produce cubes. Compressivestrength, microstructure, XRD and FTIR analysis were studied. Result showed that the processing routehas the potential to produce cement powder where SEM micrographs have proved that the geopolymer-ization process continued after addition of water forming a homogeneous structure and geopolymersbonding increased in intensity which was observed through IR analysis. It was believed that presencesof crystalline phase as seen in XRD diffractogram were good for mechanical properties.

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1. Introduction

Over the past 30 years, high quality and more clay products isproduced through many new and improved processes. This hasexpanded the traditional applications and has caused the emergingof many new applications. Kaolin is white or near white color withpseudo-hexagonal crystal along with plates, some larger books andvermicular stacks. Traditionally, kaolin has found applications inpaper coating, rubber filling, ceramic ingredients, cement and soon [1]. Properties of kaolin can be improved by thermal dehydroxy-lation process, which is manufactured by firing kaolin at 650–850 �C. The elimination of OH� group from kaolinite layer [2] in-creased the reactivity of metakaolin. The reaction of thermal dehy-droxylation is [3]:

2SiO2 � Al2O3 � 2H2O! 2SiO2 � Al2O3 � 2H2O "

Ongoing interest in use of selected clay minerals has broughtabout the use of kaolinite in construction industry. Metakaolin isutilized as an artificial pozzolanic additive for concrete to produceblended cement. This is due to the high pozzolanic reactivity ofmetakaolin, which is able to react with portlandite, Ca(OH)2 re-leased during the hydration of Portland cement and the miro-fillereffect which improved the packing of cement matrix [3]. Also,starting 1940s, the study on alkali-activated cement started. Alka-

li-activated system containing calcium silicate hydrated (CSH) andalumino-silicate phases is developed by Victor Glukhovsky andthen by Pavel Krivenko in the 1950s. It was firstly known as ‘‘soilsilicates’’ [4]. Palomo et al. have established two models of alkali-activated binding systems includes the alkali activation of blastfurnace slag (Si ± Ca) with mild alkaline solution, and alkali activa-tion (Si ± Al) using metakaolin and Class F fly ash with medium tohigh alkaline solutions. The former has calcium silicate hydrate(CSH) as main products while the latter has reaction products ofzeolite like polymers [2,5].

Later in 1972, Joseph Davidovits coined the name ‘‘geopoly-mers’’ [4] to describe the zeolite like polymers. Geopolymersare the alumino-silicates polymers which consist of amorphousand three dimensional structure formed from the geopolymeriza-tion process of alumino-silicates monomers in alkaline solution[6]. Investigations have been carried out on calcined clays (e.g.,metakaolin [7–13]) or industrial wastes (e.g., fly ash [14–18] ormetallurgical slag [19,20]). So far, geopolymerization processusing metakaolin has focused on the effect of various parameters,such as thermal treatment, concentration and time of reactionand later the comparison between different kaolinites was done[21].

Geopolymerization of geopolymers is a complex process and un-til now it is not fully understood [22]. A reaction pathway involvingthe polycondensation of orthosialiate ions (hypothetical monomer)is proposed by Davidovits [23] .

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⇑ Corresponding author. Tel.: +60 17 4968530.E-mail address: yun86_liew@yahoo.com (Y.M. Liew).

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According to researchers [4,24,25], three steps are suggested inthe geopolymerization process: (1) dissolution in alkaline solution;(2) reorganization and diffusion of dissolved ions with formation ofsmall coagulated structures; and (3) polycondensation of solublespecies to form hydrated products.

Geopolymers are well-known of its excellent properties com-pared to ordinary Portland cement (OPC), which are high compres-sive strength [26–28], low shrinkage [26,27], acid resistance[26,29], fire resistance and no toxic fumes emission [23], low ther-mal conductivity [26,27], excellent heavy metal immobilization[22], high temperature stability [22], low manufacturing energyconsumption for construction purposes and engineering applica-tion [26], etc. Thus, they are found potentially used in constructionengineering [22], fire proof [30], biomaterials [22], waste treat-ment [22] and so on. New applications are still being discovered.

Differed from the geopolymerization process, the process ofOPC manufacturing involves the calcination of limestone at hightemperature. It is an energy extensive process and emits largeamount of green house gas to the atmosphere. The production of1 ton of OPC releases approximately 1 ton of CO2 [31]. Thus, analternative material has to be found with less energy consumption,less carbon dioxide (CO2) emission and added properties to solvethe problem rose from OPC production. Now, it is accepted thatgeopolymers have emerged as an alternative to OPC.

In this paper work, a new processing route is introduced toproduce cement powder by applying geopolymerization process.Formerly, geopolymer mortars and concretes are formed directlyby the reaction of alumino-silicates sources with alkaline solution.Here, alkaline activation of calcined kaolin produces geopolymer ce-ment slurry. The solidified product is then crushed and ground intofine cement powder after the slurry being heated in oven at suitabletemperature. This processing route is developed based on the pro-duction and processing method of clay materials, where the claymaterials undergo watering, grinding, screening and de-wateringprocess to produce kaolin powder with only 10% moisture [32,33].Subsequently, in the pottery clay making, parts of hand-built vesselsare often joined together with the aid of an aqueous suspension ofclay body and water. Unfired clay from the earth can be dampenedwith water and is easy to manipulate and hold together for shapingpurposes. After a few hours, the damp clay hardens into a leather-hard yet somewhat elastic stage, at which time decorative designscan be cut or etched into the clay surface. Once air-dried, green-ware clay is very fragile and crumbles easily. When subjected tothe heat of a kiln (a high-firing oven), the clay piece becomes hardand permanent, and is capable of maintaining its form interminably.The firing process in the kiln converts the relatively weak greenceramic piece into a strong and durable product [33]. Once theceramic body is fired, the process is irreversible. As calcined kaolinis clay materials, they have the ability to mix with water and harden.For application purposes, the cement powder produced was addedwith water. Water plays crucial role in providing a transportationmedium for ions, the continual dissolution of residual startingmaterials and thus the polycondensation process as [34]:

Thus, the potential of this processing route to produce cementpowder was investigated in this work. The important parameters in-volved in the processing of cement powder such as sodium hydrox-ide (NaOH) concentration, calcined kaolin-to-activator (S/L) ratio,sodium silicate-to-sodium hydroxide (Na2SiO3/NaOH) ratio andheating conditions were studied. The relation of various molarratios, such as SiO2/Al2O3 ratios, SiO2/Na2O ratios and H2O/Na2Oratio with the compressive strength were discussed. This cementpowder, if succeeded, could be an alternative to OPC and could beused to produce mortar and concrete by addition of small amountof water.

2. Experimental method

2.1. Materials

Kaolin was purchased from Associated Kaolin Industries Sdn. Bhd, Malaysiawith minimum 40% particle size less than 2 lm. It was used as Si–Al cementitiousmaterials. Calcined kaolin was produced by calcination of kaolin at 800 �C for 2 h infurnace. Table 1 summarizes the chemical composition of kaolin as determined byX-ray Fluorescence (XRF).

Sodium hydroxide (NaOH) powder used was of 99% purity, made in Taiwanwith the brand name of Formosoda-P.

A technical grade sodium silicate (Na2SiO3) solution was purchased from SouthPacific Chemicals Industries Sdn. Bhd. (SPCI), Malaysia with chemical compositionof 30.1% SiO2, 9.4% Na2O and 60.5% H2O (modulus, SiO2/Na2O = 3.2), specific gravityat 20 �C = 1.4 g/cm3 and viscosity at 20 �C = 0.4 Pa s.

NaOH solution of desired molar concentration (6–10 M) was prepared and al-lowed to cool down to room temperature. Alkali activator solution was preparedby mixing Na2SiO3 solution and NaOH solution with ratio of 0.12–0.28, by mass un-til clear solution obtained. The solution was prepared for minimum 24 h prior to useto allow for equilibration.

2.2. Cement powder synthesis

Calcined kaolin was mixed well with alkali activator solution for 10 min byusing mechanical mixer, forming homogeneous slurry. The detail of mixture pro-portions is shown in Table 2. The fresh geopolymer paste was then poured into asteel molds measuring 50 mm � 50 mm � 50 mm. The geopolymer slurry werecompacted as described in ASTM C109 [35]. The molded samples were sealed witha thin film to prevent moisture loss. All specimens were heated undisturbed in oven

Table 1Chemical composition of kaolin.

Component Kaolin (%)

SiO2 54.0Al2O3 31.7K2O 6.05TiO2 1.41Fe2O3 4.89MnO2 0.11ZrO2 0.10LOI 1.74

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at 80 �C. The hardened geopolymer samples were then pulverized using Mortar andPestle, grinder and passed through screen mesh to obtain cement powder at fixedparticle size.

2.3. Characterization methods

2.3.1. Compressive testingThe compressive strengths were measured using the Instron machine series

5569 Mechanical Tester as refer to ASTM C 109/C 109M – 05 based on the basicprinciple of Force/Area on the 50 mm cube specimens [35]. For this research, thecompressive testing was done on cement pastes instead of cement mortars. The ce-ment powder was added with water (18%) to produce cement cube and tested withits compressive strength. The cement cube was cured in oven at 80 �C for 3 days.The samples were tested for 7th day strength. A minimum of three specimens weretested to evaluate the early strength gain for the specimens.

2.3.2. X-ray diffraction (XRD)XRD diffraction data were conducted using XRD – 6000, Shimadzu X-ray diffrac-

tometer with Cu Ka radiation and with auto-search/match software as standard toaid qualitative analysis.

2.3.3. Scanning electron microscope (SEM)JSM-6460LA model scanning electron microscope (JEOL) was performed to re-

veal the microstructure of kaolin, calcined kaolin and to observe the degree of reac-tion of geopolymerization process. The specimens were cut into small piece andcoated with platinum by using Auto Fine Coater, model JEOL JFC 1600 beforeexamination.

2.3.4. Fourier transform infrared spectroscopy (FTIR)Infrared spectra were recorded from 4000 cm�1 to 500 cm�1 using Perkin Elmer

FTIR Spectrum RX1 Spectrometer. The specimen for testing was prepared using KBrpellet technique. Small amount of potassium bromide (KBr) and geopolymer pow-der was put into a mold. By using cold press machine, mold which contains powderand KBr was pressed at 4 ton for 2 min to produce specimens for examination.

3. Results and discussion

3.1. Compressive strength

For compressive testing, cement powder was added with waterto produce cement paste. Fig. 1 presents the compressive strengthat 7th day of the resulted cement paste with different mixtureproportions as shown in Table 1. It was in evidence that the NaOHconcentration, S/L ratio, Na2SiO3/NaOH ratio and the heating condi-tions have great effect on the compressive strengths measured.

By comparing Mix 1, Mix 2 and Mix 3, it was obvious that lowNaOH concentration of 6 M does not favor the cement powderproduction. The compressive strength of resulted cement paste in-creased with NaOH concentration and dropped after an optimumat 8 M has achieved. This might probably due to lower dissolutionability of calcined kaolin at low NaOH concentration of 6 M and thuscausing insufficient Na+ ion to allow for complete polymerization ofthe network [6]. NaOH solution of 8 M provided optimum alkalinityfor dissolution of alumino-silicates sources, where sufficient Al3+

and Si4+ ions are released from the alumino-silicates and partici-pated in the geopolymerization process. Conversely, higher NaOHconcentration of 12 M showed lower compressive strength. Eventhough higher NaOH concentration has higher dissolution ability;however, it is not desired by the polycondensation process [22] asexcess Na+ ion left in the system weakens the structure [6]. Further-more, high alkaline solution caused the gel to set rapidly before itcan be transformed to a more homogeneous structure [36].

Both S/L ratio and Na2SiO3/NaOH ratio affected the workabilityof the mixture [13,22,37]. By comparing S/L ratio and Na2SiO3/NaOH ratio, as these ratios increased (from S/L ratio of 0.60 inMix 4 to S/L ratio of 1.20 in Mix 6; and from Na2SiO3/NaOH ratioof 0.16 in Mix 7 to Na2SiO3/NaOH ratio of 0.28 in Mix 10), work-ability of mixture decreased. Less workable mixture caused diffi-culty in compaction and molding process where failure inproviding good compaction may seriously reduced the compres-sive strength of the cement paste [13]. This was thus leading tothe lower compressive strength in Mix 5 and Mix 10. For Mix 6,the workability of the slurry is too low to allow for compactionprocess. No result was recorded for compressive strength of the re-sulted cement paste. In addition, for Mix 10 (higher Na2SiO3/NaOHratio), excess of Na2SiO3 may also hinders the evaporation of waterand structure formation [37]. For lower S/L ratio and Na2SiO3/NaOH ratio, excess of Na+ ions were believed to exist in structure,which will form sodium carbonate due to atmospheric carbon-ation. This will probably affect the geopolymerization processand result in decreased strength of cement paste. Nevertheless,there was no much research done on the effect of S/L ratio and Na2-

SiO3/NaOH ratio on the geopolymer synthesis and more investiga-

Table 2Detail of mixture proportions.

Mixture no. NaOH concentration (Molar) Calcined kaolin-to-alkaline activator solution ratio Na2SiO3-to-NaOHratio

Drying temperature (�C) Drying time (h)

1 6 0.80 0.24 80 32 8 0.80 0.24 80 33 10 0.80 0.24 80 34 8 0.60 0.24 80 35 8 1.00 0.24 80 36 8 1.20 0.24 80 37 8 0.80 0.16 80 38 8 0.80 0.20 80 39 8 0.80 0.20 80 4

10 8 0.80 0.28 80 311 8 0.80 0.20 80 6

Mix 1

Mix 4

Mix 7

Mix 8

Mix 2 Mix 2

Mix 8

Mix 9

Mix 3

Mix 5

Mix 2Mix 11

Mix 6Mix 10

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

NaOH concentrations

Calcined kaolin/activator

ratios

sodium silicate/NaOH

ratios

heating condition

Com

pres

sive

str

engt

h at

7th

day

(M

Pa)

Fig. 1. Compressive strength at 7th day with various mixture proportions.

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tions have to be carried out to better understanding the effect ofthese parameters.

As according to Fig. 1, heating condition at 80 �C for 4 h was morelikely to produce cement powder with higher strength (Mix 9). Thehard paste was crushed and ground to become powder after heating

process. At time, the paste was believed has just solidified. When thepaste was heated for longer period (Mix 11), it has completely hard-ened and formed well-coordinated framework, thus the addition ofwater could no more led to continuous dissolution and polyconden-sation reaction resulting in lower strength. Cement powder fromheating condition at 80 �C for 3 h (Mix 8) contained more moisturethan cement powder from heating condition at 80 �C for 4 h (Mix9). High moisture content might cause polycondensation processto continue beyond time [10] lead to hardening of the cement pow-der as in the case of OPC when exposing to atmosphere. When OPCwas left exposed to the atmosphere, Portland cement reacted withmoisture in hydration process to become hard. The process duringheating is complex and yet, the effect of heating condition on the ce-ment powder produced will be further investigated in later study.

To identify the key parameters influencing the processing of thecalcined kaolin cement powder, a gradation analysis was carriedout based on the compressive strength measured for various mixproportions. Table 3 represents the gradation analysis of thevarious important factors based on compressive strength. Fromthe result, the range of compressive strength by Na2SiO3/NaOH ra-tios was the highest followed by NaOH concentration, S/L ratiosand finally heating conditions. It can be concluded that Na2SiO3/NaOH ratios affect the mechanical properties the most; followedby NaOH concentration, S/L ratios and heating conditions.

Table 4 summarizes the effect of various molar ratios (i.e., SiO2/Al2O3, Na2O/SiO2, Na2O/Al2O3 and H2O/Na2O) on the compressivestrength of resulted cement paste. Previously, Davidovits [38]proposed that the ranges of oxide molar ratios to produce geopoly-mer may as the following: 0.2 6 Na2O/SiO2 6 0.28; 3.5 6 SiO2/Al2O3 6 4.5; and 15 6 H2O/Na2O 6 17.5. Barbosa et al. [39] whohave conducted a test on calcined kaolin geopolymers found thatNa2O/SiO2 ratio of 0.25, SiO2/Al2O3 ratio of 3.30 and H2O/Na2O ratioof 10.0 were the optimum chemical composition. Based on a studyon metakaolin geopolymers, best mechanical performance wereachieved when the ratio of SiO2/Al2O3 is 3.0 and Na2O/SiO3 ratio is0.25 [40].

Table 3Gradation analysis of the various important factors based on compressive strength.

Factors Mix. no. Compressive strength (MPa) Range

NaOH concentration 1 0.3 4.42 4.73 4.4

S/L ratios 4 1.8 4.22 4.76 0.5

Na2SiO3/NaOH ratios 7 3.8 5.48 5.7

10 0.3

Heating conditions 8 5.7 3.29 7.4

11 4.2

Table 4Effect of SiO2/Al2O3, Na2O/SiO2, Na2O/Al2O3 and H2O/Na2O molar ratios on compres-sive strength.

Mixno.

Molar ratios Compressive strength at 7thday (MPa)

SiO2/Al2O3

Na2O/SiO2

Na2O/Al2O3

H2O/Na2O

1 3.16 0.28 0.88 19.53 0.32 3.16 0.36 1.13 14.36 4.73 3.16 0.42 1.33 11.69 4.44 3.28 0.46 1.51 14.36 1.85 3.08 0.29 0.91 14.36 0.56 3.03 0.25 0.76 14.36 NA7 3.05 0.38 1.17 14.09 3.88 3.10 0.37 1.15 14.32 5.7

10 3.20 0.35 1.12 14.49 0.3

Fig. 2. XRD pattern of kaolin, calcined kaolin, cement powder and the resulted cement paste for Mix 9. (K = kaolinite; A = alunite; D = dickite; Q = quartz; Z = zeolite;HS = hydrosodalite; and S = sodalite).

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From the various important factors investigated in this study,the various oxide molar ratios of each mixture proportions werecalculated. For constant solids-to-liquid and Na2SiO3/NaOH ratios,increasing NaOH concentration increased the Na2O/SiO2 andNa2O/Al2O3 while decreased the H2O/Na2O molar ratio. The SiO2/Al2O3 molar ratio remains constant. When NaOH concentrationand Na2SiO3/NaOH ratio were kept constant, the H2O/Na2O molarratio remains constant while SiO2/Al2O3, Na2O/SiO2, and Na2O/Al2O3 molar ratios decreased as the S/L ratios were increased. Onthe other hand, for increasing Na2SiO3/NaOH ratios, the SiO2/Al2O3 and H2O/Na2O molar ratios increased whereas Na2O/SiO2

decreased. The highest compressive strength was obtained whenthe SiO2/Al2O3, Na2O/SiO2, Na2O/Al2O3 and H2O/Na2O molar ratioswere 3.10, 0.37, 1.15 and 14.23, respectively.

It was obvious that compressive strength increased when Na2O/SiO2 molar ratio was increased. With increasing Na2O concentra-tion, it was believed that the solubility of the alumino-silicatessource was enhanced as suggested by Guo et al. [16]. As accordingto the authors, the compressive strength decreased as the Na2Oconcentration was more than 10%. In this case, the strengthdegraded when the concentration of Na2O was more than only2%. As alkali cations are required for charge balancing of the Al3+

in the IV-fold coordination [41], Na2O concentration is very impor-tant for the extent of geopolymerization process.

As reported by previous researchers [40], low SiO2/Al2O3 molarratio (high Al2O3) yielded low strength products. According to theresearchers, high Al2O3 content leads to fast setting of the paste. Itwas believed that the increased setting of the paste enable the paste

to transformed into a more homogeneous structure which results ina lower compressive strength. The compressive strength was im-proved when the SiO2/Al2O3 molar ratio was increased until 3.10.

The optimum H2O/Na2O molar ratio was 14.23. Compressivestrength dropped when the H2O/Na2O molar ratios were higherthan 14.23. Based on Davidovits [42], water is carrier for the poly-condensation reaction whereby this reaction occurred betweencompounds that are soluble in water. Nevertheless, at high H2O/Na2O molar ratios, there are more water content which led toslower dissolution and reaction of the paste.

To sum up, NaOH concentration, alkali activator ratios and heat-ing condition has prominent influence on the processing of cementpowder. Na2SiO3/NaOH ratios affect the mechanical properties themost; followed by NaOH concentration, S/L ratios and heating con-ditions. The best mechanical performance to produce calcined kao-lin cement powder were achieved at SiO2/Al2O3, Na2O/SiO2, Na2O/Al2O3 and H2O/Na2O molar ratios were 3.10, 0.37, 1.15 and 14.23,respectively. These parameters must be emphasized and furtherin depth study when designing the synthesis of cement powder.

3.2. XRD analysis

Fig. 2 represents the XRD diffractogram of kaolin, calcinedkaolin, cement powder and the resulted cement paste (Mix 9).Dehydration by thermal treatment converts kaolin to calcined kao-lin, which is semi-crystalline and much more reactive than kaolin[43]. Kaolin showed two intense diffraction peaks at 2h value of12.5� and 25.2�, less intense peaks at 2h of 45.4�, 49.5�, 50.9� and

Fig. 3. XRD pattern of the resulted cement paste for various mixture proportions.

Fig. 4. SEM micrograph of (a) kaolin and (b) calcined kaolin (Mag. 5000�).

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59.9� and humps at 2h = 19.8–21.9�, 35.0–36.0� and 37.8–39.2�,which are all associated with kaolinite (K). Diffraction peaks ofquartz (Q) could be found at 2h values of 20.8�, 26.5�, 55.2� and62.3�. Alunite (A) and dickite (D) peaks appeared at 2h values of17.8� and 22.4�, respectively.

As according to previous research has found that calcined kaolinis not only a simple mixture of silica and alumina but retains somelong-range order due to the stacking of the hexagonal layers [44].After calcination, kaolin was transformed into calcined kaolin,which shows semi-crystalline to amorphous pattern. Most of thediffraction peaks of kaolinite disappeared and a halo at 2h from

15� to 30� owing to the amorphous silica appeared [45]. The XRDpattern of kaolin and calcined kaolin clearly showed the amorph-ization of kaolin produced during the thermal activation. However,some kaolinite peaks could still be seen. This implied that the kao-lin–calcined kaolin transformation was not complete [3]. Quartzand alunite phases were largely unreactive and remained in thecalcined kaolin. Here, XRD diffractogram also manifested the ther-mal stability of mineralogical impurities.

After alkali-activation of calcined kaolin with alkali activatingsolution, calcined kaolin showed marked shift in the scatteringpeak. There was a diffuse halo at about 20–40� 2h in both XRD dif-

Fig. 5. SEM micrographs of (a) cement powder and resulted cement paste for (b) Mix 2, (c) Mix 7, (d) Mix 8, (e) Mix 9, (f) Mix 10 and (g) Mix 11 and (h) zeolite crystalsobserved in the resulted cement paste.

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fractograms of cement powder and resulted cement paste. This istypical amorphous characteristic of geopolymers [34]. The diffusehalo shifted to higher angle values compared with calcined kaolinsuggesting that there is a structural arrangement occurred. Diffrac-tion peaks of quartz decreased in intensity most probably due to thedilution effect [46]. In XRD pattern of cement powder, few zeolite(Z) and hydrosodalite (HS) phases observed. After addition of waterto produce cement paste, these zeolite and hydrosodalite peaksincreased in intensity and few sodalite phases appeared. Accordingto Zuhua et al. [34], crystalline phase is destructive to the consistentdistribution of geopolymers which are contrary to Alvarez-Ayuso

et al. [47] which stated that the contact between zeolitic and amor-phous phase would increase the compressive strength. According tothe authors, presence of zeolite in geopolymers is frequent in theactivation of geopolymers. However, there is a limitation of thecrystalline phase that can be tolerated by the matrix. After the lim-itation, further increased crystallinity is detrimental to compressivestrength. In this work, it was verified that when the crystallinephase increased, the compressive strength increased (Fig. 3).

3.3. Microstructure

Microstructures of starting materials, cement paste synthesizedand the resulted cement paste were observed. SEM micrographs ofkaolin and calcined kaolin are shown in Fig. 4a and Fig. 4b, respec-tively. Both kaolin and calcined kaolin has plate-like structure. Theparticles appeared as plate forming a layer-like structure [3,48].Calcination of kaolin produces its dehydroxylation, but does notchange its plate-like structure [45]. However, the layer-like struc-ture was more open between layers [2]. The plate-like structurecontributed smaller surface area for geopolymerization processcompared to fly ash which has sphere microstructure. The changeprocess from kaolin to calcined kaolin can be conveniently moni-tored by using infrared spectroscopy (Fig. 6).

Fig. 5 displays the SEM micrographs of cement powder andresulted cement paste for various mixture proportions. Microstruc-ture of cement powder (Fig. 5a) contained large part of un-reactedraw materials in the system. After alkali activation in activatingsolution, the surface of calcined kaolin was slightly activated. Thegrowth of little sponge-like gel could be noticed on the surface ofcalcined kaolin. This proved that the geopolymerization reactionoccurred at the surface of calcined kaolin particles. After reactionwith water, the geopolymerization process continued to react form-ing more spherical aggregates (Fig. 5b–g). A more homogeneousstructure with more intervening materials were observed in Mix 9as shown in Fig. 5e and thus contributed to highest compressivestrength. Less un-reacted particle could be observed. This provedthat water takes part in the dissolution, hydrolysis and polyconden-sation reaction during geopolymerization as it provides medium forthe dissolution of alumino-silicates and the transportation of vari-ous ions, hydrolysis of Al3+ and Si4+ compounds and polycondensa-tion of different aluminate- and silicate-hydroxyl species [34]. Thisbrings to the continuous dissolution of residual solid particles andhydrolysis of generated Al3+ and Si4+ to form homogeneous struc-ture. XRD diffractogram reflected the presence of zeolite phase inthe cement paste; however, no zeolite crystals were observed by

Fig. 6. IR spectra of (a) kaolin, (b) calcined kaolin, (c) cement powder and theresulted cement paste for (d) Mix 2, (e) Mix 7, (f) Mix 8, (g) Mix 9, (h) Mix 10 and (i)Mix 11.

Table 5Summary of main FTIR absorption peak of kaolin, calcined kaolin, cement powder and the resulted cement paste.

Bonds Kaolin (cm�1) Calcined kaolin (cm�1) Cement powder (cm�1) Resulted cement paste (cm�1)

Mix 2 Mix 7 Mix 8 Mix 9 Mix 10 Mix 11

OH� 36883617

3442 33212164

3333 3326 3332 3343 3365 3368

H2O 1643 1645 1658 1656 1659 1645 1645 1645AlAO/SiAO 1391 1402 1415 1404 1395 1449 1394SiAO 1113

9941031

SiAOAT (T = Al or Si) 944 960 958 958 957 966 961AlIVAOH 907SiAO 799AlAO 781SiAOAT (T = Al or Si) 756 743 741 739 741SiAO 749

641Zeolite 664 662 662 665

554673 665

SiAOAAl IV 537 546 546 548 548 543 546 529 545

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SEM at low magnification. This suggested that these zeolite phasesoccurred as very small crystals (’2 lm) and was revealed in SEMmicrograph in Fig. 5h at magnification of 10,000�. The zeolites havecubic morphology [2,21]. It was believed that the alkaline activationof calcined kaolin caused the dissolution of silicate and aluminatespecies into solution. The dissolved Al will firstly react with thesilicate supplied by sodium silicate to form the silicate oligomers,and later grows and begins to crystallize forming zeolites [5].

3.4. FTIR analysis

Fig. 6 shows the IR spectra of kaolin, calcined kaolin, cement pow-der and the resulted cement paste for various mixture proportionsand Table 5 summarizes the main FTIR absorption peaks of kaolin,calcined kaolin, cement powder and the resulted cement paste ofvarious mixture proportions. Kaolin showed characteristic peaksat 3688 cm�1 and 3617 cm�1, corresponding to the OH- stretchingvibration. H2O stretching was also found at 1643 cm�1. Bands at1113 cm�1 and 994 cm�1 were assigned to SiAO bonds in the SiO4

molecules [49]. The other band at 907 cm�1 was attributed toAlIV-OH vibrations [2]. The bands at 799 cm�1, 749 cm�1 and641 cm�1 were SiAO symmetric stretching [50]. Absorption at537 cm�1 was assigned as SiAOAAlVI, where the Al is in octahedralcoordination [2,51].

After thermal treatment of kaolin at 800 �C for 2 h, the OH�

vibration peaks at 3688 cm�1 and 3617 cm�1 decreased suggestingthat the calcination of kaolin to calcined kaolin is not complete.The H2O stretching at 1645 cm�1 was absent after the thermaltreatment. Band of kaolin at 1113 cm�1 shifted to lower frequencyat 1031 cm�1, which was the amorphous SiO2 [52]. Absorptionband of AlIV-OH vanished owing to the distortion of the tetrahedraland octahedral sheets of kaolinite [2]. New band at 781 cm�1 ap-peared which attributed to the AlAO stretching vibration in AlO4

tetrahedral [53]. Besides, 537 cm�1 shifted to higher frequencyafter calcination process.

In cement powder, broad band was observed at 3321 cm�1

(OH� vibration). The intensity decreased in the resulted cementpaste. This meant that there are large amount of water adsorbedinto the surface or caught in the cavities in the geopolymer struc-ture in cement powder and expelled out from the structure afterthe curing process to form cement paste. The same observationwent to band at around 1645 cm�1. Thus, the addition of waterwas purposely for the continuous dissolution of the raw materialsand the hydrolysis and polycondensation process, and was ex-pelled out from the structure after curing. The absorption at2164 cm�1 corresponded to the stretching of OH under stronghydrogen bond, which disappeared in the IR spectrum of cementpaste. Bands at 1391 cm�1 and 1395 cm�1 were the asymmetricalstretching vibrations of AlAO and SiAO bonds [16], which repre-sent the absorption band of calcined kaolin. SiAOAT linkages oc-curred at 1031 cm�1 shifted to lower frequency at 944 cm�1 and957 cm�1. This indicated there are probably changes in the silicatenetwork whereby there are increasing of non-bridging oxygen insilicate sites and the increasing of Al substitution in the silicatenetwork suggested by Hajimohmmadi et al. [54]. These peaksshowed an increase in intensity from cement powder into resultedcement paste suggesting that the geopolymerization process con-tinued after the addition of water into the cement powder. Anothernew peak at 739 cm�1 presented in resulted cement paste, but ab-sent in cement powder were symmetrical vibration of SiAOATbonding of AlO4 and SiO4 tetrahedrons [47]. This again was theproof the geopolymerization. In the resulted cement paste, therepresented zeolite absorption band at 665 cm�1 and 554 cm�1

[21] which are also observable in XRD pattern in Fig. 4. Band at537 cm�1 in calcined kaolin shifted to higher frequency after the

geopolymerization process, which is 546 cm�1 in cement powder.This indicated the residual starting materials left in the system.

4. Conclusion

The main findings of this study can be summarized as follows:

(i) The processing route suggested has the potential to producecement powder. This process uses the geopolymerizationreaction. Calcined kaolin was firstly alkali-activated usingmixture of NaOH and Na2SiO3 solution and then heated inoven to get a solidified product. This was crushed into pow-der. For application purpose, the powder was added withwater (’18%). Water plays role in providing transport path-ways for continual dissolution of materials, hydrolysis andpolycondensation process, which in other words, continualgeopolymerization reaction.

(ii) NaOH concentration, calcined kaolin-to-alkaline activatorratio, Na2SiO3-to-NaOH ratio, heating temperature and timehave great effect on the mechanical properties of cementpowder. According to the gradation analysis, it was foundthat the key parameter influencing the properties of thecement powder was NaOH/Na2SiO3 ratio, followed by NaOHconcentration, S/L ratios and heating conditions.

(iii) The highest compressive strength was obtained when theSiO2/Al2O3, Na2O/SiO2, Na2O/Al2O3 and H2O/Na2O molarratios were 3.10, 0.37, 1.15 and 14.23, respectively.

(iv) SEM micrographs have proved that water plays importantrole in geopolymerization process and the geopolymeriza-tion process continued after addition of water to form amore homogeneous structure.

(v) Presences of crystalline phase are favorable to the mechani-cal properties as according to XRD analysis.

(vi) IR spectra showed that the geopolymers bonding (AlAOASiand SiAOASi) increased in intensity after the addition ofwater in producing the cement paste.

Acknowledgments

This work is supported by Center of Excellence GeopolymerSystem Research @ UniMAP. Also, the authors of the present workwish to dedicate their great thanks to KACST for funding this studythrough the collaboration between UniMAP-KACST.

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