Materials Letters 64 (2010) 2551–2554

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Materials Letters

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Influence of calcination temperature of kaolin on the structure and properties offinal geopolymer

M.R. Wang, D.C. Jia ⁎, P.G. He, Y. ZhouInstitute for Advanced Ceramics, Department of Materials Science, Harbin Institute of Technology, Harbin 150080, China

⁎ Corresponding author. Tel.: +86 451 86418792; faxE-mail address: dcjia@hit.edu.cn (D.C. Jia).

0167-577X/$ – see front matter. Crown Copyright © 20doi:10.1016/j.matlet.2010.08.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 April 2010Accepted 4 August 2010Available online 6 August 2010

Keywords:CeramicMechanical propertiesThermal propertiesX-ray techniques

In this paper, the structure of two types of metakaolins from kaolin calcined at 800 and 900 °C, respectively,and the obtained geopolymer were systematically characterized. It was found that calcination temperaturehad little effect on the environment of silicon atoms but had great effect on that of aluminum ones. 27Al NMRanalysis showed that tetrahedral aluminums in the metakaolin from kaolin calcined at 800 and 900 °C werein different environment, of the type AlQ3(3Si) and AlQ4(4Si), respectively, leading to different environmentof aluminum atoms in the resulted geopolymer. Aluminum atoms in the geopolymer based on metakaolinfrom kaolin calcined at 800 °C were in the types of tetrahedral and octahedral, and silicon atoms were in thetypes of tetrahedral Q4(3Al) together with a small amount of Q4(0Al). However, geopolymer based onmetakaolin from kaolin calcined at 900 °C consisted of Q4(4Si) unit aluminum and Q4(3Al) unit silicon. Theresults revealed that the calcination temperature had a great effect on environment of the aluminum atomsof the metakaolin, thus led to the different structure and properties including mechanical strength andthermal conductivity of the post obtained geopolymer.

Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

Table 1

1. Introduction

Studies on geopolymer have been carried out for several decades.They are of scientific interests in recent years due to the two majoradvantages: low energy consumption and no CO2 emission in thepreparation process. At present, there are mainly three kinds ofaluminosilicate sources for the preparation of geopolymer: i) metakao-line (calcined kaolin) [1–3]; ii) fly ash [4,5]; iii) chemically synthesizedkaolin [6,7]. The geopolymer is prepared by polycondensation below100 °C at ambient pressure in an alkaline solution. However, there areno literature which discuss the effect of kaolin calcination at elevatedtemperatures such as 800–900 °C on the properties of the post obtainedgeopolymer. Only Lecomte et al. [8] reported that Vickers hardnessvalues of geopolymers improved with kaolin calcination temperatures(700–800 °C), which increased from 191±6 to 217±5 MPa, and thereason for this case was not analyzed yet and was still unclear.Therefore, in this paper, metakaolins were obtained by calcining kaolinat 800 and 900 °C, respectively, and the geopolymers based on the twometakaolins were prepared. It was found that the properties ofgeopolymer based on metakaolin from kaolin calcined at 900 °C weredifferent from that reported in previous research. The effect ofcalcination temperature of kaolin on properties of geopolymer was

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analyzed by X-ray diffraction (XRD) and solid state nuclear magneticresonance spectroscopy (NMR).

2. Experimental

2.1. Sample preparation

The geopolymer used in this study had a composition withnominal molar ratios of SiO2/Al2O3=4.0, K2O/Al2O3=1.0, H2O/K2O=11.0, as described in the literature [9]. Kaolin powder with anaverage particle size of 4.08 μm (Fengcheng Reagent Factory, China,90%) was calcined at 800 °C and 900 °C for 4 h, respectively, to obtainmetakaolin, denoted as 800MK and 900MK, respectively. A typicalprocessing route for final geopolymer was as follows: a potassiumsilicate solution was made by dissolving fused silica (ShanghaiDixiang Indus, China, mass 40%) into a KOH (Tianjin Fuchen Indus.,China, Analytical pure) solution with a magnetic stirrer. The

Properties of G-800MK and G-900MK.

Sample Bulk density(g/cm3)

Flexuralstrength(MPa)

Compressivestrength (MPa)

Thermalconductivity(W/(m·K))

G-800MK 1.43±0.03 8.7±0.6 73±1.5 0.23±0.02G-900MK 2.29±0.02 32.1±1.2 111±2.4 1.10±0.02

ghts reserved.

Fig. 1. XRD patterns of the kaolin (a), 800MK (b), 900MK (c), respectively.

Fig. 2. 27Al NMR spectrums of kaolin, 800MK, 900MK, G-800

2552 M.R. Wang et al. / Materials Letters 64 (2010) 2551–2554

metakaolins were added to the potassium silicate solution and mixedfor 30 min with a high-shear rate mixer. After being poured into aplastic box, the slurry was cured at 80 °C in a standard curing box. Thegeopolymer based on 800MK and 900MK were denoted as G-800MKand G-900MK, respectively.

2.2. Sample characterization

Powders of metakaolins and geopolymers were characterized byXRD (Rigaku D/max-B computer-controlled diffractometer with CuKα radiation) and 27Al and 29Si solid state NMR at 9.4 T (BrukerAvance III400 spectrometer at 5 kHz for Al and 12 kHz for Si). 27Al and29Si NMR spectra were recorded with 2 and 1 s delay time,respectively. Chemical shifts are referenced to tetramethylsilane forthe 29Si nuclei and to AlNO3 for the 27Al nuclei.

Density of the as-prepared geopolymer was determined byArchimedes' method. Flexural and compressive strengths were testedusing an Instron-500 type universal testing machine with a crossheadspeed of 0.5 mm/min. Thermal conductivity measurements of thesamples were carried out using a laser flash diffusivity technique(Netzsch LFA-427, Laser Flash Thermal Constant Analyzer, Germany)

MK and G-900MK, respectively, * spinning side bands.

2553M.R. Wang et al. / Materials Letters 64 (2010) 2551–2554

at room temperature in an argon atmosphere. The value of thermalconductivity (λ) for samples is determined from

λ = ρacp ð1Þ

where ρ is the density, a thermal diffusivity, and cp the heat capacityof samples. Data were averaged on three measurements for eachsample.

3. Results and discussion

Bulk density, flexural strength, compressive strength and thermalconductivity values of G-800MK and G-900MK were presented inTable 1. As shown, the bulk density of G-800MK is similar to that ofliteratures [1,10]. In contrast, the bulk density of G-900MK is 60.1%higher than that of G-800MK and the properties of G-900MK also havean obvious increase compared with that of G-800MK. It implies thatcalcination temperature of the starting kaolin has a great influence onthe properties of the final geopolymer.

The 800MK and 900MK are X-ray amorphous (Fig. 1) withdifferent large diffuse peaks centered at 22 and 21°, respectively,which indicates that the short range ordering of 900MK is differentfrom that of the 800MK. In addition, small amplitude reflections areapparent at 36.4, 45.8 and 66.8° in the XRD pattern of 900MK (Fig. 1c),indicating that kaolinite in 900MK completely transforms from

Fig. 3. 29Si NMR spectrums of kaolin, 800MK, 900MK, G-800

starting crystalline phase into amorphous one. Meanwhile, aftercalcination the residual crystalline quartz still can be observed.According to a literature [11], the existence of α-quartz will notprevent the geopolymerization reaction.

In order to further prove the change of short range structure, solidstate NMR analysis was performed. In Fig. 2, the 27Al chemical shift ofkaolin is 1.3 ppm corresponding to the six-coordinate aluminum. Aftercalcination, the resonances of 800MK and 900MK contain threeoverlapping peaks corresponding to four- (δ=61.1 and 56.4 ppm),five- (δ=28.1 and 29.5 ppm) and six-coordinate aluminum (δ=1.2 and0.5 ppm), respectively (Fig. 2) [12–14]. According to Klinowski [15] andDavidovits [16], the resonance at 61.1 ppm in 800MK shows that theenvironment of aluminumatoms is tetrahedral, of the typeAlQ3(3Si). Butthe resonance at 56.4 ppmof 900MK is tetrahedral, of the type AlQ4(4Si).After geopolymerization, environment of aluminum atoms of G-800MKand G-900MK are different. The 27Al chemical shifts of G-800MK are at56.6 and6.1 ppm, respectively, corresponding to tetrahedral coordinatedwith AlQ4(4Si) type and octahedral coordinated, respectively. In contrastto G-800MK, aluminum atoms of G-900MK fully transform from four,five, and six coordinate to four coordinated aluminum at 56.7 ppmassigned to AlQ4(4Si). It implies that the reaction of G-900MK is morecomplete than that ofG-800MK, and thus thedegree of polymerization ofG-900MK is larger than that of G-800MK.

In Fig. 3, the 29Si chemical shift of kaolin is −89.8 ppm,corresponding to the SiQ3(0Al) environment [17]. As compared with

MK and G-900MK respectively, * spinning side bands.

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kaolins, the 29Si chemical shift of 800MK and 900MK reveals onedistinct Q4 silicon environments around at−106 ppm, correspondingto the SiQ4(0Al) environment. It is indicated that the environment ofsilicon atoms of kaolin changes after calcination. However, the changeof calcination temperature from 800 to 900 °C seems to have no effecton the environment of silicon atoms of metakaolin (800MK and900MK). After geopolymerization, 29Si spectra of G-800MK showsbroad peaks of the SiQ4(3Al) sites at about −90.6 ppm, together witha resonance at −103.1 ppm arising from the unreacted metakaolinSiQ4(0Al), while the structural unit in G-900MK is completelycomprised of SiQ4(3Al) species. So the structure of G-800MK ismuch more complicated than that of G-900MK.

According to the aforementioned analysis, it can be induced thatcalcination temperature has an effect on the environment of aluminumatoms rather than the environment of silicon atoms of metakaolin. Thedifferent environment of aluminumatoms between800MKand900MKleads to various properties of G-800MK and G-900MK. Under samechemical composition and synthesis condition, different properties ofthe as-prepared materials are caused by the calcination temperature ofraw materials. The molecular weight of geopolymer increases with thedegree of polymerization, leading to the increase in the bulk density ofthe final geopolymer. In G-800MK, there was considerably moreunreacted metakaolinite as indicated by the 27Al NMR spectrum.While in G-900MK, no detectable metakaolinite was observed.Metakaolin isweak andacts asdefect in thematerial, locally intensifyingthe stress in the binder and precipitating failure [18]. Thus G-900MKexhibited much higher flexural and compressive strength than G-800MK. Due to the much more complicated structure of G-800MK,according to the phonon thermal conduction theory, its phonon meanfree path are shorter than that of G-900MK, thermal conductivity is lessthan that of G-900MK.

4. Conclusions

The structure of metakaolin was greatly influenced by the startingkaolin calcination temperature. The structural changeofmetakaolinwasdue to the change of aluminum atoms environment other than that ofsilicon atoms, leading to a higher degree of polymerization of G-900MKthan that of G-800MK. As a result, bulk density and mechanical

properties of G-900MK are much higher than that of G-800MK.Meanwhile, G-800MK retained much lower thermal conductivity thanG-900MK due to its complicated structure.

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