Arab J Sci Eng (2012) 37:977–989DOI 10.1007/s13369-012-0230-5

RESEARCH ARTICLE - CIVIL ENGINEERING

Suresh Thokchom · Kalyan Kr. Mandal · Somnath Ghosh

Effect of Si/Al Ratio on Performance of Fly Ash Geopolymersat Elevated Temperature

Received: 26 February 2010 / Accepted: 7 October 2010 / Published online: 20 April 2012© King Fahd University of Petroleum and Minerals 2012

Abstract This paper presents the results of an experimental investigation on performance of fly ash-basedgeopolymer pastes at elevated temperature exposure. Geopolymer paste specimens having Si/Al in the range1.7–2.2, manufactured by activating low calcium fly ash with a mixture of sodium hydroxide and sodiumsilicate solution were subjected to temperatures up to 900 C. The effect of Si/Al ratio was studied on the basisof physical appearance, weight losses, residual strength, volumetric shrinkage and water sorptivity at differenttemperatures. Scanning electron microscopy along with energy dispersive X-ray and X-ray diffraction anal-ysis were also conducted to examine changes in microstructure and mineralogy during the thermal exposure.Specimens gradually changed in colour from grey to light red accompanied by the appearance of small cracksas the temperature was increased to 900 C. Loss of weight and volumetric strain due to elevated tempera-ture exposure were higher in specimens manufactured with lesser Si/Al ratios. Geopolymer paste specimencontaining maximum Si/Al of 2.2 performed best in terms of residual compressive strength after exposure toelevated temperatures.

Keywords Fly ash · Amorphous silica · Geopolymer · Elevated temperature · Compressive strength · XRD ·SEM · EDX

S. Thokchom (B)Civil Engineering Department, Manipur Institute of Technology, Imphal 795004, IndiaE-mail: thok_s@rediffmail.com

K. Kr. Mandal · S. GhoshCivil Engineering Department, Jadavpur University, Kolkata 700032, IndiaE-mail: kkma_ju@yahoo.co.in

S. GhoshE-mail: som_ghosh2000@yahoo.com

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1 Introduction

Alkali-activated geopolymer composites have become very popular in the last few decades due to their manyreported advantages over ordinary Portland cements. Advantageous properties include early gain of higherstrength, lesser creep and drying shrinkage, better durability in acid and sulphates, and no dangerous alkaliaggregate reaction besides its environment friendliness. Among the family of geopolymers, fly ash-based geo-polymer is gaining more attention because fly ash is available abundantly throughout the world as a wastefrom thermal power plants. Previous research on geopolymer has been basically confined mostly to its manu-facturing processes including the effects of various synthesizing parameters. Though there are quite a numberof literature available on geopolymers, those dealing with fly ash-based geopolymers are more recent butvery few. Davidovits [1] reported that geopolymer possesses high early strength, better durability and has nodangerous alkali–aggregate reaction. Geopolymers manufactured from low calcium fly ash by activation withvarious alkaline solutions have shown excellent performance against acids and sulphate [2–11]. Bakharev [2,7]showed that geopolymer materials manufactured from fly ash performed much better than OPC counterparts inacid and sulphate environment. Alkali-activated slags are reported to be more resistant than ordinary Portlandcement counterparts when subjected to elevated temperature curing [12]. Due to differential thermal expan-sion between aggregate and pastes, the behaviour of geopolymer paste and concrete at elevated temperaturewas found to be different [13]. Specimen size and aggregate size have been identified as the two main fac-tors that govern geopolymer behaviour at elevated temperatures of 800 C [14]. Geopolymer materials havelow thermal stability and these were found unsuitable for refractory insulation application [15]. Barbosa andMackenzie [16] investigated the thermal behaviour of inorganic geopolymers and composites derived fromsodium polysialate and reported that thermal expansion was influenced by the properties of the filler. Cheng andChiu [17] conducted a fire resistance test on geopolymers manufactured with granulated blast furnace slag asactive filler. Sanjayan et al. [18] compared the performance of geopolymers made with metakaolin and fly ashafter exposure to elevated temperatures and attributed the better performance of fly ash-based geopolymers onsintering reactions of unreacted fly ash particles. More recently, Sanjayan and Kong [19] conducted studies onthe effect of elevated temperatures on geopolymer paste, mortar and concrete and identified specimen size andaggregate size as the two main factors that govern geopolymer behavior at elevated temperatures up to 800 C.The authors also investigated the damage behaviour of geopolymer composites exposed to elevated tempera-ture [20]. However, due to limited number of studies on performance of geopolymer at high temperature, it isstill felt necessary to carry out more investigations in order to arrive at a concrete conclusion.

The present experimental investigation was to study the effect of Si/Al ratio on the performance of flyash-based geopolymer paste specimens at elevated temperature exposure. Geopolymer pastes manufacturedwith varying Si/Al ratios of 1.7, 1.9 and 2.2 were subjected to elevated temperature up to 900 C and theirperformance was evaluated on the basis of weight loss, strength loss, volumetric shrinkage and sorptivityrecorded after temperature exposure. In addition, mineralogical changes and microstructural changes due toelevated temperature were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) andenergy dispersive X-ray (EDX).

2 Experimental

2.1 Materials

Low calcium Class F fly ash conforming to ASTM C 618 was used in the present experimental investigationand it was obtained from Kolaghat Thermal Power Plant near Kolkata, India. It had a mineral and chemicalcomposition as in Fig. 1 and Table 1. X-ray diffractogram of the fly ash showed amorphous phases withinclusions of semi-crystalline phases such as mullite, quartz and hematite.

About 75 % of particles were finer than 45 µm and Blaine’s specific surface was 380 m2/kg. Laboratory-grade sodium hydroxide in pellet form (98 % purity) and sodium silicate solution (Na2O = 8 %, SiO2 = 26.5 %and 65.5 % water) with silicate modulus ∼3.3 and a bulk density of 1,410 kg/m3 was supplied by Loba ChemieLtd, India. A mixture of sodium hydroxide and sodium silicate solution having constant Na2O in the mix as8.5 % of fly ash and Si/Al ranging from 1.7 to 2.2 was prepared 1 day ahead and used as the activating solutionto manufacture the geopolymer paste. For Si/Al ratios higher than 1.7, amorphous silica powder was externallyadded into the activating solution to achieve the required Si/Al ratio. Water to fly ash ratio was maintainedat 0.32.

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Fig. 1 X-ray diffractogram of fly ash used, M mullite, Q quartz, H hematite

Table 1 Chemical composition of fly ash by XRF

Chemical SiO2 Al2O3 Fe2O3 TiO2 CaO MgO K2O Na2O SO3 P2O5 LOIa

composition

Percentage 56.01 29.8 3.58 1.75 2.36 0.30 0.73 0.61 Nil 0.44 0.40a Loss in ignition

2.2 Preparation of Specimen

Low calcium fly ash was first mixed with the required quantity of activator solution in a Hobart mixer for5 min to produce a uniform mix. The geopolymer mix exhibited a thick sticky nature with good workability.The mix was then transferred into 50-mm steel cube moulds and vibrated for 2 min to expel any entrapped air.Specimens were cured along with the moulds in a ventilated oven for a period of 24 h at 85 C and allowedto cool inside the oven before being removed to room temperature. The details of the samples used in thepresent study are given in Table 2. For each test, three replicate specimens were used and the average valuesare reported. The average compressive strength determined at 7 days for specimens of Si/Al ratios 1.7, 1.9 and2.2 was found to be 27.73, 36.51 and 39.47 MPa, respectively.

2.3 Test Procedure

After 7 days from manufacture, the fly ash geopolymer paste specimens were subjected to elevated tempera-tures of 300, 600 and 900 C in an electrically operated furnace having a capacity of 1,000 C. The heatingrate was maintained at approximately 8.5 C per minute till attaining the required elevated temperature and itwas held at this temperature for a period of 2 h. The specimens were then allowed to cool inside the furnacebefore removing them for further investigations and tests. A crack detection optical microscope WF 10×,C & D (Microservices) Ltd., UK was used to observe the surface for cracks and such other physical changesafter each temperature exposure. Measurements of weight were performed immediately after removal fromthe furnace. Dimensional changes after each exposure were measured with the help of a vernier caliper andvolumetric strain was obtained as product of the three dimensional strains. Three specimens from each serieswere used for determination of water sorptivity for each exposure temperature. Before the sorptivity test, suchspecimens were painted with waterproof enamel paint all around to allow unidirectional capillary rise of waterinto the specimen. XRD tests were conducted on powdered samples in a Rigaku Miniflex XRD machine. The

Table 2 Details of the geopolymer paste test specimens

Specimen Si/Al atomic Na2O content Water to fly Curing temperature 7 days compressiveID ratio (%) ash ratio and duration strength (MPa)

GP 1.7 1.7 8.5 85 C and 24 h 27.73GP 1.9 1.9 8.5 0.32 36.51GP 2.2 2.2 8.5 39.47

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Fig. 2 Photograph of GP1.9 specimens at different temperatures and standard colour indexes (http://www.w3schools.com/html/html_colornames.asp)

samples were step scanned at the rate of 1(2θ) per minute in the range of 2θ = 5−65. Microstructurechanges at various exposure temperatures along with micro-probe analysis was performed in a JEOL JSM6360 scanning machine fitted with Inca Oxford EDX analyzer. Meanwhile, unexposed geopolymer specimenswere also subjected to similar tests to make comparisons with the exposed counterparts.

3 Results and Discussion

3.1 Physical Appearance

Figure 2 shows a typical photograph of the physical appearance with regard to changes in colour at differenttemperatures for a geopolymer paste specimen series. Changes of colour have been compared with standardcolours (http://www.w3schools.com/html/html_colornames.asp) as far as possible. The colour of paste samplechanged from initially Alice blue/light grey to chocolate/light red as the temperature was increased till up to900 C. In most specimens, small microcracks began to appear at 300 C and it gradually increased withtemperature. These cracks were spread over a larger area and extended deep into the specimen after exposureto 900 C.

Images of specimen surfaces as seen through an optical microscope are given in Figs. 3, 4 and 5. Fromphysical observation of specimens as well as from observations through optical microscope, it was noticed thatSi/Al ratio of geopolymer paste showed a great influence on physical changes after elevated temperature expo-sure. Specimens manufactured with activating solution containing higher Si/Al ratio of 1.9 and 2.2 revealedlesser formations of cracks much in contrast to those prepared with lesser Si/Al ratio of 1.7. For Na2O contentof 8.5 % and water to fly ash ratio of 0.32, the maximum Si/Al ratio possible in the activator solution was1.7. It was therefore necessary to introduce external silica powder (SiO2) to achieve higher Si/Al ratios of 1.9and 2.2. Enhancement of Si/Al ratio showed a tremendous increase in sustaining the elevated temperatures inrelation to physical appearances. GP1.7 specimens prepared with the least Si/Al ratio of 1.7 suffered maximumphysical changes in terms of colour change and surface cracks. However, GP1.9 and GP2.2 which had higherSi/Al ratios of 1.9 and 2.2, respectively, exhibited much lesser corresponding changes. Moreover, the widthand depth of cracks were much larger in GP 1.7 specimen. The least changes were observed in the GP2.2specimen manufactured with the highest Si/Al ratio of 2.2.

Even after exposure to high temperature of 900 C, this specimen did not change much in colour apartfrom having fewer and smaller cracks. Measurements of crack widths were done at least at five spots alongthe cracks and reported as average. The width of the cracks was as much as 0.5 mm in GP1.7 specimensand such cracks extended long and were much deeper compared to the specimens GP1.9 and GP2.2. Inthe GP1.9 and GP2.2 specimens, the average width of the crack was observed to be 0.3 and 0.15 mm,respectively.

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Fig. 3 Surface image of the GP1.7 specimen at different temperatures as seen through an optical microscope; a unexposed,b 300 C, c 600 C and d 900C

3.2 Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX)

Small samples collected from near the surface (0–3 mm depth) were used for SEM and EDX tests. SEMmicrographs along with their corresponding EDX spectra at selected spots for GP1.9 after exposure to variouselevated temperatures and also for unexposed condition are presented in Fig. 6. The unexposed specimenGP1.9 shows an amorphous microstructure with visible pores along with some unreacted fly ash particles.However, with increase in exposure temperature, the microstructure showed some disruption due to sinteringof some phases. Micrographs of GP1.9 specimens after 600 C exposure reveals better microstructure thanthose before exposure and after 300 C exposure. At 600 C, some phase changes might have taken place assintering is evident in the microstructure. Significant changes in microstructure are prevalent in the specimenexposed to 900 C. It resembled an inhomogeneous microstructure with the presence of larger pores due todiffusion of phases caused by sintering. EDX spectra at different spots identified with letters A, B, C and Din SEM micrographs of unexposed specimen as well as for those exposed to various temperatures show majorconstituent elements such as silicon (Si), aluminium (Al), calcium (Ca), iron (Fe), oxygen (O) and titanium(Ti) in the specimens. Other minor phases of potassium (K) and sodium (Na) were also identified in most ofthe EDX spectra. However, magnesium (Mg) was also detected in a few specimens. These do not give a clearindication of changes in the constituent elements and may not be compared to one another as the selected spotsare often different.

Micrographs of GP1.7 and GP2.2 along with EDX spectra at selected spots E and F after exposure to max-imum temperature of 900 C are shown in Fig. 7. The two micrographs reveal significant differences betweenthem. In the GP1.7 specimen containing Si/Al ratio of 1.7, SEM micrograph depicts complete disruption ofthe matrix due to sintering of phases inside the specimen. Hence, the microstructure shows an interconnectedmatrix with most of the initial pores being blocked in the process. On the other hand, GP2.2 specimen manufac-tured with maximum Si/Al ratio of 2.2 present a relatively undisturbed matrix except at few locations. Further,

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Fig. 4 Surface image of the GP1.9 specimen at different temperatures as seen through an optical microscope; a unexposed,b 300 C, c 600 C and d 900 C

on comparison of microstructures of different geopolymer specimens with varying Si/Al ratios, it is apparentthat specimens containing higher Si/Al ratio have a greater resistance to elevated temperature of 900 C.

3.3 X-ray Diffraction Analysis (XRD)

Powdered sample of specimens passing through 75 µm sieve were used for XRD tests for unexposed as wellas for those exposed at different temperatures. X-ray diffractogram for the GP1.9 specimen before expo-sure and after elevated temperature exposure are presented in Fig. 8. Diffractogram for unexposed GP1.9specimen show traces of amorphous aluminosilicate gel characterized by the broad hump registered between2θ = 20–30 and some semi-crystalline phases of mullite (3Al2O3·2SiO2), hematite (Fe2O3), gismondine(Ca2Al4Si4O16·9(H2O)) and quartz (SiO2). Increasing the exposure temperature till 300 C caused marginalchanges in the diffractogram, yielding few peaks of albite (NaAlSi3O8) and nepheline (AlNaSiO4). Formationsof albite and nepheline phases at higher temperatures are due to sintering. Further heating of the specimen upto 600 C did not show notable changes. However, some peaks were observed to disappear and at the sametime few other peaks became prominent. At 900 C, X-ray diffractogram exhibited rather more disturbed peakswith few of them completely disappearing such as for mullite which was earlier present at 2θ = 15–20 andoccurrence of more peaks of albite. As a whole, the diffractogram of GP1.9 at 900 C presents lower intensitypeaks as compared to other diffractograms at lower temperatures.

Figure 9 presents X-ray diffractogram for GP1.7, GP1.9 and GP2.2 after exposure to 900 C. Onexamination of these diffractograms, some notable differences could be observed among the three geo-polymer specimens. GP1.7 specimen manufactured with the least Si/Al ratio of 1.7 reveals a number oflow-intensity broader peaks characterizing the amorphous phases distributed throughout. However, the pres-ence of such amorphous phases is less prevalent in diffractograms of both GP1.9 and GP2.2 geopolymerpastes. Narrower and more distinct peaks in the diffractogram of these specimens having higher Si/Alratios indicate a better crystalline structure. It can be concluded that mineralogical changes in fly ash

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Fig. 5 Surface image of the GP2.2 specimen at different temperatures as seen through an optical microscope; a unexposed,b 300 C, c 600 C and d 900 C

geopolymers at elevated temperatures are characterized with formation of new phases such as albite andnepheline.

3.4 Variation of Volumetric Shrinkage

Dimensional strains for each exposed specimens were measured with the help of vernier calipers and thevolumetric strain was determined. Evolution of volumetric strains with temperature for the three series ofgeopolymer paste specimen is shown in Fig. 10.

At lesser exposure temperature of 300 C, specimen GP2.2 caused maximum volumetric strain, whileGP1.7 and GP1.9 specimens recorded lesser volumetric strains. The results indicate lower strains for geo-polymer specimens with lesser Si/Al ratios at lower temperature of 300 C. However, when the temperaturewas elevated to 600 C, the volumetric strain values showed a reverse pattern such that specimens preparedwith lesser Si/Al yielded greater volumetric strain. This trend continued till up to 900 C. Beyond 600 C, thevolumetric strain increased rapidly to a final value of 20.24 and 16.8 % for GP1.7 and GP1.9 specimens at900 C. The percentage increase of volumetric strain at 900 C over those of 300 C was as much as 283 %for GP1.7 and 162 % for GP1.9. However, GP2.2 specimens containing the highest Si/Al ratio of 2.2 exhibiteda gradual increase in strain with a relatively lower final value of 8.4 % at 900 C. The percentage increaseof volumetric strain at 900 C over that of 300 C was as low as 25 %. The shrinkage data obtained showsremarkably better performance for specimens with higher Si/Al ratio.

3.5 Variation of Weight Loss

Immediately on removal from the furnace after each temperature exposure, specimens were weighed for study-ing evolution of weight with temperature. Figure 11 presents the variation of weight after exposure to different

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Fig. 6 SEM micrograph and EDX spectra of GP1.9 specimen at various temperatures; A1, A2 unexposed; B1, B2 300 C; C1,C2 600 C; and D1, D2 900 C

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Fig. 7 SEM micrograph and EDX spectra of GP1.7 and GP2.2 specimens at 900 C E1, E2 GP1.7; and F1, F2 GP2.2

Fig. 8 X-ray diffractogram of unexposed and temperature exposed GP1.9 specimen

temperatures. Up to 300 C, specimens of all series exhibited rapid loss in weight due to evaporation of mois-ture present in the specimens. Recorded loss of weight at this temperature was 15.45, 15.21 and 6.98 % forGP1.7, GP1.9 and GP2.2, respectively. However, further increase in temperature caused insignificant changes

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Fig. 9 X-ray diffractogram of geopolymer specimens after exposure to 900 C

Fig. 10 Variation of volumetric shrinkage of geopolymer specimen with temperature

in weight. As compared with the GP2.2 specimen, GP1.7 and GP1.9 showed relatively greater loss of weight.Weight loss at 900 C was 16.33, 16.05 and 8.62 % for GP1.7, GP1.9 and GP2.2, respectively. The resultsobtained indicate notable effect of Si/Al ratio on performance of fly ash geopolymer pastes subjected to ele-vated temperatures. Higher values of Si/Al in geopolymer mix caused lesser loss of weight. Loss of weight inthe specimens can be attributed to evaporation of available moisture in the specimen. Moisture gets evaporatedafter 100 C and beyond that chemically bonded water in the system gets evaporated till up to 300 C.

3.6 Residual Compressive Strength

Compressive strength tests of all specimens were conducted using a digital compression testing machine. Threeunexposed specimens were tested for compressive strength after 7 days of casting and these have been takenas the initial strength. The variation of compressive strength with exposure temperature is shown in Fig. 12.

Loss in strength was gradual up to 300 C for all the specimens. Residual strength obtained at this stagewas approximately 96 % for GP1.9 and GP2.2 specimens and 90 % for GP1.7 specimen. Specimens showed

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Fig. 11 Weight loss of geopolymer specimen with temperature

Fig. 12 Variation of Residual compressive strength of geopolymer specimen with temperature

continuous loss of strength on further increase of temperature. At the highest exposure temperature of 900 C,substantial loss in strength was recorded. The GP2.2 specimen with maximum Si/Al ratio of 2.2 still retainedapproximately 50 % of its compressive strength even after exposure to 900 C. However, the GP1.7 specimenmanufactured with the least Si/Al ratio indicated a residual strength of 63 %. Overall, geopolymer pastes man-ufactured from class F fly ash showed good performance with respect to retention of compressive strength.However, it is evident that higher Si/Al ratio perform better than those with lesser Si/Al ratios. The strengthloss can be attributed to shrinkage of specimens and disruption of microstructure at elevated temperatures.Moreover, cracks were also detected at higher temperatures in all the specimens which led to the loss ofstrength.

3.7 Variation in Water Sorptivity

Water sorptivity test of unexposed as well as those exposed to various temperatures has been conducted as perprocedures used by Sabir et al. [21]. The basic purpose was to study the changes in the pore structure of speci-mens with increasing temperature. Variation of sorptivity with temperature for the three series of geopolymerpaste specimens is shown in Fig. 13.

As expected, the GP2.2 specimen showed least sorptivity before exposure to elevated temperatures. Sig-nificant differences in sorptivity values were observed among the unexposed geopolymer specimens. WhileGP2.2 specimen measured a sorptivity of 1 × 10−4 g/mm2/min0.5, specimens of GP1.7 and GP1.9 showedconsiderably higher values at 8 × 10−4 and 16 × 10−4g/mm2/min0.5, respectively. Increasing exposure tem-perature caused sorptivity to increase up to 300 C for all the specimens. Further increase in temperaturereduced the sorptivity. However, beyond 600 C, geopolymer specimens exhibited a notable drop in sorptivity

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Fig. 13 Variation of Water sorptivity of geopolymer specimens with temperature

which could be due to sintering of some phases in the geopolymer matrix at higher temperatures. Simulta-neously, occurrence of wider and deeper cracks could have lessened the capillary effect required for sorption.After exposure to 900 C, the sorptivity values of the specimens were 4 × 10−4, 6 × 10−4, 3 × 10−4 and10 × 10−4g/mm2/min0.5 for GP2.2, GP1.9 and GP1.7 specimens, respectively.

4 Conclusion

The effect of Si/Al ratio on behaviour of fly ash-based geopolymer pastes in elevated temperature exposure hasbeen investigated. The parameters considered were changes in physical appearance, loss of weight, variationof strength, water sorptivity and volumetric strain. Besides, microstructural and mineralogical analysis at dif-ferent exposure temperatures was performed. Geopolymer specimens manufactured by activation of class F flyash with Na-based alkaline activators with varying Si/Al ratios remained physically stable without structuraldisintegration on exposure to elevated temperature, though a gradual change of colour was noticed. A rapidincrease of volumetric strain occurs in the geopolymer specimens beyond 600 C. Most of the weight losses arerecorded till 300 C. At elevated temperatures, mineralogical changes are characterized by formation of albiteand nepheline. SEM micrograph of specimens shows sintering at high temperatures. However, performanceof specimens with higher Si/Al ratios was significantly better in elevated temperature exposure. It is thereforeexpected that enhancing the Si content can improve performance of geopolymer composites to exposure at stillhigher temperatures, thereby making it a possibility for use as temperature-resistant construction materials.

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