Compressive strength of fly-ash-based geopolymer concrete at elevated temperatures

FIRE AND MATERIALSFire Mater. (2014)Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/fam.2240

Compressive strength of fly-ash-based geopolymer concrete atelevated temperatures

F. U. A. Shaikh*,† and V. Vimonsatit

Department of Civil Engineering, Curtin University, Perth, Australia

SUMMARY

This paper presents the compressive strength of fly-ash-based geopolymer concretes at elevated temperatures of200, 400, 600 and 800 °C. The source material used in the geopolymer concrete in this study is low-calcium flyash according to ASTM C618 class F classification and is activated by sodium silicate (Na2SiO3) and sodiumhydroxide (NaOH) solutions. The effects of molarities of NaOH, coarse aggregate sizes, duration of steamcuring and extra added water on the compressive strength of geopolymer concrete at elevated temperaturesare also presented. The results show that the fly-ash-based geopolymer concretes exhibited steady loss of itsoriginal compressive strength at all elevated temperatures up to 400 °C regardless of molarities and coarseaggregate sizes. At 600 °C, all geopolymer concretes exhibited increase of compressive strength relative to400 °C. However, it is lower than that measured at ambient temperature. Similar behaviour is also observedat 800 °C, where the compressive strength of all geopolymer concretes are lower than that at ambient tem-perature, with only exception of geopolymer concrete containing 10M NaOH. The compressive strength inthe latter increased at 600 and 800 °C. The geopolymer concretes containing higher molarity of NaOH solution(e.g. 13 and 16M) exhibit greater loss of compressive strength at 800 °C than that of 10M NaOH. Thegeopolymer concrete containing smaller size coarse aggregate retains most of the original compressive strengthof geopolymer concrete at elevated temperatures. The addition of extra water adversely affects the compressivestrength of geopolymer concretes at all elevated temperatures. However, the extended steam curing improvesthe compressive strength at elevated temperatures. The Eurocode EN1994:2005 to predict the compressivestrength of fly-ash-based geopolymer concretes at elevated temperatures agrees well with the measured valuesup to 400 °C. Copyright © 2014 John Wiley & Sons, Ltd.

Received 29 May 2013; Revised 19 January 2014; Accepted 20 January 2014

KEY WORDS: fly ash; geopolymer; compressive strength; elevated temperatures; fire

1. INTRODUCTION

Ordinary Portland cement (OPC) concrete is one of the most widely used construction material in theworld. It generally offers adequate fire resistance in most normal applications. It is a nonhomogeneouscomposite material whose fire performance is controlled by that of aggregates and cement paste.Concrete exhibits low thermal conductivity (50 times lower than steel) and therefore heats uprelatively slowly compared with steel in fire. It is non-combustible and does not produce any smokeand toxic gases in fire. Although concrete provides good fire resistance, it is blamed to be the mainsource of greenhouse gas emission into atmosphere. It is estimated that to produce one tonne ofcement, approximately one tonne of CO2 is released into the atmosphere [1]. Moreover, around 5–8%of global CO2 emission is due to cement manufacture and makes it the third most polluting activity ofmankind [2].

*Correspondence to: F. U. A. Shaikh, Department of Civil Engineering, Curtin University, Perth, Australia.†E-mail: [email protected]

Copyright © 2014 John Wiley & Sons, Ltd.

F. U. A. SHAIKH AND V. VIMONSATIT

In recent years, geopolymer has emerged as a novel engineering material in the construction industry[3, 4]. It is formed by the alkali activation reaction between alumina-containing and silica-containingsolids and alkali activators. Depending on the chemical composition of the raw material, the reactionproduct could be a three-dimensional aluminosilicate gel (for low-Ca systems) or a calcium silicatehydrate gel with high degree of Al substituent (for high-Ca systems) [5–7]. The raw material forgeopolymer production normally comes from industrial by-products, for instance, fly ash and blastfurnace slag. The industrial by-products can substitute cement clinker by100% in the system ofgeopolymer. For this reason, geopolymer is generally considered as an environment-friendlyconstruction material with great potential for sustainable development. Apart from the environmentaladvantages, pastes and concrete made of geopolymer can exhibit many excellent properties, forexample, high early-age strength, low creep and shrinkage, high resistance to chemical attack and goodfire resistance [8–10].

Considerable researches have been conducted on geopolymer concrete where mechanical and durabilityproperties are studied extensively [11]. However, very little is reported on the behaviour of geopolymerconcrete at elevated temperatures [12–14]. Most of the published research reported the residual strengthof geopolymer concrete measured at ambient condition after exposure to elevated temperatures up to800 °C [12–14]. Generally, during fire, the temperature reaches approximately 800 °C within 30min, andthe rate of temperature increase is very slow thereafter and becomes constant at about 1000–1100 °Cwithin 2–2.5 h (Figure 1) [15]. The standard fire curve (Figure 1) for building also shows that within15min of fire, the temperature quickly rises to 600 °C and above this temperature the concrete does notfunction at its full structural capacity [16]. Therefore, the compressive strength of geopolymer concretewhilst at elevated temperature during fire is very useful in the design and stability of reinforcedconcrete structures. Currently, little information exists on the compressive strength of geopolymerconcrete whilst at elevated temperatures [17, 18]. The results reported in [17] only present thecompressive strength of geopolymer paste at elevated temperatures. However, because of thermalincompatibility between geopolymer paste and aggregates [19], the behaviour of geopolymer concreteat elevated temperatures would be different from that of concrete. This paper presents the compressivestrength of fly-ash-based geopolymer concrete at various elevated temperatures of 200, 400, 600 and800 °C. The effects of molarities of sodium hydroxide (NaOH) solution, size of coarse aggregates,extended steam curing and effect of extra water on the compressive strength of fly-ash-basedgeopolymer concretes at above elevated temperatures are also evaluated and discussed in this paper.This paper also discussed the applicability of existing Eurocode EN1994:2005 [20] for reasonableprediction of compressive strength of fly-ash-based geopolymer concretes at elevated temperatures.

2. EXPERIMENTAL PROGRAMME

Table I shows detailed experimental programme and mix proportions of this study. In total, nine series ofmixes are considered. The first series is OPC concrete, whereas the remaining series are fly-ash-based

Figure 1. Standard fire curve [11].

Copyright © 2014 John Wiley & Sons, Ltd. Fire Mater. (2014)DOI: 10.1002/fam

Table

I.Experim

entalprogrammeandmix

proportio

ns.

Series

Mix

proportio

nsin

kg/m

3

Water/cem

ent

ratio

Alkaliactiv

ator

solutio

n/flyashratio

Ordinary

Portland

cement

Fly

ash

Sand

Coarseaggregatesize

(mm)

Alkaliactiv

ator

Added

water

710

20

Sodium

hydroxide(N

aOH)

Sodium

silicate

(Na 2Sio

3)

10M

13M

16M

1408

—660

—467

701

——

——

142

0.35

—2

—408

660

—467

701

41—

—103

——

0.35

3—

408

660

—467

701

—41

—103

——

0.35

4—

408

660

—467

701

——

41103

——

0.35

5—

408

660

389

779

—41

——

103

——

0.35

6—

408

660

—467

701

41—

—103

——

0.35

7—

408

660

—647

554

41—

—103

14.3

—0.38*

8—

408

660

—647

554

——

41103

14.3

—0.38*

9—

408

660

—467

701

41—

—103

14.3

—0.38*

*Note:

Added

water

isincluded

inthecalculationof

alkaliactiv

ator

solutio

nin

series

7–9.

COMPRESSIVE STRENGTH OF FLY-ASH-BASED GEOPOLYMER CONCRETE



geopolymer concretes. In each series, three concrete cylinders of 100mm in diameter and 200mm inheight are cast and tested. The effects of different molarities of sodium hydroxide on the compressivestrength of geopolymer concretes at elevated temperatures are evaluated in second, third and fourthseries, where 10, 13 and 16M NaOH are considered, respectively. In the fifth and sixth series, theeffects of 10- and 20-mm-size coarse aggregates on the compressive strength of geopolymer concretesat elevated temperatures are evaluated. Series seven and eight evaluate the effect of extra added wateron the compressive strength of geopolymer concretes at elevated temperatures, whereas the last series(series nine) evaluates the effect of extended steam curing of 40 h on the compressive strength ofgeopolymer concretes at elevated temperatures.

3. MATERIALS

The cement used in this study is general-purpose Portland cement, which corresponds to ASTM type I.The class F fly ash used in this study is obtained from Collie Power Station in Western Australia, whichis used to form the geopolymer binder along with alkaline liquids. The fly ash consists of an amorphouspart about 60% by wt. and a crystalline part about 40% by wt. The chemical composition of fly ash isshown in Table II. The crystalline part of the fly ash has low reactivity and acts as fine aggregate in thebinder system. The activating alkali liquid consisted of Na2SiO3 and NaOH solutions. The compositionof Na2SiO3 is (wt.%) Na2O= 14.7, SiO2 = 29.4 and water = 55.9. The other characteristics of Na2SiO3

solution are specific gravity = 1.53 g/cc and viscosity at 20 °C = 400 cp. The NaOH solution is preparedfrom analytical grade NaOH pellets. The alkaline liquid is mixed 24 h prior to mixing of the concrete.This process involved measuring the required mass of NaOH, which is then added to a measuredamount of water to make the required molarity. These are then mixed in a plastic beaker in an icebath to promote cooling as the extreme exothermic reaction of the sodium hydroxide solutionreleases a lot of heat. Once cooled, the NaOH solution is poured into a container where it is mixedwith a measured mass of Na2SiO3 solution. The mass of Na2SiO3 used is 2.5 times that of theNaOH as past research [21] showed this to be the optimal ratio. The coarse aggregates used in thisstudy are predominantly of 20mm size except series 5 where small size coarse aggregate of 10mmis used. The grading curves of both 10- and 20-mm-size coarse aggregates are shown in Figure 2.Both aggregates are crushed from granite rock and are prepared to saturated surface-dry condition.

To cast the concretes, cylinders of 200mm in height and 100mm in diameter are prepared. Thecylinders are modified in order to install 20-mm-long and 2-mm-diameter removable pins so thatholes would be cast into the specimens for the thermocouples to be installed as shown in Figure 3.

Table II. Chemical analysis and physical properties of Portland cement and flyash.

Chemical analysis Cement (wt.%) Class F fly ash (wt.%)

SiO2 20.2 51.50Al2O3 4.9 23.63Fe2O3 2.8 15.30CaO 63.9 1.74MgO 2.0 1.20MnO — —K2O — 0.84Na2O — 0.38P2O5 — —TiO2 — —SO3 2.4 0.28LOI 2.4 1.78

Physical PropertiesParticle size 25–40%≤ 7μm 40% of 10μmSpecific gravity 2.7–3.2 2.6


Figure 2. Grading curves of coarse aggregates used in this study.

Figure 3. Schematic of modified steel mould to install the thermocouples inside the cylinder.


4. SPECIMEN PREPARATION

The mixing of geopolymer and OPC concretes is carried out in a pan mixer. Firstly, the aggregates andcement or fly ash (in case of geopolymer concrete) are dry mixed for approximately 5min and then thewater or alkaline liquid (in case of geopolymer concrete) is slowly added into the mix and continues tomix for another 5min. The cylinders are then filled with concretes and compacted on vibrating table.The geopolymer specimens are then subjected to stream curing at 60 °C immediately after casting. Thegeopolymer concrete specimens are then demoulded after 24 h and stored in the laboratory in open airuntil testing at 28 days. The OPC concrete cylinders are demoulded after 24 h and stored in the curingtanks where they are subjected to standard wet curing conditions. As the specimens are tested atelevated temperatures, neither rubber nor sulphur caps are used. The ends of the specimens are sawedoff to provide a smooth and perpendicular surface for the specimens to be loaded upon. Before testing,the specimens are dried in an oven at 105 °C for 24 h to remove any free water from the concrete. Thisis to prevent the specimens from exploding in the kiln during the heating process, as a result ofextremely high pore water pressure from the superheated water. After drying, the specimens areweighed, and their diameter is measured to calculate an accurate surface area and density of the specimens.

5. IN SITU COMPRESSIVE STRENGTH TEST SETUP

The main objective of this study was to measure the compressive strength of standard fly-ash-basedgeopolymer concrete cylinders whilst at elevated temperatures. A locally manufactured kiln was used toheat the concrete cylinders, where the specimens were heated up to 800 °C. To test the specimens, the kiln



was positioned inside the Avery-DennisonUniversal TestingMachine (S.I.M.Machine Tools, England, UK)as shown in Figure 4. Two circular openings, at the top and bottom of the kiln, were made to provide accessof the loading cylinders. After installing the specimen and the loading rods, rock wool was used to seal thegap preventing any heat loss through the openings at the top and the bottom. The concrete cylinders werepositioned inside the kiln where two thermocouples were inserted into the holes in the specimen, and twomore thermocouples were also inserted into the kiln from the top holes in order to monitor the kiln airtemperature. The thermocouples were connected to the data logger and were used to monitor thetemperature inside the concrete and the kiln air as shown in Figure 5. It was recommended [22] that theconcrete be heated at a rate of 1 °C per minute for specimens of 100mm in diameter; however, standardbuilding fires can reach temperatures of 600 °C within 15min (Figure 1) [16]. Thus, a heating rate of500 °C per hour was selected. During the heating process, the temperatures of four thermocouples weremonitored. Once the specimen reached the target temperature, the testing platform (the bottom steelcylinder) was raised until the loading cylinder was in contact with the loading head at top, and the testsare conducted. The schematic detail of the test setup is shown in Figure 5, whereas a close-up view ofinside the kiln during concrete cylinder testing at elevated temperature is shown in Figure 6.

The rate of temperature increase in the kiln and in the cylinder is shown in Figure 7. As can be seenin the figure, there is a significant lag between the core temperature of concrete cylinder and airtemperature inside the kiln, particularly for the 200 and 400 °C temperature profiles. This is due tothe heat capacity of the concrete specimens and the rate at which they are able to absorb heat. This

Figure 4. Experimental setup for compressive strength test at elevated temperatures.

Figure 5. Schematic of in situ testing of cylinders inside the kiln at elevated temperatures.


Figure 6. Inside view of the kiln.

(a) (b)

(c) (d)

Figure 7. Measured temperatures–time profile of cylinders and kiln air at various elevated temperatures (a)200, (b) 400, (c) 600 and (d) 800 °C.


rate of temperature gain is consistent for both the geopolymer and OPC concrete specimens as theyhave similar densities and material properties. However, the difference in temperature between thekiln and the cylinder at 600 and 800 °C is less. Even though the difference in temperature betweenthe kiln and the cylinder existed, the target test temperatures in all cylinders were maintained duringcompression test, which can be seen in the cylinders thermocouples readings in Figure 7.

Two pairs of lines for the temperatures can be seen at any given time in Figure 7. The two dotted linesare for the thermocouples measuring the air temperature inside the kiln, whereas the two solid lines are the



readings for the thermocouples that were inserted 20mm within the concrete cylinder. For both pairs ofthermocouples in the concrete and in the kiln air, there was no more than a 30 °C temperaturedifference within the pairs on average. The significant difference between the temperatures within thekiln and the concretes’ internal temperature is to ensure rapid heating, which would better reflect theheating within a house fire. It was noted that there was no noticeable difference in heating timesbetween the geopolymer and the Portland cement concretes or between different molarities. The startingtemperature for the specimen at 400 °C was slightly above room temperature as the specimen was setup whilst the kiln was still warm from previous testing. This was to speed up the testing process somultiple specimens could be tested in a day. The elevated starting temperature had no influence on theconcretes because all specimens were exposed to 105 °C for an extended duration during the dryingprocess. Up until 500 °C, the kiln was able to maintain a constant heating rate, thereafter, the heatingrate decreased. This was due to the kiln being unable to maintain such a high heating rate because ofheat loss, most noticeably through the steel loading cylinder that acted as heat sink as can be seen in theimage where the loading cylinder is glowing red (Figure 8).

6. RESULTS AND DISCUSSION

6.1. Compressive strength of fly-ash-based geopolymer concretes at elevated temperatures

All fly-ash-based geopolymer concretes except series 7–9 experienced loss in compressive strengthfrom ambient temperature to 400 °C (Figures 9–11). The trend is similar to that observed in OPCconcrete. The reduction in compressive strength of geopolymer concrete from ambient temperaturecan be attributed to a number of temperature-induced stresses and chemical reactions. The thermalincompatibilities between the aggregate and pastes might have contributed to the reduction instresses. It is evident in Figure 12 [13] that the aggregate expands when exposed to elevatedtemperatures and the geopolymer paste contracts after 200 °C because of the loss of moisture, andthis contraction continues up to about 350 °C. It has also been reported by Cruz and Gillen [19] thatPortland cement paste follows a similar trend expanding to temperatures up to 93 °C and continuallycontracts thereafter. The thermal incompatibility induces stresses and hence cracking in bothgeopolymer and OPC concretes damaging the bond between the aggregate and the paste in theconcrete. The initial loss in compressive strength of both concretes from ambient temperature to400 °C is also attributed to the thermal gradient between the surface and core temperatures ofconcrete cylinder. Although the cylinder specimens are small in dimension compared with normalconcrete structures, the thermal gradient still exists in the cylinder. As can be seen in thetemperature profiles in Figure 7, the thermal lag between the kiln and the core temperature of theconcrete cylinder is greatest in the lower heating temperatures (e.g. at 200 and 400 °C in Figure 7).

Figure 8. Inner view of the kiln: steel loading cylinder acted as heat sink.


Figure 9. In situ compressive strength of ordinary Portland cement and geopolymer concretes at variouselevated temperatures.

Figure 10. Effect of sizes of coarse aggregates (CA) on the in situ compressive strength of geopolymerconcretes at various elevated temperatures.

Figure 11. Effect of added water and extended steam curing on the in situ compressive strength ofgeopolymer concretes at various elevated temperatures.


Kristensen and Hansen [22] reported cracking in concrete due to thermal gradient between 20 and 30 °Cover 50mm length. The thermal gradient observed in the cylinders at 200 °C is about 89.5 °C over 20mmin this study. This might generate stresses within the specimens and lead micro cracking within the


Figure 12. Thermal expansion of coarse aggregates and geopolymer paste [13].


concrete. The greater temperature differential in the lower heating stages leads to greater stress differentialand hence more cracking in the concrete specimens, which is believed to contribute to the lowercompressive strengths recorded at 200 and 400 °C in this study. However, past research [13,23] did notexperience loss of compressive strength of fly-ash-based geopolymer pastes at 200 and 400 °C due toabsence of coarse aggregate.

The increase in compressive strength from 400 to 600 °C is observed in all fly-ash-based geopolymerconcretes. The OPC concrete, on the other hand, exhibited about 46.5% loss in compressive strength at600 °C. The significant loss of strength experienced in the OPC concrete after 400 °C can be attributedto the dissociation of calcium hydroxide [Ca(OH)2], which is one of the main products of the hydrationof Portland cement [24]. Dissociation is the process in which ionic compounds split into smallerparticles and occurs between 300 and 400 °C, with the dehydration of Ca(OH)2 occurring between 500and 600 °C [24], which also leads to strength reduction in OPC concrete. The increase in compressivestrength of the fly-ash-based geopolymer concrete, on the other hand, could be attributed to the stablecontraction of geopolymer paste as observed by Rickard et al. [25] using the same source of fly ash(Collie Power Station in Western Australia).

All fly-ash-based geopolymer concretes exhibited reduction in compressive strength from 600 to800 °C temperature except that containing 10M NaOH. The rate of strength loss of OPC concrete iseven higher than that of geopolymer concretes. Past research [23] also reported similar reduction incompressive strength of geopolymer paste beyond 520 °C, which is very consistent to that observedin this study. However, the increase in compressive strength of geopolymer concrete containing10M NaOH at 800 °C is not clear and needs further investigation.

Although the exact reasons for different compressive behaviour of fly-ash-based geopolymer concretesat various elevated temperatures are not clear, the thermal incompatibility between the aggregate and thegeopolymer paste might be responsible here. By looking into Figure 12, three major trends of expansion/contraction of geopolymer paste at various elevated temperatures up to 800 °C can be noticed. The first isbetween 200 and 350 °C, where the paste contracted at extremely high rate. Other research on class F fly-ash-based geopolymer also confirms similar shrinkage of geopolymer paste up to 300 °C [26]. In thesecond phase, between 350 and 600 °C, the contraction of the paste is almost stable and shrinks atmuch slower rate beyond 600 °C than that of the first temperature range. However, the coarseaggregates, for example, those originated from basalt and slag in [13], showed continued expansion atall elevated temperatures up to 800 °C (Figure 12). Cruz and Gillen [23] also reported similar expansionof coarse aggregates of dolomite rock. By comparing these trends with the compressive strength testresults in Figure 9, it can be seen that the significant reduction in the compressive strength up to 400 °Ccould be attributed to extremely high shrinkage of geopolymer paste that damaged the bond betweenthe coarse aggregate and the paste in the geopolymer concrete. The slight gain in the compressivestrength of geopolymer concretes at 600 °C relative to 400 °C can be attributed to the stable contractionof geopolymer paste between 350 and 600 °C [9,20]. Duxson et al. [27] proposed that the slowdehydroxylation of hydroxyl groups contributed to the stable contraction in this temperature range.After 600 °C, the shrinkage of paste increases and is believed to be contributed to the reduction of



compressive strength of geopolymer concretes at 800 °C. Rickard et al. [25, 26] proposed that theshrinkage/densification of geopolymer paste is due to viscous sintering of geopolymer matrix filling thevoids in the material.

6.2. Effect of molarities of NaOH solution

The effects of different molarities of NaOH solutions (e.g. 10, 13 and 16M) on the compressive strength offly-ash-based geopolymer concretes at various elevated temperatures (200, 400, 600 and 800 °C) can alsobe seen in Figure 9. It can be seen in Figure 9 that all fly-ash-based geopolymer concretes exhibitedreduction in compressive strength at 200 and 400 °C. When the temperature is increased to 600 °C, amoderate gain in compressive strength is observed in geopolymer concretes. When the concretecylinders are further heated up to 800 °C, the geopolymer concretes exhibited reduction in compressivestrength compared with their respective ambient strength, except that containing 10M NaOH (series 2).Generally, the geopolymer concretes containing high molar sodium hydroxide solutions (e.g. 13 and16M) exhibited higher compressive strength at all elevated temperatures than that containing 10M

sodium hydroxide solution. However, interestingly, it is observed that the rate of loss of compressivestrength after 600 °C is also high in geopolymer concrete containing high molar sodium hydroxidesolution (e.g. 16M in this study). This could be attributed to the fact that the high molar NaOH solutioncontains high amount of sodium oxide, which improves the alumina-silicate networks ingeopolymerisation and hence increases the compressive strength [28]. Rickard et al. [25] recentlyreported, on the basis of X-ray diffraction analysis, sodium-based crystalline compounds such asnepheline (NaAlSiO4), albite (NaAlSi3O8) and tridymite (SiO2) in class F fly-ash-based geopolymerafter exposed to fire. These high temperature phases improve the thermal resistance of fly-ash-basedgeopolymer because of their high melting points [25]. The higher in situ compressive strength ofgeopolymer concretes containing 13 and 16M NaOH than that containing 10M NaOH at variouselevated temperatures can also be attributed to the formation of higher amount of aforementionedcompounds in the former concretes than the latter.

6.3. Effect of sizes of coarse aggregates

Figure 10 shows the effects of 10- and 20-mm-size coarse aggregates on the compressive strength of fly-ash-based geopolymer concretes at elevated temperatures. Similar to the previous section, both concretesexhibited reduction in compressive strength at elevated temperatures up to 400 °C. Geopolymer concretecontaining 20-mm-size coarse aggregates exhibited higher reduction of compressive strength than thatcontaining 10-mm-size coarse aggregate. When the specimens are heated up to 600 °C, a moderate gainin compressive strength is observed in both geopolymer concretes. It has also been observed that thegeopolymer concrete containing 10-mm-size coarse aggregate regained its ambient compressivestrength at 600 °C and maintained this gain up to 800 °C. These results are also consistent with thosereported in [23], where the geopolymer concrete containing 14-mm-size basalt coarse aggregatesexhibited higher compressive strength than that containing 20mm at elevated temperatures. Therefore,it can be concluded that the fly-ash-based geopolymer concrete containing smaller size coarseaggregates exhibits higher compressive strength at elevated temperatures irrespective of mineralogy ofthe aggregates and their origin. The slightly better compressive strength of geopolymer concretecontaining 10-mm-size coarse aggregate than that containing 20-mm-size at elevated temperaturescould also be attributed to the delayed formation of micro cracks in the interfacial transition zones(ITZ) in the former concrete than in the latter concrete [29]. Shigang et al. [29] reported that the microcracks in ITZ are formed earlier in the concrete containing larger coarse aggregates than that containingsmaller size aggregates, because of large continuous ITZ around the coarser aggregates and hugedifference in stiffness between the matrix and the coarse aggregates.

6.4. Effects of extra added water and extended steam curing

The effects of extended steam curing and extra added water on the compressive strength of geopolymerconcrete at elevated temperatures are shown in Figure 11. It can be seen that by extending the steamcuring period from 24 to 40 h, the compressive strength of geopolymer concrete at elevated



temperatures is increased by about 20%. The geopolymer concrete, which obtained longer steamcuring (40 h), exhibited higher compressive strength capacity at all elevated temperatures than thatobtained 24 h steam curing (Figure 11). This could be due to additional geopolymerisation in thatextended heat-cured concrete. Research also shows that extended heat curing of fly-ash-basedgeopolymer concretes increases the compressive strength [11]. Figure 11 also shows the effects ofextra added water on the compressive strength of geopolymer concrete at elevated temperatures. Itcan be seen that by adding extra water, the compressive strength of geopolymer concretes at bothmolarities (10 and 16M) decreased at all elevated temperatures. This could be attributed to theformation of weak polymerisation products in those concretes due to presence of extra water.Similar to ordinary concrete, the use of excessive water in geopolymer concrete also adverselyaffects its strength and durability properties, because of the formation of excessive voids throughevaporation of some unbound water during heat curing at mild temperature and the rest at highertemperatures. These effects can be confirmed by comparing the strength values at ambient andelevated temperatures of geopolymer concretes with and without extra water.

7. COMPARISON OF EXPERIMENTAL RESULTS WITH PREDICTION

Eurocode EN1994:2005 [20] provides guideline to predict the compressive strength of OPC concretes atdifferent elevated temperatures when they are exposed to fire. In this study, the measured compressivestrength of fly-ash-based geopolymer concretes and OPC concretes at various elevated temperatures arecompared with that predicted by the Eurocode EN1994:2005 and are shown in Figures 13 and 14. Itcan be seen in Figure 13 that the measured compressive strengths of OPC concrete at various elevatedtemperatures are very close to the predicted values. In the case of geopolymer concretes havingdifferent molarities of NaOH solution, it is observed that the prediction by Eurocode EN1994:2005 isvalid up to 400 °C, where the reduction of experimentally measured strength follows the same trend tothat predicted by the code. The same is also true for geopolymer concretes having 10- and 20-mm-sizecoarse aggregates (Figure 14). However, beyond 400 °C, it is observed that the code’s prediction differssubstantially from that measured in this experiment. This is true for all geopolymer concretesirrespective of different molarities of NaOH and different sizes of coarse aggregates. On the basis ofthis present study, it can be concluded that the Eurocode EN1994:2005 can be used to predict the

Figure 13. Comparison of code-predicted compressive strength of geopolymer concretes having differentmolarities of NaOH at elevated temperatures with that observed in the experiment.


Figure 14. Comparison of code-predicted compressive strength of fly-ash-based geopolymer concreteshaving different coarse aggregate sizes at elevated temperatures with that observed in the experiment.


compressive strength of fly-ash-based geopolymer concretes with reasonable accuracy up to 400 °C.Moreresearch is needed to gather data in order to confirm the applicability of the Eurocode EN1994:2005beyond 400 °C and to adjust it beyond this temperature range.

8. APPEARANCE OF GEOPOLYMER CONCRETES WHILST AT ELEVATEDTEMPERATURES

It is evident in Figure 15 that colour changes occurred in the fly-ash-based geopolymer concrete afterexposure to elevated temperatures. The geopolymer concrete changes from a normal dark grey to asalmon pink at 800 °C. This colour change is also accompanied by an increasing presence of surfacecracks on the geopolymer concrete specimens from 400 °C and above (Figure 15). These cracksresult from the thermal incompatibility between the aggregate and the geopolymer matrix asdiscussed earlier. No evidence of cracking or discoloration was seen in the OPC concrete specimens(Figure 16). The visible colour change in geopolymer concretes at elevated temperature can beattributed to the significantly higher iron oxide in fly ash and the oxidisation of the iron particles inthe fly ash at elevated temperatures [13]. By looking into the chemical compositions of fly ash and

Figure 15. Effect of elevated temperatures on the physical appearance of fly-ash-based geopolymer concrete.


Figure 16. Effect of elevated temperatures on the physical appearance of ordinary Portland cement (OPC)concrete.


cement in Table II, it can be seen that the class F fly ash contains about five times more iron oxide thancement. The iron oxide in the fly ash used in this study is also significantly higher than other class F flyashes (e.g. Richard et al. [25] and Provis et al. [30] studied class F fly ashes from different sources inAustralia containing iron oxides that ranged between 0.64% and 10.2%).

9. CONCLUSIONS

This paper presents the in situ compressive strength of fly-ash-based geopolymer concretes measuredon standard cylinders at various elevated temperatures up to 800 °C inside the kiln. Comparison is alsomade with OPC concrete. The effects of different molarities of NaOH solutions, sizes of coarseaggregates, extended steam curing and effect of extra added water on the compressive strength offly-ash-based geopolymer concretes at elevated temperatures are also evaluated. The applicability ofEurocode EN1994:2005 to predict the compressive strength of fly-ash-based geopolymer concretesat elevated temperatures is also evaluated.

On the basis of this limited experimental study of the compressive strength of fly-ash-basedgeopolymer concretes at elevated temperatures, the following conclusions can be drawn:

• Because of the thermal incompatibility between coarse aggregates and fly ash geopolymer paste [13],the compressive strength of geopolymer concretes decreased at elevated temperatures up to 400 °C,which is consistent with OPC concrete. However, the geopolymer concretes exhibited higher com-pressive strength at 600 and 800 °C because of relatively stable contraction of geopolymer paste atthose temperature ranges.

• The formation of improved alumina-silicate networks during geopolymerisation [28] and highmelting temperature phases such as nepheline (NaAlSiO4), albite (NaAlSi3O8) and tridymite(SiO2) [25] in geopolymer concretes containing high molar NaOH exhibited higher compressivestrength at all elevated temperatures.

• Geopolymer concrete containing smaller size coarse aggregates exhibited slightly higher com-pressive strength at all elevated temperatures than that containing larger coarse aggregates becauseof the likelihood of less micro cracking in the ITZ of aggregates in the former [29].

• Geopolymer concrete that received extended heat curing exhibited higher compressive strength atall elevated temperatures because of additional geopolymerisation at extended time.

• The existing Eurocode EN1994:2005 [20] can be used to predict the compressive strength of fly-ash-based geopolymer concretes up to 400 °C. However, beyond this range, it underestimates thetest results and more research is needed in this area.

• The significantly higher quantity of iron oxide in the studied fly ash contributed to the visiblecolour change in geopolymer concrete at elevated temperatures particularly at 600 and 800 °C.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the final year project students G.R. Williams and C.D. Simmons for theirassistance in casting and testing of specimens in this study.



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Compressive strength of fly-ash-based geopolymer concrete at elevated temperatures