ORIGINAL ARTICLE

Performance of blended ash geopolymer concreteat elevated temperatures

M. W. Hussin • M. A. R. Bhutta • M. Azreen •

P. J. Ramadhansyah • J. Mirza

Received: 12 May 2013 / Accepted: 7 January 2014

� RILEM 2014

Abstract This study involved laboratory investiga-

tion of the performance of blended ash geopolymer

concrete at elevated temperatures. Geopolymer con-

crete composite was prepared using blended ash,

pulverized fuel ash, and palm oil fuel ash, obtained

from agro-industrial waste along with alkaline activa-

tors. The samples were heated up to 800 �C to evaluate

mass loss, strength, and microstructural changes due to

thermal impact. Ordinary Portland cement (OPC)

concrete was prepared as control concrete. The

deterioration of concrete at elevated temperatures

was examined by X-ray diffraction, fourier trans-

formed infrared spectrometer, thermogravimetry ana-

lyser and field emission scanning electron microscope.

A comparison between the performance of geopoly-

mer and OPC concretes—the former exhibited better

performance at elevated temperature.

Keywords Agro-industrial waste � Alkaline

activator � Blended ash � Elevated temperature �Geopolymer concrete

1 Introduction

Concrete is generally believed to be an excellent fire

resistant material. Many recent studies on ordinary

Portland cement (OPC) concrete have shown exten-

sive damage at high temperature. In a recent investi-

gation, it was found that geopolymer cement could be

a possible solution for making concrete resistant at

elevated temperatures. The threat of fires has neces-

sitated search for new fire-resistant materials to be

useable in construction industry, so as to ensure the

stability of functioning properties in case of extensive

fire [1, 2]. Conventional OPC concrete fails when

exposed to elevated temperature possibly due to

dehydration and destruction of C–S–H gel and other

crystalline hydrates. A phase composition of OPC is

characterized by compounds without the mineral

polymer that may cause some problems linked to the

durability of OPC concretes, especially threat of fires

[3, 4]. The resistance of OPC concrete to degradation

resulting from exposure to elevated temperatures

depends on the ingredients used in the concrete. Some

aggregates (e.g., siliceous) present a significant

strength loss at about 570 �C because of a phase

M. W. Hussin � M. A. R. Bhutta (&)

UTM Construction Research Center (UTM CRC), Faculty

of Civil Engineering, Universiti Teknologi Malaysia,

Johor Bahru, Malaysia

e-mail: aamer.bhutta@gmail.com

M. Azreen � P. J. Ramadhansyah

Faculty of Civil Engineering, Universiti Teknologi

Malaysia, Johor Bahru, Malaysia

J. Mirza

Department of Robotics and Civil, Research Institute of

Hydro-Quebec, Varennes, QC J3X ISI, Canada

Materials and Structures

DOI 10.1617/s11527-014-0251-5

change of quartz. Chemical and physical deterioration

occur at elevated temperatures because both interlayer

and chemically bound water are destroyed due to the

decomposition of calcium hydroxide (CH) and cal-

cium silicate hydrates (C–S–H) [3]. It has been shown

that the crucial exposure temperature at which

concrete begins to fail in compressive strength is

approximately 400 �C. This is caused by the decom-

position of CH and the increase in resulting volume

which occurs during cooling due to hydration of

calcium oxide. In addition, this could lead to cracking

and may even cause explosive spalling at temperatures

between 480 and 510 �C, thus decreasing the loading

capacity of concrete structures [3].

Inorganic polymers based on aluminosilicates are

dubbed as geopolymer [1]. These can be obtained

through synthesis of pozzolanic compounds or alumi-

nosilicate source materials with highly alkaline solu-

tions [2]. Geopolymers exhibit good fire resistance,

owing to their ceramic-like features [1, 2]. Therefore,

concretes produced using geopolymers may have

superior fire resistance compared to conventional

concretes produced with OPC. In general, the main

principle for alkalis to form a durable heat-resistant

mineral is to be bonded covalently in a three-dimen-

sional alumino-silicate network. The excellent dura-

bility of ancient cements was believed to be

characterized by increased content of alkalis, which

showed a similarity to natural zeolites in the reaction

products of the cements [1]. It should also be consid-

ered that the alkaline alumino-silicates are known to be

durable and not susceptible to change as compared to

calcium silicate hydrate (C–S–H) binder gel system

[5]. The main reaction product obtained is an alkaline

alumino-silicate, which has previously been described

as a zeolite precursor and nepheline (NAS2) or albite

(NAS6) of mineral polymer demonstrate how alkaline

mixture can form heat-resistant minerals [5–7].

In order to evaluate the effect of accidental

exposure to fire in the structures, the reaction of

concrete to elevated temperature must be examined,

with reference to mechanical properties, characteriza-

tion of microstructure changes with X-ray diffraction

(XRD), fourier transformed infrared spectrometer

(FTIR), thermogravimetry analyser (TGA–DTG) and

Field emission scanning electron microscope with

energy dispersive X-ray (FESEM–EDX). Generally,

the alkali activated concretes exhibit better thermal

resistance than OPC concretes. However, no such

information exists about alkali activated concrete

incorporating blended ash (pulverized fuel ash ? -

palm oil fuel ash). The present research work focuses

on a comparative study of high temperature perfor-

mance of OPC concrete and geopolymer concrete

resulting from the alkali activation of blended ash

(PFA ? POFA) from agro-industrial waste.

2 Materials and methods

2.1 Materials

PFA acquired from the silos of Kapar Power Station,

Selangor, Malaysia, POFA acquired from the burning

of the palm oil shell and husk from Kahang Mill in

Johor, Malaysia were used. More than 95 % PFA and

POFA passed through 45 lm. The chemical compo-

sition of the ashes is given in Table 1. To activate the

blended ash (BA = PFA ? POFA), a commercial

grade sodium hydroxide (NaOH) and sodium silicate

(Na2SiO3) alkaline solution was used as alkaline

activator. Local crushed granite sand with a specific

gravity of 2.62 as fine aggregate, and coarse aggre-

gates with a specific gravity of 2.68 were used for

making concrete. In order to enhance the workability

of BA geopolymer concrete, a super plasticizer

(napthaline based) was added to the mixture [8].

2.2 Testing procedures

All blended ash geopolymer (BAG) concrete speci-

mens were prepared with an alkaline solution ash ratio

Table 1 Chemical compositions of PFA, POFA and OPC

Type SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O LOI

PFA 46.7 35.9 5.0 3.9 0.8 0.6 0.5 1.0

POFA 53.5 1.9 1.1 8.3 4.1 1.3 6.5 18.0

OPC 20.1 4.9 2.4 65.0 3.1 0.2 0.5 2.4

Materials and Structures

of 0.4 by mass. The ratio of Na2SiO3 to NaOH is 2.5 by

mass. The concentration of NaOH was 14 Molar. The

molar SiO2 to Na2O of the sodium silicate solution is

equal to 2 (SiO2/Na2O: SiO2 = 29.4 %,

Na2O = 14.7 %. The rest is water = 55.9 % by

mass). The mix proportion for BAG concrete is given

in Table 2. Both coarse and fine aggregates were used

in saturated surface dry condition. The blended ash

and the aggregates were first dry-mixed in 80 l

capacity pan mixer for 5 min. The alkaline solution

containing NaOH and Na2SiO3 was added and mixed

for another 5 min. A napthaline based superplasticizer

was added to the mixture to achieve the workability of

BAG concrete between 80 and 100 mm slump. The

cube specimens of 100 9 100 9 100 mm size were

moulded and compacted in two layers, followed by

compaction on a vibration table for 10 s to remove the

air. After casting, the specimens were covered using

vacuum bagging film to avoid the evaporation of

alkaline solution. The test specimens were subjected to

room temperature (28 �C) for 28 days.

OPC concrete was also prepared with water to

cement ratio of 0.50 by mass, as control specimens

(Table 3). The specimens were cured in water for

28 days. The initial compressive strengths were

determined to be 26 and 25 MPa, for OPC and BAG

concretes respectively, as reported previously [8].

The specimens were then heated in the furnace

(shown in Fig. 1) that was designed for a maximum

temperature of 1,000 �C. The specimens were sub-

jected to temperatures of 200, 400, 600 and 800 �C at

an incremental rate of 4.4 �C/min starting from room

temperature. As soon as the target temperature was

attained, the specimens were put inside the furnace for

about 1 h. Specimens were allowed to cool naturally to

room temperature inside the furnace. Finally, mass

loss, compressive strength test and microstructure

analysis were conducted on the hardened concrete.

OPC concrete with water/cement (w/c) ratio 0.50

was used for comparison in the tests. The specimens

containing Portland cement at w/c = 0.50 had the

same consistency as the geopolymer specimens of

alkaline solution/binder (s/b) = 0.4. Thus, the speci-

mens were compared as having the same consistency

at the time of moulding. The compressive strengths of

OPC and BAG concrete at the age of 28 days were 26

and 25 MPa, respectively. The deterioration was

examined by XRD, FTIR, TGA/DTG and FESEM–

EDX.

Samples were taken from the concrete specimens

before and after elevated heat exposure for analysis. The

crushed concrete samples were grounded in a grinding

machine to obtain it in powder form (45 lm). XRD

analysis was performed through scanning from 5� to 65�2H, with a 0.02� step size and 2 s/step count time. FTIR

analysis was performed using the potassium bromide

(KBr) pellet method (1 mg sample per 100 mg KBr) on

a spectrometer, with 32 scans per sample collected from

4,000 to 400 cm-1 at 4 cm-1 resolution. TGA was

Table 2 Mix proportions of BAG concrete

BA ratio (%) s/ba ratio (%) Mix proportions (kg/m3)

Na2SiO3 NaOH PFA POFA Sand Aggregate Admixture

70:30 0.40 119 48 290 124 530 1234 8.3

a Solution/binder ratio

Table 3 Mix proportions OPC concrete

w/c ratio Mix proportions (kg/m3)

Water Cement Sand Aggregate

0.50 192 384 898 861

Fig. 1 Heating process in the furnace

Materials and Structures

conducted by transferring samples to an alumina

crucible, held under isothermal conditions for 60 min

at 40 �C to equilibrate in a nitrogen environment (N2

flowing at 200 ml/min), and then heated to 900 �C at

10 �C/min in the same gas environment. FESEM–EDX

analysis was performed by coating samples with

platinum prior to FESEM analysis. EDX was performed

at an accelerating voltage of 15 kV.

3 Results and discussion

3.1 Visual appearance

During the heating process, some transformations,

such as moisture evaporation, chemical decomposi-

tion, and internal vapour pressure may have occurred.

At an early stage of the heating process, transforma-

tions may not be quite enough to cause any cracks.

However, as the rate of heating became higher, the

moisture content of concrete was lost and exceeded the

plastic limit, resulting in appearance of cracks.

Between 200 and 400 �C, cracks did not appear in

the BAG concrete. However, hairline cracks started to

appear at 600 �C, as well as during further heating at

800 �C. Conversely, the hairline cracks were seen on

the surface of the OPC concrete at the temperature of

200 �C. The cracks were clearly seen during heating at

400 �C until 800 �C. The crack pattern in both

concrete specimens can be seen in Fig. 2.

3.2 Mass loss

Figure 3 represents the mass loss of specimens

exposed to elevated temperatures. Test data revealed

that the mass loss occurred in both OPC and BAG

concrete specimens due to exposure to elevated

temperatures. During heat treatment, the mass of both

OPC and BAG concrete specimens gradually

decreased with an increase in temperature. The mass

loss of BAG concrete specimen was 15.9 %, consid-

erably smaller than OPC concrete specimens which

exhibited 40 % mass loss after 800 �C exposure

(Fig. 3). This can be attributed primarily to the

reaction between CH present in the OPC concrete

specimens and the elevated heat treatment, which can

induce tensile stress, resulting in cracking and scaling

of concrete.

3.3 Compressive strength

Figure 4 shows the evolution of compressive strength

of the specimens exposed to elevated temperatures.

The BAG concrete showed the best performance with

an average strength decline of 16 % compared to 50 %

decline strength in OPC concrete. The compressive

strength was referred to its original before exposure to

elevated temperature. The strength of BAG concrete

increased as the temperature increased, attaining peak

strength at 600 �C, whereas the OPC concrete attained

the peak strength only at 200 �C. It can be concluded

that the BAG concrete possessed a stable and durable

matrix than the OPC concrete. The strength of the

Fig. 2 Crack comparison of the specimens. a BAG concrete

(crack start occurred at 600 �C), b OPC concrete (crack start

occurred at 200 �C)

Fig. 3 Mass loss of concrete specimens exposed to elevated

temperature

Fig. 4 Compressive strength of concrete specimens exposed to

elevated temperature

Materials and Structures

BAG concrete increased unexpectedly when exposed

to elevated temperatures. This increase in compressive

strength is attributed to the low diffusion coefficient of

Na? at elevated temperatures which results in a higher

melting temperature of the geopolymer [15]. It

indicates that the geopolymerization process in BAG

concrete continued during exposure to high tempera-

tures, at least up to 400 �C.

3.4 XRD analysis

An XRD technique was used to obtain a better

understanding of the possible transformation in

original materials as well as the samples exposed to

elevated temperatures. Figure 5 shows the XRD

analysis results for BAG concrete which consists of

before and after elevated temperatures at 28, 200, 400,

600 and 800 �C. Appearances of semi-crystalline

alumino-silicates gel (N–A–S–H) occurred in the

sample before and after exposure to 200 �C. The term

semi-crystalline N–A–S–H is used because the XRD is

showing a peak. Santaquiteria et al. [23] also reported

that appearances of alumino-silicates gel (N–A–S–H)

occurred in the sample before and after exposure to

200 �C. Our study also confirms their findings. The

broad peaks of the BAG concrete component could be

seen in the region 25–30 2h. Zeolites formed as a

secondary reaction products which hydroxysodalite

(Na4Al3Si3O12OH) and analcime (NAS4H2) were

formed at the crystalline phase during heating until

200–400 �C. After exposure to 600 �C, hydroxysoda-

lite seemed to have disappeared but crystalline

nepheline (NAS2) was present in the specimen [9].

After exposure to 800 �C, traces of nepheline and

broad peaks of albite (NAS6) were found [10].

The XRD diffractograms obtained for OPC con-

crete sample before and after exposure to elevated

temperatures, are shown in Fig. 6. The main phases

identified in OPC concrete are C–S–H gel, CH and

calcium carbonate. C–S–H-gel along with CH and

Calcium carbonate has also been identified by Rashad

and Zeedan [4] and Morsy et al. [11]. Our study also

Fig. 5 XRD of BAG

concrete exposed to elevated

temperature

Fig. 6 XRD of OPC

concrete exposed to elevated

temperature

Materials and Structures

confirms their findings. Samples thermally treated at

the 200–800 �C mostly dominated with the presence

of CH and calcite (C). The intensity of CH peak

decreased due to its decomposition to quicklime

(CaO) as well as the partial conversion of CH to

calcium carbonate such as calcite and anorthite [4, 11].

At 400–800 �C, C–S–H completely disappeared. It

was thought to be mainly due to the transformation of a

new structure to crystalline anhydrous calcium silicate

phases i.e. calcite (C) and anorthite (An) [4].

3.5 FTIR analysis

The FTIR spectra in Figs. 7 and 8 indicate major bands

at approximately 3,445, 1,645, 1,425, 1,015 cm-1 in

OPC concrete and 3,450, 1,645, 1,430, 1,045,

780 cm-1 in BAG concrete. The structure of molec-

ular water in the system is characterized by the O–H

stretching band, from 3,200 to 3,700 cm-1, while

bending of the chemically bonded H–O–H is located at

1,645 cm-1. This could be related to water bound in

the hydrated products formed after alkaline activation

[12]. Thus, the bands at 1,010–1,040 and 780 cm-1

are assigned to quartz as the crystalline phase in both

samples [13]. In Malaysia, 100 % OPC is not

commercially available in the local market. Only the

blended OPC with some unknown % of PFA is

available. Therefore, the FTIR peaks are shifted to

1,005 cm-1 because of blended OPC ? PFA. The

carbonate in the system is characterized by absorption

at 1,425 cm-1, which is consistent with the presence

of anorthite and calcite particularly in OPC samples

[22, 23]. The main binder gel band appears at

1,015 cm-1, assigned to the asymmetric stretching

mode of the C–S–H structure formed in OPC samples.

Whereas the position at 1,045 cm-1 is consistent with

Fig. 7 FTIR spectra of

BAG concrete samples

exposed to elevated

temperature

Fig. 8 FTIR spectra of OPC concrete samples exposed to elevated temperature

Materials and Structures

N–A–S–H gels formed in geopolymer binder systems

derived from solid precursor used [14]. FTIR spectra

of the BAG concrete samples before and after heat

treatment show only minor differences (Fig. 6). The

Si–O–Si bond of the BAG based product is not

affected when exposed to elevated temperatures. The

bands at approximately 3,450 and 1,645 cm-1are

attributed to O–H stretching and O–H bending

respectively. These are being characteristic by weakly

bound molecules of water [15, 16]. The BAG concrete

sample before and after heat treatment showed only

small changes in the bands between 780 and

1,045 cm-1 and a marked decrease of chemically

bonded water at about 1,645 and 1,430 cm-1 band. It

resulted from the decomposition of calcium carbonate

by the reaction of enzymes and nepheline as identified

by XRD at temperature from 400 to 600 �C. On the

other hand, presence of albite as identified by XRD,

supported the N–A–S–H binder gel of BAG concrete

to maintain its position. It indicates that most of the

molecular chains consisting of SiO4 and AlO4 tetra-

hedra, linked alternately by sharing all the oxygens,

were not significantly destroyed by heat temperature

[12].

Conversely, the reaction of the OPC samples at

temperature up to 800 �C showed marked decompo-

sition of the C–S–H and O–H phases in the micro-

structure. Figure 8 shows distinct differences between

the spectra obtained from exposed specimens when

compared with unexposed specimens. The water

component at 3,445 cm-1 changed to 2,975 cm-1

and the chemically bonded carbonate at 1,425 cm-1

also changed to 1,460 cm-1 starting from heating

600–800 �C which are contributed by the presence of

anorthite and calcite as identified by XRD [17, 23].

Finally, the decomposition of the main binder, C–S–H

gel is associated with shifting to the new bands at

1,130 and 955 cm-1 in the sample after heating at

800 �C temperature. It is also consistent with the

degradation of the binder assigned to the presence of

anorthite and calcite [18]. It shows that the OPC

concrete was altered by high or elevated temperatures.

3.6 Thermogravimetry

Figure 9 presents differential thermogravimetry

(DTG) data. The mass loss (TGA) was determined

up to 900 �C for the BAG and OPC concrete samples.

3.6.1 BAG concrete

The samples of unexposed (not heated in the furnace)

BAG concrete were subjected to TGA analysis. A

mass loss of 8.8 % was observed at the temperature

range of 60–200 �C. However, it was 7.6 % when

these samples were exposed to 200 �C in the furnace.

For the BAG concrete exposed to 400 and 600 �C, 3.2

and 2.7 % mass loss was observed, respectively.

Between 500 and 900 �C, 1.8 and 1.4 % mass loss

was noted, thus making the cumulative mass loss equal

to 5.0 and 4.1 % respectively. It is clear that the BAG

concrete showed a mass loss at 60–200 �C which is

associated with free and/or loosely bound water

present in the samples [18, 19]. Considering the

temperature of the initial mass loss peak at 60 �C, it is

observed that the main structure of these BAG

concretes was dominated by alumino-silicate type

products (geopolymer gel such as N–A–S–H), which

usually present a mass loss at low temperatures. It is

due to the freely evaporable water present in the pores

of these gels. The identification of a peak at 200 �C is

related to reaction products with water which are more

tightly bonded to their structures than in geopolymeric

gel. At this temperature, it is common to detect the

occurrence of dehydration of zeolites and related

structures, such as hydrosodalite as identified in FTIR,

particularly when exposed to slightly elevated tem-

perature [10]. Furthermore, peak at 650 �C is attrib-

uted to the complete dehydration of zeolites present in

the binder [20], while no significant change was

exhibited at temperatures above 750 �C.

3.6.2 OPC concrete

The samples of unexposed (not heated in the furnace)

OPC concrete were subjected to TGA analysis. The

mass loss of 3.1, 8.3 and 12.9 %, respectively, was

observed at temperatures ranging from 60 to 200, 200

to 600 and 600 to 900 �C. However, the mass loss was

2.8, 6.9 and 7.1 % when the samples were exposed to

200 �C in the furnace with cumulative mass loss of

18.4 and 21.9 % respectively. The mass loss of

samples exposed to 400 �C in the furnace was

observed 11.5, 15.0 and 18.0 % at temperatures

ranging from 60 to 300, 300 to 600 and 600 to

900 �C. However, the mass loss of 4.6, 3.8 and 4.9 %,

respectively, was determined for the samples exposed

to 600 �C in the furnace at temperatures ranging from

Materials and Structures

60 to 300, 300 to 600 and 600 to 900 �C with

cumulative mass loss of 44.5 and 13.3 %, respectively.

For the concrete exposed to 800 �C in the furnace, the

mass loss was constant. It remained 1.4 % in the

temperature range of 400–900 �C.

For the OPC concrete specimens, the crystalline

phase of CH (portlandite) was decreased as the

temperature increased. A significant reduction in the

intensity of CH peak was observed after the samples

were exposed to 600 �C in the furnace. This is due to

the thermal decomposition of CH phase at about

400–500 �C, forming evaporable water steam and

calcium oxide (Fig. 9). The products formed are

porous and could absorb atmospheric water vapour

to re-hydrate and re-form Ca(OH)2 accompanied by

volume expansion. It leads to further cracking if

exposed to the atmospheric environment [11]. As can

be seen from the DTG diagram, the first peak located at

approximately 60–200 �C. This peak is mostly due to

the decomposition of free and loosely bounded water.

The second peak is at approximately 430–470 �C and

represents the decomposition of Ca(OH)2. The third

peak at approximately 650–670 �C is related to the

decarbonation of calcite as identified in XRD [4, 21].

3.7 Field emission scanning electron microscope

with energy dispersive X-ray (FESEM–EDX)

Investigations of microstructure of the samples using

FESEM–EDX showed distinct changes in morphology

as a consequence of exposure to elevated tempera-

tures. Figure 10 shows FESEM–EDX micrographs of

Fig. 9 Thermogravimetry of BAG concrete exposed to elevated temperatures. a TGA for BAG concrete, b TGA for OPC concrete,

c DTG for BAG concrete, and d DTG for OPC concrete

Materials and Structures

(a)

(b)

(c)

(d)

Fig. 10 FESEM image and

EDX spectrum of OPC

concrete exposed to elevated

temperature. a OPC

concrete exposed to 200 �C,

b OPC concrete exposed to

400 �C, c OPC concrete

exposed to 600 �C, and

d OPC concrete exposed to

800 �C

Materials and Structures

(a)

(b)

(c)

(d)

Fig. 11 FESEM image and

EDX spectrum of OPC

concrete exposed to elevated

temperature. a BAG

concrete exposed to 200 �C,

b BAG concrete exposed to

400 �C, c BAG concrete

exposed to 600 �C, and

d BAG concrete exposed to

800 �C

Materials and Structures

the fracture surface of OPC concrete samples after

exposure to 200, 400, 600 and 800 �C. It became clear

that the microstructure of OPC, heat exposure at

200 �C, was stable for thermal treatment and illus-

trated a dense structure of hydrated products as shown

in Fig. 10a. It is evidenced from the microstructure of

the hardened concrete that the C–S–H and CH exist

after 200 �C exposure. However, heat exposure of the

concrete at 800 �C displayed decomposition of the

hydration products with the formation of wide micro-

cracks and pores size ranging between 3.5 and 10 lm

(Fig. 10d). Furthermore, the cement paste was sepa-

rated from the aggregate thereby creating gaps. Poor

microstructure is associated with the generation of

undesirable configuration of C–S–H crystals, and

increased cracking at high temperature. Generally,

the C–S–H crystals grow long, thin/narrow and occupy

less space in the matrix at high temperatures, thereby

resulting in decreased densification of the microstruc-

ture [11]. The hydration of C–S–H into calcium

silicate and lime, which produce white, needle-shaped

bundles can be seen in specimens exposed between

400 and 800 �C [4]. The increased micro-cracking is

the result of high thermal stresses that are generated

due to the induced temperature gradients.

After exposure to 400 �C the microstructure of

BAG concrete specimens was transformed into a

matrix of more reacted material with crystalline

reaction products as shown in Fig. 11b. After exposure

to 600 �C, the microstructure seemed to be more

reactive than the OPC concrete samples shown in

Fig. 11c. It clearly showed that heat exposure of the

BAG concrete sample at 800 �C caused a significant

reduction in unreacted particles. A more compact

microstructure due to the sintering process at 800 �C,

the size of albite was 8.75 lm larger than 5.72 lm at

600 �C which contributed to the durable matrix of the

concrete.

Table 4 shows the elemental ratio (Ca/Si) calcu-

lated from FESEM–EDX collected after both samples

were heated to the specified elevated temperatures.

The Ca/Si ratio was calculated from EDX (Figs. 10,

11; Table 4). In OPC concrete the Ca/Si ratio tends to

be higher because of the high content of CaO

compared to BAG concrete. It is indicated that the

Ca rich gel, such as C–S–H, can have more harmful

effects compared to a lower Ca/Si ratio in the binder

system like N–A–S–H when the concrete samples are

exposed to elevated temperature up to 800 �C.

4 Conclusions

The following conclusions can be drawn from this

study:

1. The BAG concrete exhibited better structural

stability than OPC concrete after exposure to

elevated temperatures due to more stable cross-

linked alumino-silicate polymer structure. The

hairline crack appeared between 600 and 800 �C

in BAG concrete whereas they were observed on

the surface of the OPC concrete at the temperature

of 200 �C.

2. The BAG concrete had the best performance with

an average of 16 % strength decline compared to

50 % strength decline of OPC concrete. The

strength of BAG concrete increased as tempera-

ture increased, attaining peak strength at 600 �C

whereas the OPC concrete attained peak strength

at 200 �C only.

3. At elevated temperatures (200–800 �C), the zeo-

lite-like product of N–A–S–H binder gel system

in BAG concrete samples reacted as thermally

stable structures, such as hydrosodalite and anal-

cime. This improved the crystallinity of geopoly-

mer materials during heating until 200–400 �C

and maintained their structure up to 800 �C. It

recrystallized to structurally similar nepheline or

albite as proved in XRD.

4. BAG concrete had better heat resistance as com-

pared to OPC concrete, as evidenced by thermal

behaviour, increase in compressive strength and

minimal changes of the bands between 780 and

1,045 cm-1 in FTIR spectra. Compact microstruc-

ture was observed by FESEM images.

5. The Ca-rich gel such as C–S–H could have severe

effects in OPC, while N–A–S–H gel systems at

the same concentration appeared to have less

effect on the structure of the geopolymer material.

Table 4 Mean Ca/Si atomic ratio after heat exposure

Type of sample Temperature (�C)

200 400 600 800

OPC

BAG

Ca/Si ratio 2.24

0.25

2.15

0.12

2.98

0.22

2.69

0.12

Materials and Structures

6. This study suggests that it is more feasible to

utilize BAG concrete than OPC concrete as source

material for synthesizing fire resistant geopoly-

mer. This could be used as a construction struc-

tural material requiring fire-resistant performance.

Acknowledgments The authors wish to express their

appreciation to Ministry of Higher Education (MOHE) and

Research Management Centre (RMC), UniversitiTeknologi

Malaysia (UTM) for providing the Research University Grant

(RUG), VOT No. QJ130000.2522.03H36.

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Materials and Structures

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