Post on 11-Jun-2022
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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