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Magazine of Concrete Research, 2011, 63(3), 163–173
doi: 10.1680/macr.9.00110
Paper 900110
Received 25/06/2009; last revised 08/03/2010; accepted 06/04/2010
Published online ahead of print 10/01/2011
Thomas Telford Ltd & 2011
Magazine of Concrete ResearchVolume 63 Issue 3
Geopolymer and Portland cement concretesin simulated fireZhao and Sanjayan
Geopolymer and Portlandcement concretes in simulatedfireR. ZhaoDepartment of Civil Engineering, Monash University, Clayton, VictoriaAustralia
J. G. SanjayanFaculty of Engineering and Industrial Sciences, Swinburne University ofTechnology, Hawthorn, Victoria, Australia,
High-strength Portland cement concrete has a high risk of spalling in fire. Geopolymer, an environmentally friendly
alternative to Portland cement, is purported to possess superior fire-resistant properties. However, the spalling
behaviour of geopolymer concrete in fire is unreported. In this paper, geopolymer and Portland cement concretes of
strengths from 40 to 100 MPa were exposed to rapid temperature rises, simulating fire exposures. Two simulated fire
tests, namely rapid surface temperature rise exposure test and standard curve fire test, were conducted. In both
types of test, no spalling was found in geopolymer concretes, whereas the companion Portland cement concrete
exhibited spalling. This can be attributed to different pore structures of the two concretes. The sorptivity test found
that geopolymer concrete had a significantly higher sorption, therefore more connected pores, than Portland cement
concrete when compared at the same strength level. Hence, it is suggested that the water vapour can escape from
the geopolymer matrix quicker than in Portland cement concrete, resulting in lower internal pore pressure. The paper
concludes that, when compared at the same strength level, the geopolymer concrete possesses higher spalling
resistance in a fire than Portland cement concrete due to its increased porosity.
Introduction
Spalling of concrete
Concrete can spall when exposed to fire, leading to disintegration
of concrete structure in an accidental fire. Sometimes the spalling
is explosive. Explosive spalling is characterised by large or small
pieces of concrete being violently expelled from the surface
(Phan, 1996). The pieces may be as small as 100 mm or as large
as 300 mm in length and 15–20 mm deep in the concrete
structure elements. This type of spalling occurs during the early
part of a fire, usually within the first 30 min or so of a standard
furnace test. Various researches have been reported on the
spalling behaviour of Portland cement concrete and blended
Portland cement concrete. It is believed that high-strength con-
crete is more vulnerable to spalling when exposed to fire than
normal-strength concrete (Ali et al., 2001, 2004; Phan, 1996 ).
Spalling mechanisms
There are three main theories commonly used to explain the
cause of spalling.
(a) Moisture clog spalling: this was first proposed by Shorter and
Harmathy (1961), who hypothesised that spalling was caused
by the steam pressure build-up in the pores of concrete in
fire. During heating, the heat flow will increase the
temperature of the pore water in the concrete. When the pore
water reaches a sufficiently high temperature, it will begin to
vaporise, resulting in the increase of pore pressure. The
vapour will migrate along the temperature gradient, and
either escape from the concrete or move in the material until
it reaches a lower-temperature area and condenses. As this
process continues, pore water will build up in the cooler
region and form a saturated layer. The saturated layer will
impede the pore water from further migration. If vaporised
water cannot escape fast enough, the internal pore pressure in
the material will keep rising until it exceeds the material’s
tensile strength and causes spalling. This theory was later
adopted by Consolazio et al. (1998) and Kalifa et al. (2001).
(b) Bazant (1997) hypothesised that spalling results from
restrained thermal dilation close to the heated surface, which
leads to compressive stresses parallel to the heated surface,
further leading to brittle fractures of concrete. Similar
alternative theories include: that developed by Ulm et al.
(1999), the chemoplastic softening model; Stabler and Baker
(2000), the coupled thermomechanical damage model; and
Nechnech et al. (2002), the elastoplastic damage model.
(c) Thermal incompatibility between the aggregates and the
cement paste (Phan, 1996) may also cause spalling,
particularly in concrete with siliceous aggregates.
It has also been concluded by many researchers (Bazant and
Thonguthai, 1979; Harada and Terai, 1997; Khoury, 2000; Phan
et al., 2001) that concrete spalling is caused by the combination
163
of pore pressures and differential thermal stresses. In these
reports, it is believed that spalling is largely due to the moisture
presence and rapid heating rate. The rapid heating rate causes
thermal gradients and build-up of pore pressures. Pore pressure
and differential thermal stress can act solely or in combination
depending on the heating rate, concrete size, concrete structure,
moisture content and so on. Spalling may occur when the
combination of these pressures exceeds the tensile strength of the
concrete.
Spalling test methods
Spalling is difficult to predict and characterise. Spalling predic-
tion during heating has been largely an imprecise empirical
exercise. Large-scale tests are commonly used to assess the
spalling propensity of a concrete (Sanjayan and Stocks, 1993).
The specimens are usually tested in a fire furnace which is set to
follow a standard fire or hydrocarbon fire curve. This test method
is the standard method to predict risk of spalling. However, it is
costly and time-consuming.
Han et al. (2005, 2009) proposed a small-scale test using a gas
fire furnace. The standard size (100 mm diameter 3 200 mm
high) concrete cylinders were placed in the gas fire furnace
before testing. The cylinders were heated in the furnace by a
preset temperature plotted against time curve. This test method is
more economic and convenient than a large-scale test; however, it
does not consider the effect of size. Phan and Carino (2002) also
used the small-scale cylinder specimens to evaluate the concrete
behaviour exposed to the elevated temperatures. They concluded
that spalling tendency increased as the water/cementitious materi-
al ratio decreased. This tendency is consistent with the notion
that the tendency of explosive spalling is related to the resistance
to water vapour transport during heating. The present study used
this type of test for spalling assessment.
Hertz and Sorensen (2005) proposed a small-scale test by
exposing confined concrete cylinders to a pre-heated electric
muffle furnace. They developed a steel mantle to confine the
concrete cylinder. The confined concrete cylinder was exposed to
the pre-heated furnace chamber to achieve a rapid increase in
temperature. The end of the confined cylinder could be consid-
ered as a part of a fire-exposed surface of a concrete slab or wall.
A modified form of this test is also used in the present study.
Geopolymer cements
Ordinary Portland cement (OPC) is the main ingredient used in
the production of concrete – the most widely used construction
material in the world. In the past, concrete was simply a compo-
site of Portland cement paste with aggregates; however, modern-
day concrete incorporates other cementitious materials which act
as partial replacements of Portland cement. Fly ash is often used
in concrete as a supplementary cementitious material. Using fly
ash blended cement in concrete brings environmental benefits by
reducing resource, energy consumption and carbon dioxide emis-
sions.
Davidovits (1991) introduced the word ‘geopolymer’ to describe
an alternative cementitious material which has ceramic-like
properties. As opposed to OPC, the manufacture of fly-ash-based
geopolymer does not consume high levels of energy, as fly ash is
already an industrial by-product. This geopolymer technology has
the potential to reduce emissions by 80% (Davidovits, 1991)
because high-temperature calcining is not required. Geopolymer
can be produced by combining a pozzolanic compound or
aluminosilicate source material with highly alkaline solutions
(Davidovits and Davidovics, 1991). Fly ash, which is available
abundantly worldwide from coal-burning operations, is an excel-
lent aluminosilicate source material. In Australia, fly ash is
currently underutilised; according to figures taken from the year
2000, 12 million tonnes per annum were produced but only 10%
were effectively utilised in cementitious applications (Heindrich,
2002).
Fly ash was activated by the alkali to form an inorganic alumino-
silicates polymer which has a similar structure to the zeolite
minerals (Davidovits, 1991; Davidovits and Davidovics, 1991). It
hardens like organic resin, but is stable up to 1000,12008C. It
has been reported that geopolymer material had ceramic-like
properties with high strength and fire resistance (Davidovits,
1991; Davidovits and Davidovics, 1991). It has also been noted
by many researchers that geopolymer is a porous material.
Duxson et al. (2007) reported that a highly distributed pore
network existed in the geopolymer gel. Sindhunata et al. (2006)
reported that high-temperature curing increased the geopolymer-
isation extent and rate and increased mesopore volume.
Geopolymer generally requires high temperature to develop con-
siderable high strength. In the present paper, a curing temperature
of 808C is used. Therefore, it may not be practical to use for on-
site construction. However, in precast construction, 808C is used
for precast concrete slabs, beams, columns and pipes to eliminate
the lag between the time the on-site concrete is placed and the
time at which it can carry loads. Geopolymer can be prepared in
the precast construction plant.
It has also been noted by many researchers that geopolymer is a
porous material. Duxson et al. (2007) reported that a
highly distributed pore network existed in the geopolymer gel.
Sindhunata et al. (2006) reported that high-temperature curing
increased the extent and rate of geopolymerisation and increased
mesopore volume.
Research studies on concrete in fire include two main areas:
(a) residue strength performance of the concrete subjected to the
elevated temperature
(b) spalling behaviour of the concrete.
There are a few reports on the residual strength performance of
geopolymers at elevated temperature. Kong et al. (2007) noted
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Geopolymer and Portland cementconcretes in simulated fireZhao and Sanjayan
that the residual strength of fly-ash-based geopolymer paste
increased by 6% after exposure to 8008C, whereas the strength of
metakaolin-based geopolymer paste reduced 34% after exposure
to 8008C. Kong et al. (2007) concluded that during heating,
the high permeability of fly-ash-based geopolymer provides the
escape routes for moisture in the matrix, thereby decreasing the
damage to the matrix. Sintering of fly ash geopolymer increases
the strength at 8008C. Similar strength increase was also reported
by Pan et al. (2009) on fly-ash-based geopolymer mortars.
However, there has been no study reported on the spalling
behaviour of the geopolymer material subjected to rapid tempera-
ture rise.
The aim of the present paper is to study the spalling behaviour of
geopolymer concrete subjected to rapid temperature rise simulat-
ing a fire. Small-scale test methods including a surface exposure
test by using an electric furnace and a gas fire furnace test were
conducted. Several Portland cement concrete cylinders with the
same strengths were tested to compare the spalling behaviour.
Further testing was carried out to explore the reasons for the
different behaviours in fire of Portland cement and geopolymer
concretes.
Spalling test
Materials
Ordinary Portland cement conforming to the requirements of
Australian standard AS3972 was used as the binder material of
Portland cement concrete. The fly ash was sourced from Pozzo-
lanic Gladstone and it was a low-calcium fly ash (class F). The
chemical composition of the fly ash was determined by X-ray
fluorescence (XRF) and is presented in Table 1.
Alkaline activators used for making the geopolymers consisted of
alkali silicate and hydroxide solutions. The alkali silicate was
Grade D sodium silicate solution with a specific gravity of 1.53
and modulus ratio (Ms) equal to 2 (where Ms ¼ SiO2/Na2O,
Na2O ¼ 14.7% and SiO2 ¼ 29.4%). The hydroxide solution was
prepared to 8 M and 12 M concentrations using a mixture of
distilled water and a commercial grade of pellets with 90% purity,
supplied by PQ Australia.
The basalt coarse aggregate had a maximum size of 14 mm and
was sourced from Readymix, Mount Shamrock quarry, Victoria,
Australia. The fine aggregate consisted of Lynhurst sand with a
fineness modulus of 2.19. The superplasticiser used in the high-
strength Portland cement concrete was Glenium 27 (high-range
water reducer, provided by BASF).
Specimen preparation
Portland cement concrete
The sand and coarse aggregate was dry mixed in a 70 litre pan
mixer for 2 min. The cement and water were added and mixed
for 2 min. After resting for a further 2 min, the concrete was
remixed for another 2 min before sampling and testing. Three
Portland cement samples were prepared for each test result. The
samples were cured for 28 days in saturated lime water kept at
238C prior to testing.
Geopolymer concrete
Sodium hydroxide pellets were mixed with distilled water to
prepare an alkaline solution one day in advance of the day of the
mixture preparation. On the day of the mixture preparation, the
sand and coarse aggregate were initially blended with fly ash and
dry mixed first in the 70 litre pan mixer for 1 min. The alkaline
solution prepared the day before was then introduced to the
mixture, and the wet mixing continued for another 4 min. Then
the mixture was cast into standard 150 mm diameter and 300 mm
high cylindrical moulds. The fresh concrete was compacted by a
vibration table to release any residual air bubbles. The concrete
in the cylinder moulds was placed in an oven at 808C for curing.
Four batches of samples were cured in the oven for 3 h, 8 h, 48 h
and 96 h respectively to reach the different target strengths. Then
the samples were placed in a constant-temperature room (238C,
50% humidity) for 28 days before testing. Six geopolymer
samples were prepared for each test result.
The mixture proportions and curing regime are presented in Table
2. The aggregates weights shown in Table 2 are in the saturated
surface dry condition. Compressive strength was tested for the
concrete cylinders by using an AMSLER compressive strength
test machine at a loading rate of 20 MPa/min. Both Portland
cement and geopolymer concrete cylinders were tested at 28 days
after curing for compressive strength.
Surface exposure test
Based on Hertz’s method (Hertz and Sorensen, 2005), the present
study has developed a similar method to test spalling of concrete
(Figures 1 and 2). A circular hole was created on the top of the
furnace chamber and it was initially covered by a ceramic board
during the pre-heating. The furnace was preheated to a tempera-
ture of 10008C and allowed to stabilise at this temperature for
30 min. Then the top cover board was removed and one end of
the 150 mm diameter cylinder was exposed through the 100 mm
Chemical constituent: % Portland cement Fly ash
SiO2 19.90 48.8
Al2O3 4.70 27.0
CaO 63.93 6.2
Fe2O3 3.38 10.2
K2O 0.446 0.9
MgO 1.30 1.4
Na2O 0.17 0.2
SO3 2.54 0.2
LOI 2.97 1.7
Table 1. Composition of fly ash and Portland cement as
determined by XRF
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diameter hole, which transferred the heat from the furnace
chamber. This resulted in a temperature of about 8508C at the
surface of the cylinder in approximately 2 min. The temperature
plotted against time curve of the cylinder surface increased more
rapidly than the ISO standard fire curve (ISO, 1975) and was
close to the hydrocarbon fire curve (BSI, 2002) (Figure 3).
Therefore, this test can be used to simulate a standard fire or
more severe fire conditions than a standard fire. It should be noted
here that the rate of temperature rise in the first 30–40 min is the
most critical for concrete spalling (Sanjayan and Stocks, 1993);
O40* O60* O80* O110* G40y G60y G80y G100y
Water:cement 0.58 0.46 0.34 0.3 — — — —
Sand:aggregate:cement 2.4:3.8:1 1.9:3.1:1 1:2.1:1 1.4:2.9:1 — — — —
Water: kg/m3 190.00 180.00 190 143 — — — —
Cement: kg/m3 327.58 391.33 561 430 — — — —
Silica fume: kg/m3 — — — 47 — — — —
Alkaline liquid/fly ash — — — — 0.45 0.45 0.45 0.35
Fly ash: kg/m3 — — — — 381 381 381 409
NaOH solution (8M): kg/m3 — — — — 49 49 49 —
NaOH solution (12M): kg/m3 — — — — — — — 41
NaSiO4 solution (Grade D): kg/m3 — — — — 122 122 122 102
Fine aggregate: kg/m3 780.70 765.33 572.7 602 554 554 554 554
Coarse aggregate: kg/m3 1241.70 1218.33 1193 1280 1294 1294 1294 1294
Superplasticiser: l/m3 — — — 10 — — — 10
Curing condition 238C, saturated lime water 808C oven 908C oven
Curing time 28 days 3 h 6 h 48 h 96 h
* O40, O60, O80, O110 represent the ordinary Portland cement concrete specimens with a target compressive strength of 40 MPa, 60 MPa,80 MPa, 110 MPa respectively.y G40, G60, G80, G100 represent the geopolymer concrete specimens with a target compressive strength of 40 MPa, 60 MPa, 80 MPa, 100 MParespectively.
Table 2. Concrete mix proportions (kg/m3) of the concretes
Finance chamber
Thermocouple100 mm
Ceramic board cover
Cyl
inde
r
150 mm
Stee
l mou
ld
Neo
pren
e
8 m
m20
0 m
m
Figure 1. Testing rig for the surface exposure test method
Figure 2. Test furnace and specimen confined by steel mould
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unlike other materials such as steel, performance is determined
by the level of absolute temperature and length of exposure.
The cylinder size was 150 mm in diameter and 300 mm high.
Before the test, the cylinder was confined by a 25 mm thick steel
cylindrical mould tightened by high-strength bolts. A layer of
neoprene was placed in between the cylinder and the mould to
compensate for the irregularities of the concrete cylinder surface.
A thermocouple was placed on the surface of the cylinder that
was subjected to temperature exposure. The temperature rate of
change on the surface of the cylinder was recorded by a
thermocouple. The test was conducted for a duration of 1 h.
The moisture content of the specimen was measured by a
TRAMEX electronic moisture meter before the test. Also,
identical specimens were dried and weighed to determine the
moisture content of the specimens at the time of test (Table 3).
After the test, the specimens exposed to simulated fire test also
were weighed and the degree of spalling was observed.
Gas fire furnace test
A hydrocarbon gas fire furnace can generate rapid temperature
rise during a short time (BSI, 2002). The gas fire furnace test
using small cylinders has been used previously to assess con-
cretes for spalling (Han et al., 2005, 2009; Phan and Carino,
2002). In the test used in this study, concrete cylinders 100 mm
in diameter and 200 mm high were placed in the furnace (Figure
4), which was heated by a preset ISO standard fire temperature
plotted against time curve (ISO, 1975). In contrast with the
surface exposure test (described in the previous section), the
cylinders were not confined. Therefore, the confining effect of
surrounding cool concrete in a large specimen was not simulated
in these tests. However, this test was used to investigate the
spalling behaviour of geopolymer concrete in standard fire curve
exposure and also to confirm the effects of the surface exposure
test method by comparing the same samples.
Results and discussion
Surface exposure test results
No spalling was observed on the surface of any of the geopoly-
mer concrete cylinders. The heat exposure 100 mm diameter
circular area on the cylinder surface turned a brown colour. This
may have been caused by the oxide of the iron from the fly ash.
1200
1000
800
600
400
200
0
Tem
pera
ture
: °C
0 5 10 15 20 25 30 35 40 45 50 55 60Time: min
Hydrocarbon fire curve
Surface exposuretest curve
Standard fire curve
Figure 3. Temperature plotted against time curves in the spalling
test
O40 O60 O80 O110 G40 G60 G80 G100
Compressive strength: MPa 42 63 82 110 37 62 78 98
Moisture content: % 5 5.3 5.5 5.5 5.2 5.1 5.2 5.3
Spalling specimen number* 1 3 3 3 0 0 0 0
No spalling specimen number* 2 0 0 0 6 6 6 6
Average spalling depth: mm 0.1 0.3 0.7 2.5 0 0 0 0
Spalling area percentage: %y 1.6 30 83 100 0 0 0 0
* Spalling and no spalling specimen number means the numbers of concrete specimens that showed spalling or no spalling after the test.y Spalling area percentage represents the percentage of spalling area in the total heated area.
Table 3. Compressive strength and spalling occurrence of the
test samples
Figure 4. Concrete cylinders placed in the gas fire furnace before
testing
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The outer unheated 50 mm layer remained intact and acted as a
restraint to the heat exposure area (Figure 5).
High-strength Portland cement concrete samples (O80, O110)
showed signs of spalling (Figure 5). Significant spalling occurred
on the O110 Portland cement concrete cylinders. Minor spalling
occurred on O60 specimens. The spalling area or depth of O60
was much smaller compared with the high-strength Portland
(a) (b)
(c) (d)
(e) (f)
(g) (h)
Figure 5. Geopolymer and Portland cement concrete cylinders
after surface exposure test: (a) G40; (b) O40; (c) G60; (d) O60; (e)
G80; (f) O80;(g) G100; (h) O110
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cement concretes. One cylinder spalled out of three tested in
40 MPa specimens; however, the degree of spalling was very
small (Figure 5).
The spalling percentages are presented in Table 3. The numbers
represent the percentage of spalled area of the total exposed area.
O80 and O110 specimens had 83% and 100% spalling percen-
tages, which are the maximum spalling patterns in the test. Other
Portland cement cylinders with lower compressive strengths
exhibited significantly lower spalling percentages.
Cracking or popping sounds were noticed during the test, which
were understood to be accompanying the incidences of spalling.
The times of the sounds were recorded. Higher-strength concretes
(O80, O110) were accompanied by louder noise and much more
severe spalling than low-strength concretes. The recording of the
times of the sounds indicated that spalling occurred in the initial
2–3 min of the rapid heating. No sound was recorded after the
first 5 min (Figure 6).
Gas fire furnace test results
The gas fire temperature rate was set to follow the ISO standard
fire (ISO, 1975). Figure 7 shows the test results of geopolymer
and Portland cement concrete cylinders with strength levels
varying from 40 to 110 MPa subjected to the standard fire. No
spalling occurred on any of the geopolymer concrete specimens.
High-strength Portland cement concrete specimens exhibited
severe spalling. Normal- and low-strength Portland cement con-
crete specimens exhibited minor spalling. These results are
consistent with the surface exposure test results reported above.
Sorptivity testThe spalling test results demonstrate that geopolymer concrete
has a better resistance to spalling than Portland cement concrete.
In order to explore further the reason for this phenomenon,
sorptivity tests on the geopolymer and Portland cement concretes
were carried out.
Sorptivity represents the material’s ability to absorb and transmit
water through the matrix by capillary suction. Compared with
permeability, which is used to measure the flow of water under
pressure in a saturated porous material, sorptivity is a more
suitable parameter for evaluating the pore connectivity and
capillary network, which is a major factor influencing water
transmission in the concrete when subjected to fire (Consolazio et
al., 1998; Kalifa et al., 2001; Shorter and Harmathy, 1961). Thus,
the sorptivity test was conducted to compare the pore structure
characteristics of both types of concrete (geopolymer and Port-
land cement concretes).
Test method
Following 28 days of curing, three concrete cylinders for the
water sorptivity test were prepared for each batch. The sorptivity
tests were conducted according to the test method specified by
ASTM C1585 (ASTM, 2004). According to this test method, the
test specimens were first dried until constant weight at 238C in a
desiccator before testing. The test specimen was exposed to the
water at one end by placing it in a pan (Figure 8). The water in
the pan was maintained at about 5 mm above the base of the
specimen. The lower surface on the sides of the specimen was
coated with paraffin to achieve unidirectional flow. At certain
times (0, 5, 10, 20, 30, 60, 180, 360, 1440 min), the weight of the
specimen was measured. The volume of water absorbed was
calculated.
The sorptivity coefficient (Collins and Sanjayan, 2008; Gonen
and Yazicioglu, 2007; Igarashi et al., 2005; Khan, 2003; Olivia et
al., 2008) was obtained by the following equation
Q
A¼ k
ffiffi
tp
1:
where Q is the volume of water absorbed (mm3); A is the cross-
sectional area of specimen that was in contact with water (mm2);
t is the time (min); and k is the sorptivity coefficient of the
specimen (mm/min1=2) – this is the sorptivity measured per mm2
of wetted area per min1=2. To determine the sorptivity coefficient,
Q/A was plotted against the square root of time (ffiffi
tp
), then k was
calculated from the slope of the linear relation between (Q/A) andffiffi
tp
.
Results and discussion
As shown in Figure 9, the sorptivity coefficients (k) of geopoly-
mer concrete specimens were significantly higher than for the
Portland cement concrete specimens. When compared at the same
strength level, the k value of G40 is about twice that of the k
value of O40. On the higher strength levels, the difference of k
value was increased. The k value of G90 is almost three times
greater than the k value of O110.
Portland cement concrete from the O60 to O110 specimens
showed a higher compressive strength the lower the sorptivity
coefficient pattern (Figure 10). This is consistent with past
reports (Gonen and Yazicioglu, 2007; Igarashi et al., 2005;
O110
O80
O60
O40
Con
cret
e m
ixtu
re t
ype
Spalling testing time: min0 1 2 3 4 5
Loud crack
Pop
Figure 6. Sound recording of spalling time of Portland cement
concretes in the surface exposure test
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(a) (b)
(c) (d)
(e) (f)
(g) (h)
Figure 7. Geopolymer and Portland cement concrete cylinders
after the gas fire furnace test: (a) G40; (b) O40; (c) G60; (d) O60;
(e) G80; (f) O80; (g) G100; (h) O110
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Khan, 2003). The addition of silica fume in the O110 specimen
acts as a fine filler to fill the pores in the paste and interfaces
between aggregate and paste (Gonen and Yazicioglu, 2007;
Igarashi et al., 2005). Therefore, the O110 concrete processes a
densified structure resulting in a significantly low sorptivity
coefficient. O40, however, showed a similar value of sorptivity
coefficient compared with O60, instead of a higher value. Olivia
et al. (2008) proposed that variation of aggregate content,
grading and binder content can influence the sorptivity coeffi-
cient of concrete. It was observed (Olivia et al., 2008) that
increasing the aggregate/binder ratio can result in a decrease of
the water absorption. Therefore, it is suggested that the high
water/binder ratio was not the major factor to influence O40’s
sorption. It was mainly influenced by its high aggregate/cement
ratio, which was 6.2 compared with O60 with an aggregate/
cement ratio of 5.
The sorptivity coefficient of geopolymer concrete decreased with
increasing strength from G40 to G80 (Figure 10). This pattern is
also consistent with the Portland cement concrete. However,
G100 showed a significantly high sorptivity coefficient. This
phenomenon can be explained by the continuous pore structure
development of geopolymer gel during the extended oven-curing
regime, resulting in a more highly developed porous structure
compared with that produced by the normal length of oven curing
(Sindhunata et al., 2006).
A high sorptivity coefficient indicates the existence of a highly
connected porous structure of the material. When geopolymer
concrete is subjected to fire, according to the moisture clog
theory (Shorter and Harmathy, 1961), it is suggested that the
highly porous structure will be beneficial to decrease the thermal
gradient because the high volume of pore water in the structure
can absorb heat and distribute the heat flow in the matrix. A
highly porous structure can also accelerate the water flow in the
concrete skeleton, thereby slowing the temperature rise in the
concrete. It is also suggested that the highly connected porous
structure will slow the pressure build-up by releasing the water
vapour from the concrete.
ConclusionsThis paper has compared the spalling behaviour of geopolymer
and Portland cement concretes by using the surface exposure test
and standard gas furnace fire test. No spalling occurred on any of
the geopolymer concrete specimens, while spalling was observed
on some of the companion Portland cement concrete specimens.
The high-strength Portland cement concrete cylinders (O110,
O80) displayed severe spalling. Normal-strength Portland cement
concrete cylinders (O60, O40) exhibited minor spalling. The test
results on both Portland cement and geopolymer specimens by
using the surface exposure test and standard gas fire test are
consistent. These results showed that the geopolymer concrete
had a better spalling resistance to rapidly rising temperature
exposure than Portland cement concrete.
The sorptivity test demonstrated that the geopolymer concrete
specimen’s structure is more porous than the Portland cement
concrete specimens. The highly porous structure of geopolymer
concrete facilitates the release of the internal steam pressure
during heating. Hence, less tensile stress is imposed in the
Concrete
Paraffin
Water
Figure 8. Sorptivity test set-up
20 40 60 80 100 120Compressive strength: MPa
Geopolymer
OPC
12
10
8
6
4
2
0
Sorp
tivity
coe
ffic
ient
,:
10m
m/m
ink
��
30·
5
Figure 9. Sorptivity coefficient plotted against strength of
concrete
3·5
3·0
2·5
2·0
1·5
1·0
0·5
0
Sorp
tion:
mm
T 0·5 0·5: min
0 50 100 150 200 250 300 350
G100G80
G60G40
O60O40O80
O110
Figure 10. Sorptivity plots for Portland cement and geopolymer
concrete
171
Magazine of Concrete ResearchVolume 63 Issue 3
Geopolymer and Portland cementconcretes in simulated fireZhao and Sanjayan
geopolymer concrete than for Portland cement concrete during
heating, thereby reducing the geopolymer’s risk of spalling. This
comparison was carried out at the same strength levels of the two
concretes.
It can be concluded that, at the same strength level, geopolymer
concrete has a significant advantage over Portland cement con-
crete when exposed to fire.
AcknowledgementsThe authors gratefully acknowledge the financial support from
the Australian Research Council Discovery Grant No.
DP0664309 for this research work. Laboratory assistance from
Jeff Doddrell and Mr Long are also acknowledged.
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