Compressive strength and microstructural analysis of fly

Materials and Design 59 (2014) 532–539
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Materials and Design
journal homepage: www.elsevier.com/locate/matdes
Compressive strength and microstructural analysis of fly ash/palm oil
fuel ash based geopolymer mortar
Navid Ranjbar a,⇑, Mehdi Mehrali b, Arash Behnia a, U. Johnson Alengaram a,⇑,
Mohd Zamin Jumaat a
a
b
Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
Department of Mechanical Engineering and Centre of advanced Material, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Article history:
Received 1 November 2013
Accepted 15 March 2014
Available online 27 March 2014
Keywords:
Geopolymer
Fly ash
Compressive strength
Palm oil fuel ash
Microstructure
a b s t r a c t
This paper presents the effects and adaptability of palm oil fuel ash (POFA) as a replacement material in
fly ash (FA) based geopolymer mortar from the aspect of microstructural and compressive strength. The
geopolymers developed were synthesized with a combination of sodium hydroxide and sodium silicate
as activator and POFA and FA as high silica–alumina resources. The development of compressive strength
of POFA/FA based geopolymers was investigated using X-ray florescence (XRF), X-ray diffraction (XRD),
Fourier transform infrared (FTIR), and field emission scanning electron microscopy (FESEM). It was
observed that the particle shapes and surface area of POFA and FA as well as chemical composition affects
the density and compressive strength of the mortars. The increment in the percentages of POFA increased
the silica/alumina (SiO2/Al2O3) ratio and that resulted in reduction of the early compressive strength of
the geopolymer and delayed the geopolymerization process.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
The contribution of carbon dioxide (CO2) to global warming
by the cement industry is well established. CO2 emission is about
0.7–1.1 tonnes per ton of cement produced and about 50% of this
can be attributed to limestone calcination, 40% to fuel combustion
in the kiln, and the remaining 10% to manufacturing processes and
product transportation [1]. After aluminium and steel, Portland
cement is among the most intense energy constructional material
used [2]. CO2 reduction by minimizing the use of Portland cement
has recently realized; the use of cementitious materials such as fly
ash (FA), silica fume (SF), ground granulated blast furnace
slag (GGBS) and metakaolin as partial replacements for cement
were efforts taken seriously by the cement industry to decrease
cement consumption. Agro-industrial by-products such as rice
husk ash (RHA) and palm oil fuel ash (POFA) are available in
abundance in the South-East Asia; countries like Indonesia,
Thailand, Vietnam, Philippines and Malaysia produce these waste
by-products and many researchers focus on utilizing the above
⇑ Corresponding authors. Tel.: +60 107014250 (N. Ranjbar). Tel.: +60 379677632
(U. Johnson Alengaram).
E-mail addresses: [email protected] (N. Ranjbar), [email protected]
(U.J. Alengaram).
http://dx.doi.org/10.1016/j.matdes.2014.03.037
0261-3069/Ó 2014 Elsevier Ltd. All rights reserved.
products as cement replacement material. Waste from the palm
oil industry is not limited to POFA, but includes empty fruit
bunches (EFB), oil palm shells (OPS) and oil palm clinker [3–5].
OPS has been used as lightweight coarse aggregate in the development of structural grade concrete [6,7]. POFA is produced by the
palm oil industry due to the burning of EFB, fibre and OPS as fuel
to generate electricity and the waste, collected as ash, becomes
POFA. About 3 million tons of POFA was produced in Malaysia in
2007 and 100,000 tons of POFA is produced annually in Thailand,
and this production rate is likely to increase due to increased
plantation of palm oil trees [3,8,9]. POFA disposal problems cause
environmental pollution and recent research on the use of silica
rich POFA as cementitious material paves the way for the development of sustainable material. Global annual ash production is
about 500 million tons, consisting mainly, about 75–80%, of FA;
environmental issues are due to disposal problems associated with
FA which has led researchers to utilize it in the production of
Portland cement. Further, many researchers have focused their
attention on the use of FA as a source material for the development
of cementless concrete, widely known as geopolymer concrete
[10–12].
Fly ash is the most common source material for geopolymer because it is available in abundance throughout the world. It also
contains amorphous alumina silica. Malaysia uses coal for power
N. Ranjbar et al. / Materials and Design 59 (2014) 532–539
generation and that process consumes about 8 million tonnes of
coal annually. However, in 2010, the Government of Malaysia
decided to increase the percentage of coal powered electricity by
about 40% from 28%, raising FA production [13]. The use of FA in
the development of geopolymer concrete could lead to sustainable
concrete. The introduction of geopolymer in 1978 by Joseph
Davidovits generated interest in the development of sustainable
material through geopolymeric reaction of alkali activated solution
such as appropriate combinations of sodium hydroxide and sodium silicate with rich sources of silica and alumina. A new inorganic environmentally friendly geopolymer binder, which is free
of Portland cement, has enhanced properties such as high early
strength, durability against chemical attack, high surface hardness,
and higher fire resistance [10,14,15].
Geopolymer synthesis requires an aluminosilicate source, alkali
metal hydroxide and/or alkali silicate material plus water to increase workability and to mediate the reaction. The accepted sequence of geopolymerization shows it can partitioned into
approximately two periods: I dissolution–hydrolysis, II hydrolysis–polycondensation. However, these two steps probably occur
simultaneously once the aluminosilicate material is mixed with
the activator solution. The first step in the reaction is the release
of aluminate and silicate monomers by alkali attack on the solid
aluminosilicate sources; this step is required for the conversion
of solid particles to geopolymer gel. Hydrolysis reaction occurs
on the shell of the solid particles, exposing the smaller particles
which are trapped inside the larger ones. This is followed by the
formation of dissolved species that cross-link to form oligomers,
which in turn produce sodium silicoaluminate [16]. The second
stage is reaction of the remaining particles and gel generation coincide with setting and hardening which is attributed to polycondensation and the formation of a three dimensional aluminosilicate
network [17]. In this stage, the silicon/aluminium (Si/Al) ratio of
the main reaction product is increased [16]; the hardened alkaliactivated aluminosilicate gels contain SiAOASi and SiAOAAl
bonds in a highly cross-linked amorphous network [17]. Geopolymer structures can be defined in three types: poly (sialate)
(ASiAOAAlAOA), poly (sialate–siloxo) (SiAOAAlAAOASiAO) and
poly (sialate–disiloxo) (SiAOAAlAOASiAOASiAO) based on the
values of 2,4 and 6 of silica/alumina (SiO2/Al2O3) molar ratios,
respectively [18]. Geopolymer setting (hardening) is believed to
be the result of hydrolysed aluminate and silicate specie polycondensation. The composition of a typical geopolymer is generally
expressed as nM2OAl2O3xSiO2yH2O, where M is an alkali metal
[19].
The availability of silica rich POFA and silica–alumina rich FA
paves way for the use of these materials as geopolymer binders.
Some research work has been done on the use of POFA and FA in
conventional concrete to replace Portland cement [3,20–22]. There
is very few literature available on the utilization of low alumina
pozzolanic materials such as RHA and POFA in geopolymer concrete or as a replacement in FA geopolymer concrete [23,24]. Recently, another by-product of oil palm industry, OPS, has been
used as lightweight coarse aggregate in the development of FA
based geopolymer concrete [25–27].
Research on the development of sustainable geopolymer binders using local waste such as POFA, FA, RHA, and GGBS. is on the
rise. However, there is no literature available on the compressive
strength of POFA/FA using micro-structural analyses. In this investigation, different contents of POFA were introduced into the FA
based geopolymer mortar. The variable, POFA content, varied from
0% to 100% and its effect on the FA based geopolymer mortar was
studied using X-ray diffraction (XRD), Fourier transform infrared
(FTIR) and field emission scanning electron microscopy (FESEM).
In addition, the bulk density of the mortar using Archimedes method was also investigated and reported.
533
2. Specimen preparation and testing methods
2.1. Materials characterization
Raw POFA was obtained from the local palm oil industry and
oven dried for 24 h; the POFA was then sieved in 300 lm sieve
and the material which passed through the sieve was ground using
Los Angeles abrasion machine for 30,000 cycles.
Low calcium FA (class F) used in this research was supplied by
Lafarge Malayan Cement Bhd, Malaysia. The particle size distribution test was performed by Mastersizers Malvern Instruments and
the result is shown in Fig. 1. The physical properties of both FA and
POFA are given in Table 1. The oxide composition of the materials
as determined by XRF using PANalytical Axios mAX instrument is
provided in Table 2. The Si to Al molar ratios for FA and POFA were
recorded as 4.07 and 34.03, respectively. FA and POFA morphology
was studied by means of FESEM (CARL ZEISS-AURIGA 60) and the
images of both source materials used in the preparation of geopolymer mortar are shown in Fig. 2. Mining sand with a maximum
particle size of 1.19 mm was used as fine aggregate.
Sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) solution are widely recommended alkali activators. NaOH in a pellet
form with a specific gravity and purity of 2.13 g/cm3 and 99%,
respectively was used. The Na2SiO3 in liquid form with a density
of about 1.5 g/ml at 20 °C, a modulus ratio of 2.5 (SiO2/Na2O,
SiO2 = 30% and Na2O = 12%) and specific gravity of 1.5 was used
along with NaOH as alkali activator.
2.2. Specimen preparation and tests
The molarity of the NaOH and the ratio of Na2SiO3 to NaOH
were 16 and 2.5, respectively. The alkali activator solution was prepared by dissolving NaOH pellets in water and then adding the
Na2SiO3 solution. The activator to binder ratio was kept at 0.5 for
all the mixes. The mixture proportions of eight mixes are given
in Table 3. The variable studied in this investigation is the replacement of POFA by FA in increments ranging from 0% to 100%. Dry FA,
POFA and mining sand were mixed in a cake mixer for about 2 min
at a low rates of speed to blend the source materials and the sand
uniformly. The alkali activator solution was then gradually added
into the mix for about 7 min [11]. Additional water was added to
the mix simultaneously with the alkali activator to increase the
workability as well as homogeneity of the mortar. The mortar
specimens were cast in 50 mm cube steel moulds in three layers
of equal height and compacted. The samples were vibrated to remove entrained air and bubbles. Immediately after vibration, all
samples were covered and kept in the oven for hot curing for
24 h at a temperature of 65 °C; the specimens were then taken
out of the oven and kept in ambient condition with an average
Fig. 1. Cumulative particle size distribution of POFA and Fly ash.
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N. Ranjbar et al. / Materials and Design 59 (2014) 532–539
Table 1
Physical properties of POFA and Fly ash.
Materials
Median particle size (lm)
Specific gravity (g/cm3)
BET (m2/g)
POFA
FA
22.78
16.23
1.897
2.180
12.92
2.96
Table 2
Chemical composition of POFA and Fly ash.
Composition
POFA%
FA%
SiO2
K2O
Fe2O3
CaO
P2O5
MgO
Al2O3
SO3
TiO2
Na2O
MnO
CuO
Rb2O
ZnO
Cr2O3
SrO
NiO
ZrO2
Y2O3
LOI
64.169
8.250
6.333
5.800
5.182
4.874
3.734
0.719
0.187
0.184
0.180
0.078
0.059
0.030
0.025
0.020
0.016
0.009
0.003
16.3
54.715
1.003
5.149
5.306
1.115
1.103
27.280
1.008
1.817
0.434
0.099
0.010
0.007
0.031
0.042
0.361
0.022
0.162
0.017
6.8
temperature and humidity of 28 °C and 70%, respectively until testing day.
Based on other reports, the optimum geopolymer oven curing
temperature is between 60 and 65 °C in order to gain high early
strength of fly ash geopolymers with acceptable physical and
mechanical properties [28–31]. The selection of 65 °C is justified
as the 50 mm cube with high surface to volume ratio is more susceptible to curing heat and to moisture loss compared to that of
larger specimens. This could result in the reduction in strength
for curing at high temperature [28]. It has been reported previously
that the temperature at which geopolymer samples are cured
greatly affects its final compressive strength [31,32].
A compressive strength test was done using ELE Auto Compressive Testing Machine at the rate of 0.9 kN/s in accordance with
ASTM: C109. The compressive strength of the specimens was tested
after 3, 7, 14, 21, 28 and 56 days. The bulk density of the mortar
specimens was measured using the Archimedes method at 28 days.
Similarly, microstructure analysis using XRD, FESEM and FTIR
(Perkin–Elmer System 2000 series spectrophotometer (U.S.A.))
were conducted on the 28-day specimen.
3. Result and discussion
3.1. Pozzolanic particles
The XRF results as presented in Table 2 show that the major
pozzolanic components of SiO2 + Al2O3 + ferric oxide (Fe2O3) for
POFA and FA are 74.24% and 87.14%, respectively, hence satisfying
the requirements of ASTM: C618-12a. Fig. 2 shows the micrograph
of the pristine POFA and FA; it can be seen that the FA consists of a
series of spherical vitreous particles of different sizes. While usually hollow, some of these spheres might be almost intact or appear
within other small size spheres in their interior [16,33]. The FESEM
morphology of the POFA powder suggests that the sample consists
of both agglomerated and angular particles. As shown in Table 1,
the specific gravity of POFA is about 87% that of FA. The
Brunauer–Emmett–Teller (BET) surface area of the POFA and FA
are 12.92 and 2.96 m2/g, respectively. The higher BET of POFA would
lead to the requirement for additional water to achieve the same
degree of workability as that of FA based geopolymer mortar.
3.2. XRD analysis
Fig. 3 shows the XRD patterns of different compositions of FA
and POFA based geopolymer mortars which were oven cured at
Fig. 2. FESEM images of (a) fly ash, and (b) POFA particles.
Table 3
Mix proportion of specimens.
Mix order
POFA:FA (%)
SiO2/Al2O3
Na2O/SiO2
Na2O/Al2O3
POFA content (kg/m3)
Additional water (g)
ET1
ET2
ET3
ET4
ET5
ET6
ET7
ET8
0:100
5:95
10:90
15:85
25:75
50:50
75:25
100:0
4.07
4.28
4.52
4.78
5.38
7.68
12.79
34.03
0.19
0.19
0.18
0.18
0.18
0.17
0.17
0.17
0.76
0.80
0.83
0.88
0.97
1.33
2.13
5.47
0
36
73
109
181
363
544
725
66
69
71
73
78
89
100
111
N. Ranjbar et al. / Materials and Design 59 (2014) 532–539
Fig. 3. XRD patterns of different composition of FA–POFA based geopolymer
mortar.
65 °C for 24 h and kept in ambient condition until tested on day 28.
The patterns show that FA based geopolymer mortar consists of
main crystalline phases of quartz, mullite and goethite which originated from FA as well as trace levels of albite which has been confirmed and discussed by FTIR later on. However, the
geopolymerization of POFA with alkali activators produced quartz,
albite and sodalite. The amorphous to partial crystalline phases are
attributed to the reaction of the POFA, FA and alkali activators. The
increase in the percentages of POFA in the FA based geopolymer
mortar and the resulting geopolymerization show the presence of
N–A–S–H phase, albite (NaAlSi3O8). This might be the result of Al
spice deficiency in POFA which causes the replacement of mullite
phase as seen in the FA based geopolymer mortar into albite phase.
Comparison of the XRD patterns of the thermal and the ambient
cured specimens of POFA and FA based geopolymer mortars at
the 28 day mark are shown in Figs. 4 and 5, respectively. It is seen
from these figures that the thermal and ambient cured specimen
peaks show similar peak trend. Further, the chemical composition
of the specimens cured at 65 °C and ambient condition in different
curing environments had no effect on the geopolymerization.
3.3. Density
Fig. 6 shows the 28-day density of the specimens measured
using Archimedes’ method. The specimen density with POFA
replacement of 0%, 25%, 50%, 75% and 100% were measured and reported. Specimen density varies between 1671 and 1772 kg/m3,
and specimen ET8 with 100% POFA produced the lowest density.
535
The reduction in density of about 13.4% of ET8 compared to ET1
which had no POFA content might be attributed to specific gravity,
particle shape & size and water demand. In the case of POFA, particles cannot easily roll over one another due its agglomerated and
crushed shape, increasing inter-particle friction. Thus, this illuminates the need for more evaporable water in mixes with high POFA
content in order to obtain a workable mix. However, this additional
water leads to more pores, which reduces density. As seen in
Table 3, the additional water required by mix ET8 to achieve a
workable mix was higher than other mixes. Similar findings on
the requirement for more water to achieve workable mixes with
POFA as replacement material for OPC have been reported
[34,35]. In contrast, the spherical shaped FA particles reduce the
friction between the binder and fine aggregate resulting in an increase in workability of fresh mortar [36]. The spherical shape also
minimizes the particle’s surface to volume ratio, resulting in low
fluid demand. The most common reason for poor workability is
that the addition of a fine powder increases demand for water as
the surface area has increased [37]. A higher packing density was
obtained with spherical particles compared to crushed particles
in a wet state, resulting in lower water retention in the spherical
case and a subsequently lower water demand for specific workability’s [38]. The specific gravity of POFA is about 94% that of FA and
hence the use of a higher dosage of POFA decreases specimen density. In addition, the relatively smaller size of spherical FA particles
fills the voids and makes for denser packing. In the cases of specimens containing POFA and FA, increase in POFA content shows an
approximately linear decrease in the specimen density.
3.4. Field emission scanning electron microscopic analysis
FESEM micrographs of specimens ET1, ET5, ET7 and ET8 cured
at a temperature of 65 °C for 24 h and kept in ambient condition
for 28 days are shown in Fig. 7a–h. Two different magnifications
of FESEM micrographs are shown for each specimen. Fig. 7a shows
the micrograph of a specimen with 100% fly ash mortar (ET1). The
micrograph indicates that there is a strong bond between the unreacted or partially reacted fly ash and polymerized binder with no
cracks detected along the interface. Some small fly ash particles
which have reacted with the alkali activator solution are observed
to co-exist with part of the remaining unreacted spheres and even
with some other particles partially covered with reaction products;
the Si content in the binder particles are substantially higher than
that of the geopolymer gel [16] and these unreacted/partially reacted particles will react slower. The micrographs of mortar specimens ET5 and ET7, which consist of 25% and 75% POFA,
respectively, show bonding similarities. Mortar specimens ET1
and ET5 which consist predominantly of FA show the presence of
Fig. 4. XRD patterns of different ambient and thermal cured POFA based geopolymer mortar.
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N. Ranjbar et al. / Materials and Design 59 (2014) 532–539
Fig. 5. XRD patterns of different ambient and thermal cured FA based geopolymer mortar.
Fig. 6. Density of the geopolymer mortars.
a congested bulk of nanofibers produced on the surface of unreacted fly ash particles as seen in Fig. 7a, b and d. Previous study
showed that hydrolysis reactions occur on the surface of the solid
particles [17]. Fly ash is known to contain a significant proportion
of particles with hollow spheres. When these hollow spherical particles are partially dissolved they create porosity in the matrix containing highly dispersed small sized pores. These un-reacted
particles were found in hollow cavities as seen in Fig. 7a–d [33].
Increasing percentages of POFA change the microstructure of
POFA/FA based geopolymer mortar. The FESEM results of sample
ET7 shows unreacted fly ash particles appeared in the porous form
(Fig. 7f). The microstructure of mortar ET8 which contains only
POFA (Fig. 7g and h) illustrate the porous structure of POFA particles as well as increased dispersion of unreacted or partially reacted particles. The unreacted POFA particles have ability to trap
air because of its inherent crumbled shape as can be clearly observed in Fig. 3b. The density results also confirm the effect of porous POFA as it produced lower density compared to the compact
image of FA based specimen ET1.
3.5. Fourier transform infrared spectra analysis
Fig. 8 shows the Fourier transform infrared spectra with the major bands at approximately 2300, 2110, 1450, 990, 845, 670 cm1
and 445 for geopolymer specimen with 100% POFA (ET8) and
1450, 990, 670 and 445 cm1 for the fly ash based one (ET1). The
distinct intensity band at 445 cm1 is associated with the SiAOASi
bending vibration and another intense band at 990 cm1 is associated with the SiAOASi and SiAOAAl asymmetric stretching vibration for all of the materials [39,40]. It is notable that with increases
in the POFA/FA ratio, the band at the bond around 990 cm cm1 decreased. The intensity variation of this band is indicative of the variation of a mean chain length of aluminosilicate polymers. A weak
band at about 845 cm cm1 was observed for POFA/FA based
geopolymer mortar; its growth has been observed as the POFA content is increased (ET5, ET7 & ET8) and for the mortar without POFA,
the FTIR graph shows no evidence of this weak band. This band is
assigned to the bending vibration of the SiAOH [41]. According to
previous literature, the presence of SiAOH will decrease the degree
of matrix condensation and is responsible for low mechanical
properties. The band at about 760–770 cm1 is characterized as
the crystalline phase of quartz in all the samples. Consequently,
other spectra bands at about 1450 cm1 appear in all the geopolymer specimens. This band is assigned to carbonate asymmetric
stretching, which suggests the presence of sodium carbonate due
to the atmospheric carbonation of alkaline activation media. In
NaOH rich geopolymer, common atmospheric carbonation is revealed for OACAO stretching vibration [40]. The small band at
around 670 cm1 represents the functional group of AlO2 [42].
The bands at 560 cm1 indicate the presence of Al in octahedral
coordination [13,43]. The band 2100 cm1 is attributed to physically absorbed CO and H bonded CO. The band at approximately
2300 cm1 is assigned to C„N [44]. The intensity bands 2100
and 2300 cm1 are increased by increments in the POFA/FA ratio
and these bands are observable in zeolite structures.
3.6. Compressive strength
Fig. 9 shows the compressive strength of the hardened geopolymer specimens. As can be seen the FA based geopolymer mortar
gains about 70% and 98% of its 28-day compressive strength at
the 3 and 7 days, respectively. After 7 days, there is no tangible
strength gain in the ET1 mix. The high initial strength might be
attributed to low SiO2/Al2O3 in this geopolymeric system. The existence of relatively lower Si component in ET1 to ET5 mixes with
SiO2/Al2O3 = 4.07–5.38, make it possible that more ALðOHÞ4
4 species were available for condensation in these materials at early
stages. Further, the Al component tends to dissolve easier than
the silicon components, and this enables a higher rate of condensation between silicate and aluminate species than the condensation
between just silicate species [45], resulting in high initial compressive strength. However, initial SiO2/Al2O3 ratio will not be constant
throughout the geopolymerization process. The SiO2/Al2O3 increases during different stages of geopolymerization [16]. Changes
in the Si/Al ratio in the original particles, the reactive ones, and the
reacted product during the reaction process affects the trend of
compressive strength development [46]. Fig. 9 shows that geopolymerization was almost complete after 7 days and the strength gain
beyond this period was found insignificant.
The strength development of the POFA based mortars can be
seen from the ET6–ET8 curves. In contrast to the FA based mortars,
N. Ranjbar et al. / Materials and Design 59 (2014) 532–539
537
Fig. 7. High and low magnification FESEM images of the (a and b) ET1, (c and d) ET5, (e and f) ET7 and (g and h) ET8.
the POFA based mortar achieved only about 40% and 62% of the
112-day strength after 3 and 28 days, respectively. After 3 days,
the strength gain was found to ascend linearly and it continued until day 112. The slow strength development in the POFA based geopolymer mortar can be analysed from the SiO2/Al2O3 aspect. Mixes
ET6-8 correspond to the family of geopolymers characterized by
high Si/Al ratio. The increment in the SiO2/Al2O3 ratio results in
the reaction of Al content in the earlier stages. Therefore, the gradual increment of the Si content in further stages provides more silicate for condensation and reaction between the silicate species
and this causes the dominance of oligomeric silicates. The domination of Si content reduces the rate of condensation resulting in
gradual hardening of the geopolymers. As a result the increase in
the POFA/FA ratio of the geopolymer mortars delays the ultimate
compressive strength.
Compression tests showed increasing amounts of SiO2/Al2O3 ratio enhance elastic behaviour deformation rather than the brittle
crushing noted for the specimens of FA based specimens. This effect was more explicit in the case of ET8 which has the highest
SiO2/Al2O3 ratio of 34.03. Similar phenomenon was reported by
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N. Ranjbar et al. / Materials and Design 59 (2014) 532–539
The increase in the POFA/FA percentage ratios result in increases in the SiO2/Al2O3 ratios. High SiO2/Al2O3 cause the reaction
of aluminate species in the early stages and there is a scarcity of Al
species for further reactions at later stages. Hence it can be concluded that geopolymerization is dominated by the reaction and
condensation of silicate species leading to gradual strength gains
at later stages. In addition, continuous development of the compressive strength of POFA based geopolymer mortar between 28
and 112 days is about 38%, which contrasts with the FA based mortars which gained 97% of their ultimate compressive strength by
day 7. The increase of POFA content which resulted in high SiO2/
Al2O3 up to about 34 and the elastic behaviour of compression failure was observed to be elastic in the POFA based cube specimens
compared to brittle failure of FA based geopolymer specimens.
However, due to high silica content of POFA, it is worth investigating the properties of POFA based geopolymer as a high temperature resistant material. Hence, an independent study has been
conducted to address POFA based geopolymer properties at elevated temperatures.
Acknowledgement
Fig. 8. Fourier transform infrared spectra of POFA–FA base geopolymer.
This research work is funded by University of Malaya under the
research fund: High Impact Research Grant (HIRG) No. UM.C/625/
1/HIR/093 (Synthesis of novel geopolymer oil palm shell lightweight concrete).
References
Fig. 9. Compressive strength of the geopolymer samples.
Fletcher et al. [47] where they found that the high silica content in
the aluminosilicate geopolymers increased the elasticity [47].
4. Conclusions
In this investigation, eight geopolymer mortars mixtures incorporating sustainable binders such as FA and POFA were prepared.
The tests included microstructural analysis of XRF, XRD, FESEM
and FTIR to study the feasibility of replacement of POFA in FA
based geopolymer mortar. The compressive strength of the mortars was investigated for a period of 112 days. Based on the tests
conducted, the following conclusions were drawn:
The shape, particle size and surface area of POFA and FA
particles affect the hardened mechanical properties of geopolymer
mortar. The POFA particles with higher BET and crumpled shape
increased the water demand to produce a workable geopolymer,
while spherical FA particles enable workable mix with reduced
amounts of water. Consequently, the increase in the evaporable
water in POFA based geopolymer resulted in slightly more porous
material. The low specific gravity of the POFA along with its inherent raw material density and shape are capable of trapping air,
resulting in a porous geopolymer mortar with about 6% less
density compared to FA based mortar.
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