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Sulphate Resistance of Aerated Concrete Containing Palm Oil
Fuel Ash as Partial Sand Replacement
Fadzil Mat Yahaya
1
, Khairunisa Muthusamy
2
, Saffuan Wan Ahmad
3
and Mohd Warid Hussin
4
1,2,3
Faculty of Civil Engineering and Earth Resources, Lebuhraya Tun Razak, 26300 Gambang Universiti
Malaysia Pahang, Malaysia
4
Faculty of Civil Engineering, 81310 UTM Skudai, Universiti Teknologi Malaysia, Malaysia
Abstract: This research investigated sulphate resistance of aerated concrete containing palm oil fuel ash as
partial sand replacement. Plain aerated concrete with 100% river sand was used as control specimen. Aerated
concrete containing palm oil fuel ash was prepared by integrating 30% of the ground palm oil fuel ash as partial
sand replacement. For strength determination, both mixes which cast in form of cubes were subjected to water
curing up to 90 days. The compressive strength test was carried out in accordance to BS EN 12390-3 at 7, 28
and 90 days. The sulphate resistance of concretes was investigated by measuring the length change of mortar
bars which immersed in 10% Sodium sulphate solution after water cured for 28 days. The measurement of length
change was taken on weekly basis following the procedure outlined in ASTM C1012-13. Aerated concrete
containing palm oil fuel ash exhibits continuous strength development as curing age become longer. Integration
of palm oil fuel ash makes the concrete microstructure to be denser through formation of secondary C-S-H gel.
The pozzolanic reaction also reduces amount of calcium hydroxide that can react with sulphate ion to form
gypsum and ettringite which cause concrete deterioration. It is concluded that using ground palm oil fuel ash as
partial sand replacement assist aerated concrete to exhibit higher compressive strength and better durability to
sulphate attack.
Keywords: aerated concrete, palm oil fuel ash, sand cement replacement, sulphate resistance, mortar bar
expansion
1. Introduction
Due to flourishing Malaysian palm oil industry, palm oil fuel ash which is a solid waste was generated
abundantly and the disposal of this waste needs to be managed by the industry. Mohamed et al. [1] reported that
approximately 4 million tonnes of palm oil fuel ash (POFA) were produced every year. This light greyish ash
does not have sufficient nutrient to be used as fertilizer, thus it is dumped in the vicinity of the factory or at
landfill. Disposing this waste in huge amount has negative impact to the environment and local community
living nearby. Winds can easily carry the ash which can cause air pollution and affect the health of people.
Recent researcher, Aprianti et al. [2] addressed POFA as a pollutant which ends up in the atmosphere without
being used and disturbs the environment. In addition, the continuous generation of palm oil fuel ash is expected
to create demands for more landfill or area to be allocated for its disposal which may increase the waste
management cost of the industry. The conventional approach applied to dispose this waste is seen to create
bigger problem in terms of environmental pollution, health of local community and higher expenditure for waste
management by industry. Unless this solid waste is used for production of material in industry, larger quantity of
this solid waste is expected to be discarded as environmental polluting waste [3].
In the meantime, the demand for concrete which produced using a large amount of natural resources that is
water, aggregate, and sand harvested from the nature is increasing throughout the years globally. Meyer [4]
highlighted this construction material which is used worldwide has an enormous impact on environment. As the
use of concrete increases, more natural resources were obtained from the environment to cater the need of
construction industry. In relation to this issue, excessive use natural sand which is one of the concrete mixing
ISBN 978-93-84422-22-6
Proceedings of 2015 International Conference on Innovations in Civil and Structural Engineering
(ICICSE'15)
Istanbul (Turkey), June 3-4, 2015
246
ingredients may cause destruction of habitat for living things thus leading to ecological imbalance. High
dependency of the industry on natural sand may result in the depletion of this material in future which finally
affects the concrete industry. Approach taken to reduce utilization of natural sand by partially replacing it with
other material would prolong the availability of this material for future generation and contribute towards well
balanced ecosystem. As such, with the understanding that the best alternative to attain sustainable development
of the concrete industry is through use of by-product instead of raw materials in concrete [5], there are
researchers [6,7,8] who has attempted to integrate waste material as partial sand replacement in concrete.
In Malaysia, the freely available palm oil fuel ash has initiated Mat Yahaya [9] to incorporate 30% of this
solid waste as partial sand replacement in aerated concrete production suitable for non-structural application.
The mechanical properties of this aerated concrete containing fine palm oil fuel ash have been studied. However,
the durability performance of this concrete upon exposure to sulphate environment yet to be investigated. Thus,
this paper presents and discusses the behaviour of aerated concrete produced using palm oil fuel ash as partial
sand replacement when subjected to sulphate attack.
2. Experimental Programme
2.1. Materials
A single batch of ordinary Portland cement (OPC) was used as binder throughout the experiments. The sand
used was initially oven dried for 24 hours before sieved passing 300µm sieve. Potable water was used for
concrete preparation and curing purpose. Aluminium powder was used to produce air voids inside the concrete
to make it lower density. Palm oil fuel ash used as partial sand replacement in this research was collected from a
local palm oil mill located in Bukit Lawang, Johor. The ash collected is the end product of pressed palm oil fiber
and shell which burned in the incinerator shown in Fig. 1. It is then disposed in the vicinity of the mill as
illustrated in Fig. 2. The collected ashes were sieved passing 300µm before oven dried for 24 hours. After that,
modified Los Angeles Abrasion Machine was used to grind the coarse ash to be finer particle and ready to be
used for aerated concrete preparation. Based on the chemical composition of palm oil fuel ash tabulated in Table
1, this pozzolanic ash is categorised as Class F in accordance to ASTM C618-12 [10].
Fig. 1: Incinerator producing palm oil fuel ash Fig. 2: Palm oil fuel ash dumped as waste
TABLE I: Chemical Composition of Palm Oil Fuel Ash
Chemical Composition
%
Silicon dioxide (SiO
2
)
82.07
Aluminium oxide (AL
2
O
3
)
6.04
Ferric oxide (Fe
2
O
3
)
2.70
Calcium oxide (CaO)
5.11
Magnesium oxide(MgO)
2.28
Sodium oxide (Na
2
O)
1.34
Pottasium oxide (K
2
O)
2.90
Sulphur oxide (SO
3
)
2.20
Loss of ignition (LOI)
5.30
247
2.2. Mix Proportion
Two types of aerated concrete mixes have been used in experimental work. Plain aerated concrete was
produced using 100% river sand and function as control specimen. The second mix, aerated concrete containing
palm oil fuel ash was prepared by adding 30% ash as partial sand replacement. The ash was mixed as a weight-
for-weight replacement of sand in a constant quantity. Table 2 presents the details of mix proportion used to
produce aerated concrete containing palm oil fuel ash.
TABLE II: Mix Proportion of Aerated Concrete Containing Palm Oil Fuel Ash
Details
Binder Sand Ratio
30 : 70
Ordinary Portland cement (%)
100
Sand (%)
30
Palm oil fuel ash (%)
70
Water dry mix ratio
0.45
Aluminium powder (%)
0.2
2.3. Testing Method
During the casting of aerated concrete specimens, all the dry mixing ingredients were mixed uniformly
before finally adding the water. The mixture is mixed uniformly before poured filling 2/3 of cube mould (100 x
100 x 100mm). Then, mixture left to rise like a cake in the oven. After expansion of the concrete mix shown in
Fig. 3 ceased to rise and the concrete become hardened, excess of concrete is trimmed. The specimens then
covered with wet gunny for 24 hours as illustrated in Fig.4 before demoulded and immersed in water for curing
process.
Specimens to be tested for compressive strength were subjected to water curing until testing age.
Compressive strength test was conducted in accordance to BS EN 12390 – 3 [11] at the age of 7, 28 and 90 days.
Specimens to be used for sulphate resistance test were prepared in form of mortar bars (25 x 25 x 250mm). After
water cured for 28 days, the specimens were immersed in 10% Sodium Sulphate solution for duration of 9 weeks.
The elongation of mortar bars were measured every week following the procedures outlined in ASTM C1012 -
13 [12].
Fig. 3: Expanded aerated concrete before trimming Fig. 4: Aerated concrete covered with gunny sack
3. Results and Discussion
3.1. Compressive Strength
Fig. 5 shows the strength of aerated concrete mix containing palm oil fuel ash as partial sand replacement is
higher than control specimen throughout curing age. It is interesting to note, in this research aerated concrete
containing palm oil fuel ash performs better than plain specimen even at early curing age. There is no reduction
in the amount of cement used and this causes the hydration process take place in aerated concrete with POFA as
in plain concrete resulting same strength development. Since pozzolanic ash in finely divided form able to
chemically react with calcium hydroxide at ordinary temperature to produce compounds possessing cementitious
248
properties [10] inclusion of fine ground ash as partial sand replacement has allowed pozzolanic reaction to occur,
when the silica from ash reacted with calcium hydroxide to produce secondary C-S-H gel. Occurrence of both
hydration process and pozzolanic reaction in aerated concrete produced using palm oil fuel ash as partial sand
replacement manage to reduce the voids in concrete microstructure through formation of extra C-S-H gel making
it denser and stronger unlike to plain concrete which the strength just depends on hydration process. In addition,
probably incorporation of fine ash has allowed the unreacted ash to fill in the existing voids which reduces the
pores in concrete.
Continuous availability of moisture through water curing has provided conducive medium for uninterrupted
pozzolanic reaction to take place thus producing aerated concrete containing POFA with denser internal
structure enabling it to exhibit better strength performance up to later curing age. The significant role of water
curing has been highlighted by Ozer and Ozkul [13] who reported that enough water is essential on the strength
development for hydration and pozzolanic reaction to take place. Similarly, previous researchers [14, 15] has
reported that concrete containing POFA is sensitive to the curing method applied and it only exhibit better
compressive strength as compared to plain concrete upon subjected to continuous water curing. Conclusively,
aerated concrete containing palm oil fuel ash as partial sand replacement able to exhibit better strength
performance than plain concrete owing to the use of palm oil fuel ash which has been ground to be fine and
application of continuous water curing.
Fig. 5: Compressive strength development of aerated concrete mixes at the age of 7, 28 and 90 days
3.2. Sulphate Resistance
On overall, the result illustrated in Fig. 6 shows that both mortar bars made of plain aerated concrete and
aerated concrete containing palm oil fuel ash exhibit expansion as the immersion period in sulphate solution
become longer. Comparing the performance of both mixes, concrete produced using ground palm mil fuel ash as
partial sand replacement demonstrate lower expansion value throughout the experimental period. The plain
concrete specimen exhibit obvious increment in expansion value beginning from the first of week immersion
period and so on. As the immersion period become longer, appearances of growing crack were noticed on the
specimen until it broke apart at week 10
th
. The stages in the crack development which begins from the surface of
aerated concrete mortar bar and then further deeper into the material is a normal occurrence towards specimen
exposed to sulphate. Basically, the present finding is similar with Binici and Aksogan’s [16] findings which
showed that sulphate attack is a layer by layer chemical reaction starting on the surface and moving inwards.
Not only that, visible elongation as well as deformation of OPC specimen in comparison with the ones
consisting POFA is also detected, as can be seen in Figures 7. Apparently, all OPC mortar bars suffered greater
developing expansion, cracking and fractures on the whole specimen with the longer period of immersion, in
contrast with POFA mortar bars. On the other hand, mortar bar produced using aerated concrete mix containing
palm oil fuel ash exhibit very slight expansion value since the early days of experimental work and margin
difference in the expansion between these two mixes remain big until the end of testing period.
249
Basically, utilization of ground palm oil fuel ash has transformed calcium hydroxide into secondary C-S-H
gel through pozzolanic reaction that contributes towards densification of microstructure resulting in
improvement of concrete durability. Most importantly, the lower amount of calcium hydroxide which the
sulphate ion can react to form gypsum and ettringite in aerated concrete with palm oil fuel ash has resulted in
lesser formation of ettringite that is responsible for expansion. On the other hand, plain concrete is rich with
calcium hydroxide from hydration process which promotes generation of larger amount of ettringite leading to
significant expansion value and cracks as well. This fact has been confirmed by Rasheeduzzafar et al. [17] who
stated that the hardened cement pastes containing greater amount of calcium hydroxide would react more rapidly
and to a greater extent with sulphates than those which contain less calcium hydroxide. The significant
contribution of POFA towards improving resistibility of sulphate attack of aerated concrete is further verified by
findings of Cao et al. [18] who proved that reduction in calcium hydroxide would lessen the effect of gypsum
formation and the tendency of ettringite recrystallization. Besides that, occurrence of pozzolanic reaction leads
to development of extra C-S-H gel making the internal structure denser in comparison with OPC specimen. The
effectiveness of palm oil fuel ash in enhancing the durability of concrete towards sulphate attack has been
reported by previous researcher [14, 19, 20]. This finding also support the idea of Santhanam et al. [21] who
discovered that integration of mineral admixtures in concrete would increase the resistance of this material
against sulphate attack. Conclusively, palm oil fuel ash is suitable to be used as partial sand replacement to
produce aerated concrete possessing higher durability towards sulphate attack.
Fig. 6: Expansion of mortar bars when immersed in 10% Sodium Sulphate solution for 9 weeks
Fig. 7: Evident cracks and elongation of plain aerated concrete mortar bar (greyish specimen) in contrast to mortar bar
containing palm oil fuel ash (black specimen)
250
4. Conclusion
Using finely ground palm oil fuel as partial sand replacement assist aerated concrete to exhibit higher
compressive strength and better durability to sulphate attack than plain concrete. Application of continuous
water curing has promoted better pozzolanic reaction which consumes the by-product of hydration process,
calcium hydroxide that can react with sulphate ion to form gypsum and ettringite (an expansive gel) thus causing
concrete deterioration. Occurrence of pozzolanic reaction contributes towards formation of secondary C-S-H gel
that is responsible towards concrete strength development and better durability.
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