Waste ashes are the by-product from the energy generating system and need to
be managed and disposed. A large number of investigations have been directed
towards the utilization of waste ashes. Waste ashes (derived from the industrial
and agricultural by-products) such as fly ash, palm oil fuel ash, bagasse ash
and rice husk ash are now receiving a high attention for use as pozzolans in
blending with Portland cement as they offers several advantages (Chindaprasirt
and Rukzon, 2008). These ashes generally improve the properties of blended
cement. The costs are also inexpensive. Furthermore, the environmental problems
can be reduced if these ashes are reused instead of disposal. Owing to the growth
of energy demand, the output of waste ashes increases annually. As a result,
a large amount of ashes is discarded in the landfills which cause unpleasant
environment and other associated problems.
Recently, the other form of cementitious materials using silicon and aluminum
activated in high alkali solution was developed (Davidovits,
1999). This material is usually based on fly ash as source material and
is termed geopolymer or alkali-activated fly ash cement (Palomo
et al., 1999; Hardjito et al., 2004).
The geopolymer mortar and concrete possess similar strength and appearance to
those from normal Portland cement (Chindaprasirt et
al., 2007). It is also well known that geopolymers possess excellent
mechanical properties such as fire resistance and acid resistance properties
(Davidovits, 1999; Hardjito et
al., 2004). These materials are formed by the alkali-silicate dissolution
at high pH in the presence of soluble alkali metal silicate. The manufacturing
of geopolymer neither needs cement nor create high CO2 emission (Davidovits,
1999). For agricultural ash, the silica content is usually very high and
hence additional alumina is needed (Detphan and Chindaprasirt,
2009). For the high silica to alumina ratio source materials, the geopolymer
is useful in terms of fire resistance (Davidovits, 1999).
The use of industrial and agricultural by-products as source materials in making
geopolymer is, therefore, very attractive.
Attempts have been made to utilize the bagasse ash (Aigbodion
et al., 2010; Worathanakul et al., 2009).
However, the utilization of bagasse ash is still very low and a large amount
is disposed of by Landfill and is still the problem for the power plants in
Thailand. Therefore, the objective of this research is to utilize the bagasse
ash as a source material for producing geopolymer mortar in order to reduce
negative environmental effects. The benefits of applying this material to concrete
are cost saving, reduction of green house gas emission and improvement of mechanical
properties of cement matrix.
MATERIALS AND METHODS
Materials: Bagasse ash (from Singhaburi Province sugar mill in central
Thailand) was used as a source material. Ground Bagasse Ash (BA) were obtained
using ball mill grinding until the 3% weight were retained on a sieve No. 325
(45 μm). The physical properties and chemical constituents are given in
Table 1 and 2, respectively. The river sand
from Nonthaburi Province in Thailand with specific gravity of 2630 kg m-3
and a fineness modulus of 2.82 in saturated surface dry condition was used for
making mortar. Sodium hydroxide (NaOH) with 10 and 15 M concentrations and sodium
silicate (with 15.3% Na2O, 32.8% SiO2 and 15% water) were
used as liquid solutions. Sodium hydroxide was obtained from O.V. Chemical and
supply and sodium silicate was obtained from Eastern Silicate Limited.
Mix design and mixing of mortar: Geopolymer mortars were made with BA
to sand ratio of 2.75, liquid to ash ratio of 1.3 and sodium silicate (Si) to
NaOH ratio by mass (or Si/OH) of 0.50, 1.50 and 2.50 were used. The mix proportions
of geopolymer mortars are given in Table 3. The mortar flow
of 110±5% was used as the mortar was workable and easily placed into
mould. In order to produce the workable geopolymer mortar, a minimum base water
content of 5% by mass of geopolymer paste (BA, sodium silicate, NaOH and water)
was included in the mix. The mixing was done in a 25°C controlled room.
For the mixing procedure, NaOH solution, base water and BA were first mixed
for 5 min in a pan mixer. Sand was then added and mixed for 5 min. Finally,
sodium silicate solution and water were included and were mixed for another
5 min. This mixing procedure was tested and found to produce high strength geopolymer
(Chindaprasirt et al., 2007). The flow test
was done in accordance with (ASTMC 230, 1997). The flow
table was 110±5%.
Strength and porosity test: The 50x50x50 mm specimen cube was prepared
for the compressive strength test of geopolymer mortar. The test was done in
accordance with (ASTM 39, 2001) using an Instron Universal
Testing Machine. For porosity test, cylindrical specimens of 100 mm diameter
and 200 mm height were prepared in accordance with (ASTM
C109, 2001). An additional vibration of 10 sec using a vibrating table was
given to ensure the uniform compaction. The mortars and moulds were wrapped
with vinyl sheet in order to prevent moistures loss. The mortars were then put
in the oven for heat curing at 45, 65 and 90°C for 24 h. After curing at
an elevated temperature, the mortars were put in laboratory to cool down and
demoulded the next day and kept in 25°C room until testing age.
|| Physical property of pozzolanic materials
||Chemical composition of pozzolanic materials (Oxides, % by
|| Mix proportions of geopolymer mortar with BA
|Sand-to-binder ratio 2.75, Liquid-to-ash ratio 1.3, Flow 110±5%
The specimens were tested at the age of 7 days. The reported results were
the averages of three samples.
For porosity test, mortars were cut into 50 mm thick slices and the 50 mm ends
were discarded. They were dried at 100±5°C until the weight was constant.
They were then placed in desiccators under vacuum for 3 h. The set-up was finally
filled with de-aired and distilled water in order to measure the effective porosity
of mortar at the age of 7 days. The porosity was calculated by using Eq.
1 (Rukzon and Chindaprasirt, 2008, 2009):
||Vacuum saturated porosity
||The weight of specimen in the air at saturated condition (g)
||the dry weight of the specimen after 24 h in oven at 100±5°C
||the weigh of the specimen in water (g)
RESULTS AND DISCUSSION
Characteristics of BA: The chemical constituents of BA as given in Table
2 showed that the main chemical composition of BA was 65% of SiO2,
9% of CaO and 5.0% of Al2O3 with Loss on Ignition (LOI)
||Effect of curing temperature on strength of mortar with 10
The sum of SiO2, Al2O3 and Fe2O3
was 73% which was slightly higher than 70% as required for natural pozzolan
according to (ASTM C 618, 2001). According to the X-ray
Analysis (XRD) pattern of BA (Fig. 1), the BA was mainly amorphous
silica as indicated by the hump around 25°θ with a small amount of
Effects of curing temperature: The results of the effect of curing temperature
on the strength of geopolymer mortar at NaOH 10 and 15 M (Molar) are given in
Fig. 2 and 3. Test results indicated that
the strength of mortar was affected by curing temperature. As shown in Fig.
2, at 45°C curing, the strength of 2.5 Si/OH-BA-10 M mortar was 11.6
MPa. At 65°C curing, the strength increased to 15.8 MPa. However, when the
temperature was 90°C, the strength dropped to 12.5 MPa. When curing temperature
was high, the heat curing adversely affected the strengthof the samples.
||Effect of curing temperature on strength of mortar with 15
||Effect of curing temperature on porosity of mortar with 10
The small 50 mm cube specimen with high surface to volume ratio was more susceptible
to the high curing temperature and loss of moisture than the large specimen.
The optimum temperature (with 10 M NaOH mix) was 65°C and the maximum strength
was 15.8 MPa. The initial curing at appropriate temperature improved the geopolymerization
and the compressive strength of BA geopolymer mortar.
Figure 4 and 5 shows the effect of curing
temperature on the porosity of geopolymer mortar at 10 and 15 M NaOH. As shown
in Fig. 4, the porosity of geopolymer mortar was also dependent
on the curing temperature. For example, at the temperatures of 45, 65 and 90°C,
the porosities of 2.5Si/OH-BA-10M mortar were 22.2, 20.5 and 23.5%, respectively.
||Effect of curing temperature on porosity of mortar with 15
||Effect of Si/OH on strength of mortar with 10 M NaOH
The optimum curing temperature of 65°C produced a high strength and low
porosity geopolymer mortar. The effect of curing temperature on the strength
and porosity of mortars is therefore very significant.
Effects of sodium silicate (Si) to NaOH ratio: The results of strength
of geopolymer mortar as affected by sodium silicates (Si) to NaOH ratio (Si/OH
ratio) at the curing temperature of 45, 65 and 90°C is shown in Fig.
6 and 7. The strength of mortar with Si/OH ratio of 2.5
was higher than that with Si/OH ratio of 1.5 and of 0.5.
||Effect of Si/OH on strength of mortar with 15 M NaOH
||Effect of Si/OH on porosity of mortar with 10 M NaOH
For instance, at the curing temperature of 65°C, the strength of 0.5Si/OH-BA-15
M mortar with Si/OH ratio of 2.5 was 12 MPa, whereas, the strength of that mortar
with Si/OH ratio of 1.5 and 0.5 were 6.5 MPa and 3.5 MPa, respectively as shown
in Fig. 7.
The results of porosity of geopolymer mortars with different Si/OH ratios are
shown in Fig. 8 and 9. The porosity of mortar
with Si/OH ratio of 2.5 was lower than that with Si/OH ratios of 1.5 and 0.5.
The mortar with low Si/OH ratio resulted in high porosity. On the other hand,
the mortar with high Si/OH ratio resulted in low porosity. For example, at curing
temperature of 45, 65 and 90°C, the porosities of the 2.5Si/OH-BA-10 M mortar
with high Si/OH ratio of 2.5 were 22.2, 20.5 and 23.5%, respectively.
||Effect of Si/OH on porosity of mortar with 15 M NaOH
||Effect of NaOH concentration on strength of mortar at 45,
65 and 90°C
The porosities of 2.5Si/OH-BA-15 M mortar at the same emperature with Si/OH
ratio of 2.5 were 23.5, 22.4 and 27.6%, respectively. It was obvious that the
geopolymer mortar was stronger with lower porosity. In addition, the increase
in Si/OH ratio to 2.5 increased the strength and lowered the porosity of mortar.
This was due to the increase in Si/OH ratio and the associated increase in Na
ions content of the mixture (Sathonsaowaphak et al.,
2009). The sodium silicate (Si) to NaOH ratio it was therefore important
as it affected the strength and porosity of mortars.
||Effect of NaOH concentration on porosity of mortar at 45,
65 and 90°C
Effect of concentration of NaOH: The strength results of BA geopolymer
with different concentrations of NaOH solutions cured at 45, 65 and 90°C,
are shown in Fig. 10. It described the strength development
of BA geopolymer mortar at 10 M NaOH, The strength of 2.5Si/OH-BA-15M mortar
was 11.8 MPa while the strength of mortar 2.5Si/OH-BA-10 M was good at 15.8
MPa. The increase in NaOH concentration decreased Na ions in the system which
was important to the geopolymerization since Na ions were used for balancing
the charges and forming the alumino-silicate networks (Sathonsaowaphak
et al., 2009). So, the concentration of NaOH had an influence on
the strength of geopolymer mortar.
The results of porosity of geopolymer mortars with NaOH solution at 10 and
15 M concentrations are shown in Fig. 11. The porosities
of mortar with 10 M NaOH concentration was lower than that with 15 M NaOH (for
all mixes). Moreover, the relationship between the compressive strength and
porosity of mortar followed the conventional pattern as shown in Fig.
12. When all ages were considered, the porosity of mortar ranged from 20.5
to 29.8%. It should be noted that this range of porosity values corresponded
to the compressive strength over the range 15.8 to 1.9 MPa (Fig.
12). The use of BA as a source material in making geopolymer mortar is desiderable
because of the fine particle size, high surface area, silica content and degree
||Relationship between strength and porosity of mortar at 45,
65 and 90°C
It was possible to use the finely ground bagasse ash to produce medium-strength
and low porosity geopolymer with high silica to alumina ratio. Curing temperatures,
sodium silicate to NaOH ratio and concentration of NaOH are the essential factors
that influence the strength and porosity of geopolymer mortar. The good bagasse
ash geopolymer mortar could be produced using high sodium silicate (Si) to NaOH
ratio of 2.5, 10M NaOH concentration and cured at 65°C. The reasonable strengths
of 11.6-15.8 MPa and porosity of 20.5-23.5% were obtained.
This study was financially supported by the TRF Research Grant for New Scholar
No. MRG5580120, Office of the Higher Education Commission (OHEC), the Thailand
Research Fund (TRF) under the TRF Senior Research Scholar Contract No. RTA5480004
and Rajamangala University of Technology Phra Nakhon (RMUTP).