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Theeffect of sodium silicate and sodium hydroxide on the strength of aggregates made from coal fly ash using the geopolymerisation method.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2012; 7: 73–79
Published online 23 August 2010 in Wiley Online Library
(wileyonlinelibrary.com) DOI:10.1002/apj.493
Research article
The effect of sodium silicate and sodium hydroxide
on the strength of aggregates made from coal fly ash using
the geopolymerisation method
Hamzah Fansuri,1,2 * Didik Prasetyoko,1 Zezhi Zhang2 and Dongke Zhang2
1
2
Department of Chemistry, Institut Teknologi Sepuluh Nopember (ITS), Kampus ITS Sukolilo, Surabaya 60111, Indonesia
Centre for Energy (M473), University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
Received 28 February 2010; Revised 28 May 2010; Accepted 11 June 2010
ABSTRACT: Geopolymerisation of coal fly ash to produce synthetic aggregates as a potential means of utilising coal
combustion by-product has been investigated. It has been revealed that the geopolymerisation strongly depends on
the physicochemical properties of fly ash, the availability of soluble silicates and aluminates, and the concentration of
added sodium hydroxide. The presence of sodium hydroxide increases the amount of soluble silicates and aluminates
in the mixture through fly ash solubilisation. Solubility tests on various fly ash samples have shown that solubility
increases as the concentration of sodium hydroxide in the fly ash increases, which also increases the strength of the
resulting geopolymer aggregates. The compression strength of the geopolymer aggregates also increases to a maximum
before decreasing again as the amount of sodium silicate is increased.  2010 Curtin University of Technology and
John Wiley & Sons, Ltd.
KEYWORDS: fly ash; geopolymerisation; silicates; aluminates
INTRODUCTION
Utilisation of coal combustion product (CCP) or coal
ash from coal-fired power stations is of great interest.
Each year a huge amount of coal ash is produced.
According to the Ash Development Association of
Australia (ADAA), in 2005, 13 Mt (million tonnes)
of coal ash was produced from coal-fired utilities
in Australia and New Zealand[1] and this figure is
increasing year by year. The estimated worldwide
annual coal ash production is around 600 Mt, with fly
ash constituting about 500 Mt.[2]
One of the desirable coal ash utilisation approaches
is to convert fly ash into aggregates as replacements for
natural aggregates. The reasons favouring this approach
are: (1) fly ash constitutes up to 90% of the total
ash produced, (2) the natural aggregate resource is
depleting, and (3) demand for aggregates is large and
increasing continuously.[3] Aggregate may account for
70-80% by mass of concrete. Aggregates can also be
used as soil conditioners, water savers, and soil and sand
stabilisers. Therefore, the successful and economical
manufacture of aggregates from fly ash will not only
*Correspondence to: Hamzah Fansuri, Department of Chemistry,
Institut Teknologi Sepuluh Nopember (ITS), Kampus ITS Sukolilo,
Surabaya 60111, Indonesia. E-mail: h.fansuri@chem.its.ac.id
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Curtin University is a trademark of Curtin University of Technology
reduce the impact of fly ash disposal on the environment
but also provide a great benefit to the economy.
There are five common ways of converting fly
ash into aggregate, namely high-temperature sintering, adhesive material binding, cementitious reaction,
high-pressure mechanical compression and geopolymerisation.[4] Geopolymers are rapid-setting binders
and there are claims[5 – 7] that geopolymer binders have
the potential to replace ordinary Portland cement (OPC)
in construction materials as low-CO2 cements for a sustainable future.
Fly ash geopolymerisation has been reported widely
in the literature since 1990. In 1998, van Jaarsveld
et al .[8] reported the utilisation of fly ash geopolymer to immobilise heavy metals. Since then, van
Deventer’s group has published many reports on ash
geopolymerisation.[9 – 11]
To make geopolymer from fly ash, scientists normally
refer to the preparation of geopolymer from soluble
reactants such as soluble silicate and aluminates, as well
as from kaolinite. Factors that control geopolymerisation from these soluble reactants have been reasonably
understood, some of which are related to the metal
oxide ratios of the soluble reactants such as SiO2 /M2 O,
SiO2 /Al2 O3 , M2 O/H2 O and M2 O/Al2 O3 , where M is
either sodium or potassium. The best geopolymer can
be made if these ratios fall within the limits defined by
74
H. FANSURI et al.
Asia-Pacific Journal of Chemical Engineering
Eqn (1):[12,13]
0.2 < M2 O/SiO2 < 0.48
3.3 < SiO2 /Al2 O3 < 4.5
10 < H2 O/M2 O < 25
(1)
When the formula is applied to a broad range of fly
ash geopolymerisations, it gives products with variable
strengths and a clear relationship has not yet been found
between the compositions and the properties of the
geopolymer products. Some researchers[14,15] have since
discovered that these molar oxide ratios are just an indication of the approximate composition and are not very
critical, particularly when dealing with Si–Al minerals
from waste materials such as fly ash. This is the case
because, although these molar ratios are based on chemical analyses, it is highly unlikely that all of the silica
and alumina will actually take part in the synthesis reaction. An extensive geopolymerisation study over a wide
range of fly ash compositions, using the oxide molar
ratios in Eqn (1), revealed that the fly ash geopolymerisation depends more strongly on the internal properties
of the fly ash itself[16] rather than the oxide molar ratio.
In a recent publication, Chindaprasirt et al .[17] related
the workability and strength of geopolymer from a
mixture of sand and high-calcium (class C) fly ash to the
sodium silicate and sodium hydroxide concentrations in
the geopolymer gel. They found that the workability of
the gel and the geopolymer strength were dependent on
the amounts of sodium silicate and sodium hydroxide
present in the mixture.
The relationship between the effects of sodium silicate and sodium hydroxide and the strength of fly ash
geopolymer can be rationalised in terms of the mechanism of geopolymer formation proposed by Provis and
van Deventer,[18] based on the aluminosilicate weathering model of Faimon.[19] The geopolymerisation is
initiated by the dissolution of silicate and aluminate
monomers from the source materials (here, fly ash),
which occurs at high alkaline concentration.[20] These
soluble monomers then polymerise into the long-chain
geopolymer.
The dissolution of silicates and aluminates from fly
ash is not as simple as the dissolution of these materials
from kaolinite as in the Provis and van Deventer
model[18] since kaolinite is very close to a single phase
and is less heterogeneous than fly ash. As a result,
the silicate and aluminate composition in a geopolymer
gel from fly ash cannot be determined merely from
the chemical composition of the fly ash and other
reactants. It is therefore not possible to use oxide molar
ratios as in Eqn (1) as guidance for making a fly ash
geopolymer because the ratio depends strongly on the
typical physicochemical properties of the fly ash.
To understand the relationship between the
physicochemical properties and fly ash geopolymerisation, researchers at the Centre for Fuels and Energy have
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
investigated geopolymerisation for a wide range of type
F fly ashes under various reaction conditions.[16] In this
paper, we report the roles played by sodium silicate and
sodium hydroxide solution in fly ash geopolymerisation
in relation to the physicochemical properties of the ash.
EXPERIMENTAL
Materials
Fly ash samples were collected from several power
stations in Australia. The chemical composition of the
ash is given in Table 1.
XRD analyses, together with the use of the Rietveld
refinement method, were used to quantify the mineral
phases in fly ashes. The ashes consisted mainly of
an amorphous phase along with quartz and mullite.
Some of them also contained hercynite and maghemite.
The phase composition determined by quantitative XRD
analysis is shown in Table 2.
Sodium silicate solution was obtained from BDH
Chemicals. The SiO2 /Na2 O ratio, SiO2 concentration
and density were 3.5, 6.03M, and 1.35 ± 0.07 g mL−1 ,
respectively. Laboratory grade NaOH and distilled
water were used in all geopolymerisation experiments.
Table 1. Chemical composition (in wt%) of fly ashes
by the XRF method.
Components
FA-A
FA-B
FA-C
FA-D
FA-E
SiO2
Al2 O3
Fe2 O3
CaO
MgO
Na2 O
K2 O
TiO2
Mn2 O3
SO3
P2 O5
BaO
SrO
ZnO
V2 O5
69.60
24.40
1.80
0.29
0.30
0.22
2.50
1.00
0.03
<0.02
0.09
0.05
0.03
<0.02
0.02
67.20
26.40
1.40
0.35
0.30
0.29
2.90
1.10
0.02
0.03
0.08
0.06
0.03
<0.02
0.03
66.10
30.70
0.54
0.06
0.13
<0.05
0.28
2.10
<0.02
<0.02
0.07
0.04
<0.02
0.08
0.05
55.70
26.60
10.80
1.10
0.65
0.23
0.47
1.60
0.03
0.03
1.40
0.32
0.33
0.05
0.03
52.30
32.40
11.00
1.00
0.80
0.10
0.22
2.10
0.20
<0.02
0.07
0.04
<0.02
<0.02
0.02
Table 2. Phase compositions (in wt%) of fly ashes.
Mineral/phase
FA-A
FA-B
FA-C
FA-D
FA-E
Quartz
Mullite
Hercynite
Maghemite
Amorphous
Total
6.4
10.1
–
–
83.5
100
13.8
21
0.6
–
64.6
100
6.8
23.3
0.2
–
69.7
100
9.9
25.9
0.1
3.9
60.2
100
5.7
25.3
0.3
4.1
64.6
100
Asia-Pac. J. Chem. Eng. 2012; 7: 73–79
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
STRENGTH OF AGGREGATES MADE FROM COAL FLY ASH
Silicon and aluminium leaching tests
Table 3. An example of variation in the amounts of
sodium hydroxide and water added to the geopolymer
matrix in preparations with 45 g of added sodium
silicate.
Silicon and aluminium leaching tests were carried out
using a solid-to-liquid ratio of 1 : 5. As the alkaline
solution, various concentrations of NaOH were used,
specifically 0.1, 0.5, 1, 2 and 4 mol L−1 . To leach the
silicon and aluminium, the fly ash was stirred in the
alkaline solution for 24 h at room temperature. The
leached silicon and aluminium were analysed by means
of atomic absorption spectrometry (AAS).
Geopolymer preparation and compressive
strength tests
All geopolymers were prepared on the basis of 300g samples of fly ash. The mixtures contained fly ash,
sodium silicate solution, sodium hydroxide (NaOH) and
distilled water. A certain amount of water was added to
each mixture to produce a workable geopolymer gel.
In the mixtures, both fly ash and sodium hydroxide are
regarded as solids, while additional water and sodium
silicate solution are regarded as liquids.
The amount of sodium silicate added during geopolymer preparation was varied from 45 to 120 g. The
amounts of sodium hydroxide and water added were
also varied, as indicated in Table 3. In the table, the
SiO2 /Na2 O ratio indicates the ratio in the liquid phase,
i.e. in the solution containing sodium silicate, sodium
hydroxide and water. The concentrations of silicate and
alkali metal ions in the fly ash were considered constant. Therefore, their influence on the total SiO2 /Na2 O
ratio was assumed to be the same in each experiment.
The combined materials were mixed in a mechanical
mixer for 10 min to form a geopolymer gel. The thus
formed gel was then moulded in a cylindrical plastic
mould with 2 : 1 height to diameter ratio and left in
an oven at 60 ◦ C for 4 days to cure the geopolymer
into aggregate pellets. Following the curing process,
the formed aggregates were left at room temperature
and humidity for a further 24 days for ageing prior to
carrying out the compressive strength tests. The total
time for curing and ageing thus amounted to 28 days.
The strengths of the geopolymer aggregates were
tested using an INSTRON 1196 universal testing
machine operating at a speed of 2 mm min−1 . The
strength of the aggregate pellets is reported in terms
of the maximum load needed to break the aggregate.
RESULTS AND DISCUSSION
Variation in the liquid-to-solid (L/S) ratio
and strength of FA (fly ash)-geopolymers
Each fly ash used in this research was unique, especially
in its ability to adsorb water and interact with liquid
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
NaOH
Code
SiO2 /
Na2 O
mol
g
H2 O
g
A-1
A-2
A-3
A-4
A-5
A-6
B-1
B-2
B-3
B-4
B-5
B-6
C-1
C-2
C-3
C-4
C-5
C-6
D-1
D-2
D-3
D-4
D-5
D-6
E-1
E-2
E-3
E-4
E-5
E-6
3.36
3
2.5
2
1.5
1
3.36
3
2.5
2
1.5
1
3.36
3
2.5
2
1.5
1
3.36
3
2.5
2
1.5
1
3.36
3
2.5
2
1.5
1
0.00
0.01
0.04
0.08
0.14
0.27
0.00
0.01
0.04
0.08
0.14
0.27
0.00
0.01
0.04
0.08
0.14
0.27
0.00
0.01
0.04
0.08
0.14
0.27
0.00
0.01
0.04
0.08
0.14
0.27
0.00
0.55
1.59
3.15
5.75
10.95
0.00
0.55
1.59
3.15
5.75
10.95
0.00
0.55
1.59
3.15
5.75
10.95
0.00
0.55
1.59
3.15
5.75
10.95
0.00
0.55
1.59
3.15
5.75
10.95
0.00
0.08
0.24
0.47
0.86
1.64
30.75
30.75
30.75
30.75
30.75
30.75
69.75
69.75
69.75
69.75
69.75
69.75
69.75
69.75
69.75
69.75
69.75
69.75
39.75
39.75
39.75
39.75
39.75
39.75
reactant, which affected the workability of the resulting
geopolymer gel (here it is discussed as L/S ratio). Fly
ashes have very narrow ranges of L/S ratio. Fly ash A,
for example, can only be used to prepare a geopolymer
at an L/S ratio of 0.15, not more or less. In contrast,
fly ash B–E adsorb much more water than fly ash and,
therefore, more water need to be added. Hence, the L/S
ratios were much higher than 0.15. In attempting to
prepare geopolymers outside of this L/S range, their
workability was either very poor or the paste became
too thin. In such cases, the geopolymer aggregates were
ultimately very weak. The suitable L/S ratio ranges of
fly ashes are shown in Table 4.
Geopolymers with lower L/S ratio are normally
stronger than those prepared with higher L/S,[21 – 23] as
is also the case upon the hydration of OPC. Analogous results were also seen with fly ash geopolymer, as
shown in Fig. 1. The strength of the geopolymer aggregates increases with decreasing L/S, and aggregates
prepared from fly ash A with the lowest L/S ratio were
much stronger than the others. Water provides a medium
Asia-Pac. J. Chem. Eng. 2012; 7: 73–79
DOI: 10.1002/apj
75
H. FANSURI et al.
Asia-Pacific Journal of Chemical Engineering
Table 4. L/S ratio ranges of fly ash for preparing geopolymer gel.
Fly ash
L/S (mass ratio) ranges
A
B
C
D
E
0.15
0.25–0.27
0.37–0.47
0.37–0.47
0.27–0.32
added NaOH concentration, most of the fly ash particles
no longer exist as individual particles, with only a
few of them remaining. Instead, they become a blend
of soluble ash particles. Figure 2(b) clearly shows the
effect of solubilisation of the ash particles at the highest
added sodium hydroxide concentration.
50
Compressive Strength (MPa)
76
45
40
35
Fly Ash A
30
25
20
15
Fly Ash C
Fly Ash E
10
The solubility of silicates and aluminates
from fly ashes in alkaline solution
Fly Ash B
5
Fly Ash D
0
0.5
1
1.5
2
2.5
3
3.5
Molar ratio of SiO2 to Na2O
Figure 1. The compressive strength of geopolymer mortars
from fly ashes A to E.
for the soluble silicates and aluminates to polymerise.
The polymerisation is actually a condensation reaction
that produces water molecules. An excessive amount of
water will reduce the condensation rate as it modifies the
equilibrium state of the reaction. Furthermore, excess
water causes segregation in the geopolymer mixture.
At this stage, it was not clear as to why fly ash A
geopolymer was stronger than the other products. The
strengths of geopolymers FA-B and E, which have the
same L/S ratio, were found to be similar and therefore it
was concluded that the strength is controlled by the L/S
ratio. For geopolymers FA-C and D, however, which
also have the same L/S ratio, the strengths were found to
be different. Geopolymer FA-C was apparently stronger
than geopolymer FA-D. This indicates that there must
be other factors besides the L/S ratio that affect the
strength, which may also affect the strengths of other
FA geopolymers.
Chindaprasirt et al .[17] reported that the sodium
hydroxide concentration also affects the strength of
the resulting geopolymer. The same trend can also be
seen in Fig. 1, which shows an increase in geopolymer
strength as a result of an increase in sodium hydroxide
concentration. It can be concluded that the increase in
the FA geopolymer strength depends on the availability of soluble silicates and aluminates, which are more
soluble in solutions with higher alkaline concentrations.
Further evidence of the effect of increasing sodium
hydroxide concentration on the geopolymer strength
is provided by the SEM images in Fig. 2. With no
additional NaOH, the fly ash particles are bonded by a
matrix of geopolymer binder and they show no sign of
dissolution. As the concentration of NaOH is increased,
the dissolution becomes more evident. At the highest
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
The solubilities of the silicates and aluminates from all
of the fly ashes used in this research are shown in Fig. 3.
It can be seen that the solubility of the silicates and
aluminates increases with increasing sodium hydroxide
concentration, resulting in stronger fly ash geopolymer.
For example, if the curve shown in Fig. 1 were extrapolated to an SiO2 /Na2 O ratio of 0.5, the compressive
strength of the FA-A geopolymer would reach up to
70 MPa. Geopolymers made from fly ashes B, C and
E would have strengths around 20 MPa, and that made
from fly ash D would have a strength of about 6 MPa.
It is, however, impossible to obtain such materials since
sodium hydroxide or silicate would start to precipitate
at the required composition under the conditions used
in the experiments.
The effect of sodium silicate and sodium
hydroxide solution on geopolymer strength
Figure 4 shows the effect of increasing added sodium
silicate concentration on the fly ash geopolymer. Not all
of the fly ashes have been included in the figure since
only three of them, namely fly ashes C, D and E can
be treated over a wide range of S/L ratios. The figure
shows an increase in the compressive strength of the fly
ash geopolymer as more sodium silicate was incorporated into the geopolymer preparation. The increase in
compressive strength of the FA-C geopolymer was less
pronounced than the increases seen for FA-D and E.
Fly ash E proved to be the most sensitive raw material
to the increase in sodium silicate concentration and the
FA-E geopolymer was the strongest one.
An interesting result was shown by fly ash C geopolymer upon increasing the amount of added sodium silicate solution. The strength of the geopolymer was
expected to increase with increasing sodium silicate
concentration, as shown by the dashed purple line in
Fig. 4(a). However, it can be seen in the figure that the
strength diminished when the highest sodium silicate
Asia-Pac. J. Chem. Eng. 2012; 7: 73–79
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
STRENGTH OF AGGREGATES MADE FROM COAL FLY ASH
(b)
(a)
(c)
Figure 2. Effect of increasing NaOH concentration in the geopolymer mixtures.
concentration was added to make the FA-C geopolymer. The figure seemingly shows a saturation trend
with respect to the sodium silicate concentration, which
shows a limitation when making FA-C geopolymer
aggregate.
The saturation process may be explained schematically, as illustrated in Fig. 5. When less sodium silicate is available in the mixture, empty spaces between
the particles in the geopolymer matrix will result
(Fig. 5(a)). These particles are fly ash particles that were
not soluble during the geopolymerisation process. At
optimum sodium silicate addition, these gaps are filled
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
(Fig. 5(b)). The geopolymer has its ideal composition
when the geopolymer gel acts as a binder of the insoluble ash particles and fills the gaps between them.
The strength of the geopolymer stems mainly from
the strength of the particles. When more sodium silicate is available, most of the space is occupied by
the geopolymer binder and the particles merely act
as fillers (Fig. 5(c)). The geopolymer matrix becomes
the main contributor to the strength of the aggregate.
Under the conditions depicted in Fig. 5(c), the
strength of the geopolymer aggregate depends on the
Asia-Pac. J. Chem. Eng. 2012; 7: 73–79
DOI: 10.1002/apj
77
H. FANSURI et al.
Asia-Pacific Journal of Chemical Engineering
1600
Concentration of Si in the
solution (ppm)
1400
1200
1000
F
800
E
A
C
600
400
200
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
NaOH concentration (mol.L-1)
strength of geopolymer matrix. The matrix consists
mainly of solid sodium silicate, which has a strength
of 9.8 MPa.[24] When the geopolymer is in a state as
depicted in Fig. 5(b), bonded fly ash particles contribute
to the strength of the aggregate. The particles consist
mainly of mullite and/or quartz, which are stronger than
sodium silicate (1310 and 1110 MPa, respectively). On
the other hand, a lack of particle binding, as shown
in Fig. 5(a), results in the weakness of the aggregate.
Thus, it can be seen why geopolymer aggregate is weak
at low added sodium silicate concentration, reaches a
maximum, and then becomes weaker when there is too
much silicate.
500
450
Concentration of Al in the
solution (ppm)
CONCLUSIONS
400
A
E
350
300
250
200
150
C
100
F
50
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
NaOH concentration (mol.L-1)
160
(a)
140
120 g (expected)
120
100
80
105 g
60
75 g
40
45 g
20
120 g
0
0.5
1
The geopolymerisation strongly depends on the physicochemical properties of the fly ash, the availability of
soluble silicates and aluminates, and the concentration
of added sodium hydroxide, which is able to increase
the amounts of soluble silicates and aluminates in the
mixture through fly ash solubilisation.
The amount of water required for fly ash geopolymerisation strongly depends on the nature of the fly
ash. There are wide variations in the amounts of water
required for the various fly ash sources. However, for
each fly ash, the less water added, the stronger the
geopolymer product. This is consistent with the effect
of water on OPCs.
Compressive Strength (MPa)
Figure 3. Si and Al leaching from fly ash.
Compressive Strength (MPa)
1.5
2
2.5
3
140
(b)
120
100
75 g
80
60
40
20
45 g
0
0.5
3.5
1
1.5
120
2
2.5
3
3.5
Molar ratio of Na2SiO3 to Na2O
Molar ratio of SiO2 to Na2O
Compressive Strength (MPa)
78
(c)
100
75 g
80
60
40
60 g
20
45 g
0
0.5
1
1.5
2
2.5
3
3.5
Molar ratio of Na2SiO3 to Na2O
Figure 4. Compressive strength vs molar ratio of SiO2 in the sodium silicate solution and Na2 O in NaOH
solution of geopolymers made from (a) fly ash C, (b) fly ash D, and (c) fly ash E. The amounts (in g) of sodium
silicate solution added to 300 g of fly ash is indicated.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2012; 7: 73–79
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
STRENGTH OF AGGREGATES MADE FROM COAL FLY ASH
REFERENCES
Figure 5. The effect of increasing sodium silicate
concentration on the binding of insoluble fly ash
particles by geopolymer matrix.
The amounts of sodium silicate and sodium hydroxide
that can be added are limited by the intrinsic properties
of the fly ash and the workability of the geopolymer
mixture. The strength of the geopolymer increases with
increasing amounts of sodium hydroxide added. This
increase is related to the dissolution of silicates and
aluminates from the fly ash. Solubility tests on various fly ashes have shown that the solubility of silicates
and aluminates from fly ashes increases with increasing
sodium hydroxide concentration and at the same time
the strength of the resulting geopolymer also increases.
The strength of the geopolymer also increases with
increasing amount of sodium silicate solution added,
up to a certain limit. After this point, further addition
of sodium silicate solution reduces the strength of the
geopolymer.
Acknowledgements
This research has been funded by the CRC on Coal
in Sustainable Development (CCSD) and partly funded
by the Indonesian State Ministry of Research and
Technology under the Insentif Riset Terapan program
on fly ash leaching tests. The writing of this manuscript
was funded by the Indonesian Ministry of Education
under the 2009 PAR-B project. H. Fansuri wishes to
thank Abdul Hakim, who carried out most of the
leaching tests in the Inorganic Laboratory, Department
of Chemistry, Institut of Teknologi Sepuluh Nopember
(ITS) Surabaya.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
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[3] H.W. Wu, A.D. Elliot, A. Rossiter, J.N. Zhu, D.K. Zhang. An
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DOI: 10.1002/apj
79
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