The cement industry is a substantial contributor to the global greenhouse gases emissions, accounting for approximatley 6% of the total global CO2 emission. Geopolymer, an inorganic polymer consisting primarily of Si-Al-O covalent chains, is an attractive alternative to the conventional portland cement due to its much smaller carbon footprint. This research is an early work aimed at elucidating the techno-economic feasibility of geopolymer production in Indonesia, utilizing domestic aluminosilicate minerals and waste materials as feedstocks. Kaolin from the Belitung island and Class F coal fly ash from an electric powerplant in East Java were selected as the geopolymer precursors. The kaolin was initially calcined at 750 oC for 6 hours to convert it to the much more reactive metakaolin phase. Besides the type of aluminosilicate raw materials, the type of alkali solution was also varied between NaOH and KOH. The aluminosilicate materials were each reacted with 10.0 M alkali hydroxide solution at a solid-to-liquid mass ratio of 1.2 and 2.8 for the case of metakaolin and fly ash, respectively. The effect of these variables was evaluated on mortars prepared by using the obtained geopolymers, which involved the measurement of settling time in accordance to an Indonesian standard Vicat apparatus method, and compressive strength according to the ASTM C 109-80 method. The setting time of fly ash - KOH/NaOH geopolymer mortars is shorter than those obtained using metakaolin, due to the higher reactivity of the amorphous fly ash. The higher reactivity of fly ash also promotes better crosslinking of the Si-Al-O bonds, resulting in a higher compressive strength compared to the metakaolin-based geopolymer samples.

1. Proceeding for "Moving towards the New Chapter in Chemical Engineering amongst ASEAN Region ",
the 5th
Regional Conference on Chemical Engineering, 7 – 8 February 2013, PATTAYA, THAILAND
Synthesis of Geopolymer from Indonesian Kaolin and Fly Ash as a
Green Construction Material
Tjokorde Walmiki Samadhi1
, Pambudi P. Pratama1
1: Chemical Engineering Program, Bandung Institute of Technology (ITB), Bandung, Indonesia
twsamadhi@che.itb.ac.id
Abstract—The cement industry is a substantial contributor to
the global greenhouse gases emissions, accounting for
approximatley 6% of the total global CO2 emission. Geopolymer,
an inorganic polymer consisting primarily of Si-Al-O covalent
chains, is an attractive alternative to the conventional portland
cement due to its much smaller carbon footprint. This research is
an early work aimed at elucidating the techno-economic
feasibility of geopolymer production in Indonesia, utilizing
domestic aluminosilicate minerals and waste materials as
feedstocks. Kaolin from the Belitung island and Class F coal fly
ash from an electric powerplant in East Java were selected as the
geopolymer precursors. The kaolin was initially calcined at 750
o
C for 6 hours to convert it to the much more reactive metakaolin
phase. Besides the type of aluminosilicate raw materials, the type
of alkali solution was also varied between NaOH and KOH. The
aluminosilicate materials were each reacted with 10.0 M alkali
hydroxide solution at a solid-to-liquid mass ratio of 1.2 and 2.8
for the case of metakaolin and fly ash, respectively. The effect of
these variables was evaluated on mortars prepared by using the
obtained geopolymers, which involved the measurement of
settling time in accordance to an Indonesian standard Vicat
apparatus method, and compressive strength according to the
ASTM C 109-80 method. The setting time of fly ash -
KOH/NaOHgeopolymer mortars is shorter than those obtained
using metakaolin, due to the higher reactivity of the amorphous
fly ash. The higher reactivity of fly ash also promotes better
crosslinking of the Si-Al-O bonds, resulting in a higher
compressive strength compared to the metakaolin-based
geopolymer samples.
Keywords-geopolymer; metakaolin; fly ash; green chemistry
I. INTRODUCTION
In recent years, global warming has become a major
research and development focus, especially in the context of
anthropogenic greenhouse gases emissions. As much as 65% of
the total global greenhous gas emissions is contributed by CO2.
From the overall CO2 emission, approximately 6% originates
from the cement industry [1], in which each ton of Portland
cement produced emits approximately 1 ton of CO2 to the
atmosphere [2]. Considering the massive consumption of
Portland cement and other similar civil construction materials,
greenhouse gases emissions reduction in this industrial sector
will likely to give a substantial impact on global warming
mitigation.
Geopolymer is a term coined for a class of inorganic
polymers consisting mainly of Si-Al-O chains obtained by
reacting aluminosilicate solids with alkali solutions [3].
Development of this material initated in the 1970's as a
construction binder that can supplement or replace ordinary
Portland cement (OPC). Due to the low temperature
requirement for the hardening reactions, geopolymers consume
much less energy to manufacture compared to OPC, which
translates to a much smaller carbon footprint. As reviewed by
van Deventer, Provis, and Duxson [4], the replacement of OPC
by geopolymers may reduce the equivalent CO2 emission by as
much as 80%. For Indonesia, motivations for the development
of geopolymer not only stems from the abundance of several
classes of aluminosilicate raw materials, but also from the
commitment of the nation to reduce greenhouse gases
emissions by 26% by 2020 in accordance to the UNFCCC
framework stated in 2009 [5].
The objective of this research is to measure the effects of
aluminosilicate source type, alkali type, and curing temperature
on key engineering characteristics of geopolymer as a
construction binder material. Domestically available raw
materials are utilized, namely Belitung island kaolin and coal
fly ash obtained from a large base-load powerplant in East Java
province. This raw materials selection is aimed at maximizing
the impact of the research to include not only the reduction of
greenhouse gases emissions, but also the reuse of industrial
waste materials in Indonesia.
The term 'geopolymers' represents solid materials with a
chemical composition similar to zeolites, but which structurally
consist of macromolecular chains consisting of silicon,
aluminum, and oxygen atom. These silica-alumina
macromolecules incorporate polysialate groups, which are
themselves chains and rings of tetrahedrally coordinated Si4+
and Al3+
ions and oxygen atoms, the structure of which is
amorf to semi-crystalline. This macromolecule is represented
by the following empirical formula [6]:
n(−(SiO2)z−AlO2)n wH2O
In the above formula, z has a value of 1 to 32, M represents
monovalent cations such as K+
or Na+
, and n expresses the
degree of polycondensation. Polysialate bonds in geopolymers
are further classified into 3 types, namely polysialate (-Si-O-
Al-O-), polysialate-siloxo (-Si-O-Al-O-Si-O-), and polysialate-
disiloxo (-Si-O-Al-O-Si-O-Si-O-) [3]. In the gepolymer
synthesis process, the dissolution of aluminosilicate by
concentrated alkali solution produces Si(OH)4 and Al(OH)4
monomers, which subsequently undergo polycondensation to
form an alkali-aluminosilicate polymer with three-dimensional
crosslinking [3].

2. Proceeding for "Moving towards the New Chapter in Chemical Engineering amongst ASEAN Region ",
the 5th
Regional Conference on Chemical Engineering, 7 – 8 February 2013, PATTAYA, THAILAND
3.13
7.92
6.71
11.63
5.63
11.79
12.00
16.88
8.54
15.54
0
2
4
6
8
10
12
14
16
18
20
Init. Final Init. Final Init. Final Init. Final Init. Final
OPC FA+NaOH FA+KOH MK+NaOH MK+KOH
Settingtime,hours
The synthesis of geopolymers may virtually consume any
aluminosilicate solid material, encompassing raw natural
minerals to inorganic waste materials. Xu and van Deventer [7]
classified these raw materials as: (1) calcinedaluminosilicates,
such as fly ash ,metakaolin, slag, construction residues, etc. (2)
non-calcinedaluminosilicates, such as kaolinite, feldspars, mine
tailings, etc. Calcinedaluminosilicates tend to produce higher
early compressive strength due to faster reactions with the
alkali solution [8]. Conversely, non-calcinedaluminosilicates
exhibit higher increase in strength in the later stage of
geopolymerization [9].
II. METHODOLOGY
Due to the scarcity of geopolymer research in Indonesia,
this preliminary study is aimed primarily at proving the
technical feasibility of utilizing several domestically abundant
aluminosilicate materials in geopolymer synthesis. As such, a
simple 22
factorial experiment is performed, with
aluminosilicate source type and alkali solution type selected as
experimental variables.
Calcinedaluminosilicates, namely Class F coal fly ash and
metakaolin, are selected in this study to ensure faster reactions
in the initial stage of geopolymerization. The fly ash is a waste
material from the Paiton base-load powerplant in East Java
province, while the metakaolin is obtained by calcining a
Belitung kaolin sample at 750 o
C for 6 hours. Table 1 outlines
the oxide compositions of the raw kaolin and fly ash used in
this study, as measured by the X-ray fluorescence (XRF)
method. Two types of alkaline solutions are selected, namely
NaOH and KOH solutions at a fixed concentration of 10.0 M.
These solutions are prepared by dissolving solid alkali
hydroxide flakes. Purity of the NaOH flakes is 97-98%, while
that of the KOH flakes is in the 90-95% range.
TABLE I. OXIDE COMPOSITION OF ALUMINOSILICATE SOURCES
Oxide Belitung
kaolin
Fly ash
SiO2 48.1 40.1
Al2O3 36.1 30.7
Fe2O3 0.6 35.7
CaO 0.014 18.7
MgO 0.016 5.4
Na2O 0.04 0.59
K2O 0.33 2.1
Geopolymer cement pastes are prepared by first hand-
mixing the aluminosilicate powder into the alkaline solution in
a steel bowl. The metakaolin – alkali solution mass ratio is 1.2,
while the fly ash – alkali solution mass ratio is 2.8. The mixture
is then transferred to a planetary mixer, where it is mixed at
low speed until a smooth, sticky consistency is obtained. Time
required to achieve such consistency is approximately 3
minutes. The geopolymer paste is then shaped into a small ball
by hand and inserted into the sample chamber of the Vicat
apparatus. The setting time is then measured in accordance to
the Indonesian SNI 15-3500-2004 standard method, which is
based on the measurement of the force required to pull out a
standard needle from a setting cement paste specimen.
Aside from geopolymer cement pastes, mortar samples are
also prepared according to the ASTM C 109-80 method, in
which reference OPC mortars are prepared by mixing cement,
water, and sand at a cement – sand ratio of 0.36 and water –
cement ratio of 0.485. For the geopolymer mortars, a cement –
sand ratio of 0.74 is selected. The geopolymer mortars are cast
into 50 x 50 x 50 mm cubic specimens, vibrated to remove
trapped air, then cured at 60 o
C in an electric oven for 24 hours.
Following this curing step, the specimens are subjected to
compressive strength measurement by uniaxial loading
according to the ASTM C 109/109M-02 method, and surface
morphology characterization by scanning electron microscopy
(SEM).
III. RESULTS AND DISCUSSION
Figure 1 presents the initial and final setting times of the
four geopolymer paste specimens as measured by the Vicat
apparatus. As a comparison, data for the reference OPC paste
are included. Each paste formulation is tested in 8 replicates to
enable proper statistical analysis of the data. Average values
and error bars of the setting times at a 95% confidence level are
included in the graph.
Figure 1. Initial and final setting times of geopolymer pastes
measured by the Vicat apparatus (FA = fly ash, MK = metakaolin)
Overall, the setting times of the geopolymer pastes are
significantly longer than the OPC paste, both in the initial and
final stage of polymerization, despite the use of calcined
aluminosilicates [8]. The shortest setting time of the
geopolymer paste is slightly less than twice the setting time of
the OPC paste. For the same alkali type, fly ash exhibits a
significantly faster setting than metakaolin. On the other hand,
for the same aluminosilicate source type, KOH produces a
faster initial setting. However, the effect of alkaline type
becomes much less pronounced at the final stages. This is
especially true for fly ash, in which the alkaline type does not
influence the final setting time in a statistically significant
manner. As the fly ash undergoes fusion during its generation
in powerplant boilers, it is expected to be more amorphous than
the semi-crystalline metakaolin, hence its higher reactivity
towards the alkaline solution.
Figure 2 presents the compressive strength of the
geopolymer – sand mortars after curing at 60 o
C for 24 hours.

3. Proceeding for "Moving towards the New Chapter in Chemical Engineering amongst ASEAN Region ",
the 5th
Regional Conference on Chemical Engineering, 7 – 8 February 2013, PATTAYA, THAILAND
29.61
28.04
39.98
9.36
14.23
0
10
20
30
40
50
OPC FA+NaOH FA+KOH MK+NaOH MK+KOH
Compressivestrength,MPa
The figure clearly indicates that all geopolymer samples exhibit
lower compressive strength than OPC, with the exception of fly
ash – KOH geopolymer mortar, which has a 35% higher
average strength than the OPC mortar. This observation is in
agreement with the setting time measurement (see Figure 1), in
which the fly ash – KOH geopolymer exhibits the fastest initial
setting among all geopolymer samples.
For both fly ash and metakaolin, KOH produces a
significantly higher compressive strength than NaOH. KOH
has a higher basicity compared to NaOH due to the larger
cationic radius of K+
, enabling it to dissolve silicate at a higher
rate than Na+
does [10]. This behavior is also confirmed by
Yao, Zhang, Zhu, and Chen using an isothermal calorimetry
method [11]. It is also argued that the larger cationic radius of
K+
favors the formation of larger silicate oligomers in the
geopolymer gel phase, resulting in a higher degree of
polycondensation compared to Na+
[12].
Figure 2. Compressive strength of geopolymer mortars after
curing at 60 o
C for 24 hours
Figure 2 also indicates that, for both alkaline solutions,
metakaolin geopolymer mortars exhibits much lower strength
than fly ash geopolymer mortars. The finer particles of the
metakaolin demands lower solid to liquid ratio during the
preparation of the geopolymer phase, which compromises the
strength of the cured product as observed by Kong, Sanjayan,
and Sagoe-Crentsil [13]. In addition, these authors also argue
that fine pores formed as relics of reacted hollow fly ash
microspheres may act as channels for moisture diffusion during
curing at elevated temperatures, thereby resulting in less
internal development and higher strength at high ambient
temperatures [13].
Figures 3 and 4 presents the SEM images of the metakaolin
- KOH and fly ash - KOH geopolymer mortars, respectively,
both taken at 5000x magnification. The platelet-like relics of
the metakaolin is still visible in Figure 3. However, the figure
also suggests that the metakaolin has reacted extensively with
the alkaline solution, as indicated by the extensive formation of
interparticle bridges. Figure 4 clearly indicates several
unreacted fly ash particles, and the finer microstructure of the
phase surrounding the spherical particles. The finer
microstructure suggests a better condensation or crosslinking of
this sample compared to the metakaolin geopolymer mortar.
The sphere on the right hand side of the figure is partially
reacted, revealing the hollow nature of these spheres. Work is
currently in progress which addresses the impact of high-
temperature exposure on the microstructure of these metakaolin
and fly ash geopolymer mortars.
Figure 3. SEM image of metakaolin - KOH geopolymer mortar at
5000x magnification
Figure 4. SEM image of fly ash - KOH geopolymer mortar at
5000x magnification
IV. CONCLUSIONS
The preliminary technical feasibility of utilizing kaolin and
fly ash from domestic resources in Indonesia to synthesize
geopolymeric construction materials has been proved, which
may potentially contribute to the reduction of greenhouse gases
emission from the cement industry. Overall, fly ash produces
geopolymer with setting time and compressive that are
comparable to OPC. The use of KOH as the alkaline activator

4. Proceeding for "Moving towards the New Chapter in Chemical Engineering amongst ASEAN Region ",
the 5th
Regional Conference on Chemical Engineering, 7 – 8 February 2013, PATTAYA, THAILAND
in the geopolymerization increases the setting rate and early
compressive strength. However, its higher price compared to
NaOH may offset the better mechanical properties.Higher
solids loading of the fly ash geopolymer provides it with a
higher compressive strength than the metakaolin geopolymer.
Extensive reaction between the aluminosilicate solids and
alkaline solutions is evident in microstructure of the
geopolymer mortars. Partially reacted hollow microspheres are
observed in the fly ash geopolymer microstructure, which are
likely to disappear at elevated temperatures.
ACKNOWLEDGMENT
This research is partially funded by the 2013 ITB Research
Group Research and Innovation (Riset dan Inovasi Kelompok
Keahlian ITB 2013) program.
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