Factors Affecting the Setting Time of Fly Ash-Based Geopolymer
ANTONI
1,a
*, STEPHEN Wibiatma Wijaya
2,b
and DJWANTORO Hardjito
1,c
1
Department of Civil Engineering, Petra Christian University, Indonesia
2
Post Graduate Program in Civil Engineering, Petra Christian University, Indonesia
a
antoni@petra.ac.id,
b
stephen.wibi@yahoo.com,
c
djwantoro.h@petra.ac.id
Keywords: Geopolymer, Setting time, CaO content, pH level, SSA
Abstract. Fly ash is a waste from coal burning, that are generated with fluctuation both in its
physical and chemical characteristics. This characteristics of fly ash when used in the making of
geopolymer concrete will greatly affect the final products obtained. The pH value measured in fly
ash, according previous research, can influence the setting time of geopolymer and fly ash with high
pH values can cause flash-setting in the concrete. Understanding more clearly about the factors that
affect the setting time of fly ash based geopolymer is important for further progress and
development of the material. It was found that factors that influence the setting time of geopolymer
was not only from the physical and chemical properties of the fly ash itself. Other factors such as
composition and mix design, manufacturing process and environmental conditions can also affect
its setting time. The experimental results showed that fly ash particle size, CaO and MgO content,
in addition to ratio of sodium silicate and sodium hydroxide in the alkali solution, molarity of
NaOH, initial temperature of the mixture, curing temperature, and mix volume could potentially
influence the setting time of the geopolymer mixture.
Introduction
In the manufacturing of geopolymer concrete, the fly ash are to be used both its physical and
chemical properties to be reacted with the alkaline solution and be compacted to make a dense
matrix. In general, its physical properties is closely related to the type of coal burning equipment,
combustion conditions and techniques in the collection while the chemical composition is related to
the source of the coal, unit energy obtained and the burning temperature [1]. Therefore, any
different fly ash can produce different geopolymer characteristic depending on its physicochemical
properties.
According Davidovits [2], pH value measured in fly ash can affect setting time of fly ash based
geopolymer concrete. Fly ash as a raw material which has a pH value above 11 have high
possibility of flash-set that is harden after 5 minutes of mixing, while those with a pH value
between 8 and 11 tend to experience rapid setting. The pH value was closely associated with CaO
content in fly ash [3,4]. Also stated in another study [5], that fresh geopolymer concrete has a
tendency to have a short setting time, especially on geopolymer-based high calcium fly ash. Yet it
was also found that the high levels of CaO in the fly ash actually have an advantage because it can
produce a high compressive strength concrete [6]. This could be because high calcium fly ash based
geopolymer would have both polymerization and hydration reaction [7]. For comparison,
geopolymer concrete made with low calcium fly ash does not have indication of setting time up to
120 minutes after mixing [8].
This study aims to investigate the factors in causing the difference in setting time of the fly ash
based geopolymer by using fly ash from different sources. Factors that are investigated includes the
physical and chemical properties and the mixing composition and condition. If the factors causing
flash-set that occurs in fly ash-based geopolymer can be identified and addressed, then the use of fly
ash with a high pH can be possible so as to produce geopolymer concrete with high compressive
strength.
Materials Science Forum Vol. 841 (2016) pp 90-97 Submitted: 2015-09-30
© (2016) Trans Tech Publications, Switzerland Accepted: 2015-10-17
doi:10.4028/www.scientific.net/MSF.841.90
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications, www.ttp.net. (ID: 139.228.83.117, Petra Christian University, Surabaya, Indonesia-20/11/15,14:43:59)
Experimental Method
Materials. This study used five samples of fly ash from different coal fired power plant, the fly
ash code and its physical properties are listed in Table 1. Alkalinity of the fly ash (pH) is measured
on 20 gr fly ash in 80 ml deionized water solution. Particle size analysis (PSA) was carried out on
the fly ash with to see the gradation distribution of fly ash type. Fig. 1 showed the distribution of
particle size of the fly ash. It was shown that fly ash P10.3 and R9.6 had larger particle size
compared to other fly ash samples. Apparently different plant origin, might produce a different
particle shapes and gradation.
Chemical compounds from each fly ash samples were tested using x-ray fluorescence (XRF) and
the results are shown in Table 2. For all fly ash, the cumulative of SiO
2
, Fe
2
O
3
and Al
2
O
3
compound
were greater than 70%, and only one fly ash (Y11.2) have CaO content higher than 10%. Making
the fly ash Y11.2 can be classified as high calcium fly ash (type C) while others can be classified as
type F fly ash. Amount of Lost on Ignition (LOI) is also shown in the table.
Table 1. Physical properties and measured pH of the fly ash sample.
Fly ash code
Source pH
SSA
(kg/m²)
D
v
(90)
(μm)
D
v
(50)
(μm)
D
v
(10)
(μm)
Specific
Gravity
FA Y11.2 Paiton unit 5 & 6
11.2
2618 60.8 6.98 1.13 2.630
FA J10.6 Tanjung Jati 10.6
2078 63.5 8.03 1.46 2.915
FA P10.3 Paiton unit 1&2 10.3
1370 136 20.1 2.38 2.489
FA Y9.8 Paiton unit 5 & 6
9.8 1785 84.3 10.3 1.73 2.360
FA R9.6 Rembang 9.6 1169 104 19.2 3.08 2.245
Figure 1. Particle Size Analysis of the fly ash sample
Table 2. XRF test result of the fly ash sample.
Oxide (%)
Fly ash Y11.2
Fly ash J10.6
Fly ash P10.3
Fly ash Y9.8
Fly ash R9.6
SiO
₂
39.78 38.24 51.03 51.12 50.14
Al
₂
O
₃
17.87 15.28 25.13 18.9 29.08
Fe
₂
O
₃
15 24.28 9.66 17.71 9.66
CaO 15.47 9.15 5.49 5.54 4.03
K
₂
O
1.32 0.78 1.58 0.82 1.53
MgO 6.45 5.19 3.25 3.17 1.11
SO
₃
1.32 0.61 0.51 0.47 0.77
LOI 0.49 3.9 1.44 6.96 0.63
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0.1 1 10 100 1000
Volume Density (%)
Size Classes (μm)
FA Y11.2 FA J10.6 FA P10.3 FA Y9.8 FA R9.6
Materials Science Forum Vol. 841 91
The alkaline activator for geopolymerization process used sodium hydroxide solution mixed
with sodium silicate solution at predetermined ratio. Sodium hydroxide solid is dissolved in
distilled water one day prior making the mixture. Fine aggregate used is sand from Lumajang, East
Java with water absorption of 1.37% and a fineness modulus of 2.52. Distilled water was used in the
mix to prevent impurities that can affect the level of acidity.
Mix Proportion. Study of the setting behavior of geopolymer concrete was conducted on mortar
mixture. Fly ash and fine aggregate’s ratio of 1:2 and water to binder ratio of 0.25 was used for all
mixture. Concentration of sodium hydroxide solution used are 8M, 10 M and 12 M. Ratio between
of sodium silicate solution and sodium hydroxide solid (S/N) was 1; 1.5; and 2. Other variable of
the experiment include curing temperature, alkaline activator ratio and concentration and mix
volume. When the mixture and curing method is not specifically mentioned in the discussion, the
proportion used are 8 M of sodium hydroxide solution, S/N ratio of 2 and curing was done at oven
temperature of 60°C.
Mixing and setting time. Geopolymer mortar-making process begins with the manufacture of
alkaline solution in accordance with a predetermined ratio. Fine aggregate and fly ash mixed in dry
condition and then alkaline activator was added, and all material were thoroughly mixed. The
mixture was then poured into 150 mm cube mould and the measurement of setting time was
conducted on the sample at 60°C oven cured temperature or at room temperature, until initial and
final setting time is obtained. Testing of setting time testing was done using penetrometer testing
according to ASTM C403 [9]. Initial setting time is reached when the penetration resistance shows
the pressure of 500 psi (3.5 MPa) and for 4000 psi (27.6 MPa) for the final setting time.
Result and Discussion
From making of geopolymer mortar with different fly ash source materials, several factors that
affect the setting time of the mixture can be identified. The changes of setting time can be seen from
the internal factors which are its physical and chemical properties and from external factors such as
the mixture composition and manufacturing method and temperature.
Particle size and distribution is related to the specific surface area (SSA) and mean particle
size measured from PSA analysis. The initial setting time from different fly ash based geopolymer
and its relation to SSA and Dv50 is shown in Fig. 2. There seem to be correlation with the setting
time with exception of fly ash R9. There was an indication that fly ash with finer particle size tends
to have accelerated initial setting time.
Figure 2. Relationship of initial setting time of fly ash with (a) specific surface area,
(b) mean particle size of the sample.
Chemical compound of fly ash could also have an effect on its setting time. It can be seen from
Fig. 3 that higher content of CaO and MgO would produce increasingly faster initial setting time.
However, the effect of MgO content in fly ash is still not known whether gives a direct influence on
0
20
40
60
80
100
120
140
1000 1500 2000 2500 3000
Initial Setting Time (mins)
Specific Surface Area (m²/kg)
FA Y11.2 FA J10.6
FA P10.3 FA Y9.8
FA R9.6
0
20
40
60
80
100
120
140
0 5 10 15 20 25
Initial Setting Time (mins)
Mean Particle Size (Dv50) (μm)
FA Y11.2 FA J10.6
FA P10.3 FA Y9.8
FA R9.6
92 Properties and Application of Geopolymers
setting time because until now the effect of Mg is known only to reduce shrinkage in geopolymer
[11]. As mentioned [2], the pH level of fly ash could have great influence on the setting time of the
mixture. Fly ash Y11.2 with pH level of 11.2, the highest of all fly ash have very short initial setting
time.
Carbon content (LOI). High carbon content affects the absorption of liquid the mixture and
making more stiff mix. Fig. 4 shows the initial setting time of the geopolymer mortar with its
carbon content and SiO
2
/Al
2
O
3
ratio. There seem to be no seem to correlate of both properties with
its initial setting time for higher carbon content. However for fly ash with a carbon content below
1%, there are indication of flash-set. Further research is needed to determine the mechanism.
Figure 3. Relationship of initial setting time of fly ash with
(a) CaO content, (b) MgO content.
Figure 4. Relationship of initial setting time of fly ash with (a) Carbon content (LOI),
(b) SiO
2
/Al
2
O
3
ratio of the fly ash sample.
Sodium hydroxide concentration. The strength of alkaline solution to dissolve fly ash have
significant effect in the process of setting of geopolymer mortar. Fig. 5 shows initial and final
setting time for fly ash from the same source but taken at different time, Y11.2 and Y9.8. Different
NaOH molarity (8M, 10M, and 12M) seem to have different time, however this could be due to the
constant ratio of sodium silicate and sodium hydroxide ratio, making higher molarity of NaOH also
have higher content of silicate and thus making longer setting time. It also should be noted that the
time scale of the two fly ash are significantly different. Initial setting of fly ash Y11.2 were
achieved at about 15 minutes from the start of mixing while fly ash Y9.8 have more than 2 hours of
initial setting time.
0
20
40
60
80
100
120
140
0 5 10 15 20
Initial Setting Time (mins)
CaO content (%)
FA Y11.2 FA J10.6
FA P10.3 FA Y9.8
FA R9.6
0
20
40
60
80
100
120
140
0 2 4 6 8
Initial Setting Time (mins)
MgO content (%)
FA Y11.2 FA J10.6
FA P10.3 FA Y9.8
FA R9.6
0
20
40
60
80
100
120
140
0 2 4 6 8
Initial Setting Time (mins)
Carbon content (LOI) (%)
FA Y11.2 FA J10.6
FA P10.3 FA Y9.8
FA R9.6
0
20
40
60
80
100
120
140
1.5 2 2.5 3
Initial Setting Time (mins)
Fly ash SiO
₂
/Al
₂
O
₃
ratio
FA Y11.2 FA J10.6
FA P10.3 FA Y9.8
FA R9.6
Materials Science Forum Vol. 841 93
Figure 5. Setting time of geopolymer mortar for (a) fly ash Y11.2 and (b) fly ash Y9.8,
with molarity variation in the NaOH solution.
Sodium silicate to sodium hydroxide ratio. Fig. 6 showed the increase of penetration
resistance with time for all fly ash with different sodium silicate solution to sodium hydroxide solid
ratio (S/N). Molarity of NaOH solution used were 8 M. the result showed that with increase of S/N
ratio the setting time was faster, showing that the geopolymer reaction rate was faster with higher
S/N ratio up to 2. Fly ash with a higher pH level have a tendency to experience fairly more rapid
setting time compared to the others, especially fly ash on S/N ratio of 1.5 and 2. However there is
one fly ash R9.6 that did not follow the trend. Despite having a low pH value it also have a fast
setting time. This could be due to the particle shape of fly ash that absorb the alkaline liquid and
making the mixture more stiff, hence causing faster initial setting time.
0
10
20
30
40
50
60
70
80
12M 10M 8M
Setting time (mins)
NaOH concentration (M)
Initial
Final
0
50
100
150
200
250
300
350
400
12M 10M 8M
Setting time (mins)
NaOH concentration (M)
Initial
Final
0
100
200
300
400
500
600
0 30 60 90 120 150 180
Penetration Resistance (psi)
Time (mins)
FA Y11.2
S/N = 1
S/N = 1.5
S/N = 2
0
100
200
300
400
500
600
0 30 60 90 120 150 180
Penetration Resistance (psi)
Time (mins)
FA J10.6
S/N = 1
S/N = 1.5
S/N = 2
0
100
200
300
400
500
600
0 30 60 90 120 150 180
Penetration Resistance (psi)
Time (mins)
FA P10.3
S/N = 1
S/N = 1.5
S/N = 2
0
100
200
300
400
500
600
0 30 60 90 120 150 180
Penetration Resistance (psi)
Time (mins)
FA Y9.8
S/N = 1
S/N = 1.5
S/N = 2
94 Properties and Application of Geopolymers
