The Use of Borax in Deterring Flash Setting of
High Calcium Fly Ash Based Geopolymer
Antoni
1,2,a
, Stephen Wibiatma Wijaya
1,b
, Juan Satria
1,c
, Agung Sugiarto
1,d
and Djwantoro Hardjito
1,2,e
1
Department of Civil Engineering, Petra Christian University, Indonesia
2
Center of Excellence Geopolymer & Green Technology (CEGeoGTech), School of Materials
Engineering, Universiti Malaysia Perlis, Malaysia
a
antoni@petra.ac.id,
b
stephen.wibi@yahoo.com,
c
juansatria94@gmail.com,
d
agung_91101@yahoo.com,
e
djwantoro.h@petra.ac.id
Keywords: flash setting, borax, geopolymer, high calcium, fly ash, admixture
Abstract. Geopolymer that was made with high CaO content fly ash was found to have higher
compressive strength than the low CaO fly ash, using the same mixture composition. This effect
could be due to the physico-chemical properties of the fly ash, in respect to its particle size or the
chemical composition. Although it was not widely published, the occurrence of flash setting of
geopolymer was known to occur when using high CaO content fly ash as the precursor.
Geopolymer paste may solidify within minutes after the addition of alkali activators, making it very
difficult to cast in big volume. This paper investigate the effect of borax addition to the high
calcium fly ash-based geopolymer mixture to reduce the occurrence of flash setting. It was found
that the setting time can be extended significantly, with the addition of 5% borax, by mass, of fly
ash. The addition of borax also have positive effect on increasing the compressive strength of
geopolymer.
Introduction
In recent years, the use of high calcium fly ash as cement replacement have been investigated
widely. Fly ash possesses beneficial properties, i.e. it can undergo hydration and pozzolanic
reaction in the Portland cement matrix. The use of only high calcium fly ash, without any Portland
cement, can produce concrete with strength of 33 MPa at 28 days, with the addition of water [1].
Variation of fly ash quality, such as the chemical composition, is directly related to the variation
of the coal itself, as the combustion source. This causes concern on the reliability of the final
product, when large portion of fly ash is used to make concrete [2]. Rapid method to detect the
variation of fly ash has been suggested by the authors by measuring its pH level in aqueous
solution, as an indicator of the CaO content in fly ash [3].
High calcium fly ash has been used as precursor for making geopolymer by several
researchers [4-7]. In their studies, fly ash with CaO content ranging from 15-20%, by mass, was
used to produce geopolymer with compressive strength of 50 to 65 MPa. Faster setting time was
reported when compared with the low calcium fly ash-based geopolymer [8]. Under normal curing
condition, setting time of 28 to 68 minutes was reported [6].
The phenomenon of flash or rapid setting, defined as rapid hardening of geopolymer mixture of
only several minutes after the addition of alkaline activator, has rarely been reported. Several
authors mentioned the occurrence of flash setting when making geopolymer [9-11], however the
phenomenon has not yet been investigated thoroughly. Davidovits stated that fly ash with higher pH
level tends to have higher CaO content, and also to cause higher possibility of flash setting
occurrence [12]. Rapid setting of geopolymer can cause problem when casting large volume
geopolymer, and hence longer setting time should be ensured.
The use of additive to prolong the setting time and to increase the workability of geopolymer
mixture has been suggested by several authors. Naphthalene based superplasticizer [4], dipotassium
phosphate [11] and sucrose [13] have been mixed in the geopolymer mixture to slow down the
Materials Science Forum Submitted: 2016-01-13
ISSN: 1662-9752, Vol. 857, pp 416-420 Accepted: 2016-01-25
doi:10.4028/www.scientific.net/MSF.857.416 Online: 2016-05-20
© 2016 Trans Tech Publications, Switzerland
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: 111.94.153.3, Petra Christian University, Surabaya, Indonesia-11/03/16,09:46:59)
polymerization reaction rate. However, reduction of strength was observed on the final geopolymer
product, showing that they have detrimental effect.
Borax as retarding additive was not well known to be used in Portland cement or geopolymer,
but several researchers have mentioned that it could be used to retard the setting time of concrete
[1,9]. Using 100% high calcium fly ash as cement paste, Cross et al. [1] encountered flash setting. It
could be reduced with borax addition of 1.25%, by mass of fly ash. The use of borax in geopolymer
mixture with the aim to increase the compressive strength has been reported by Tailby and
MacKenzie [9]. The addition of borax of 10% by mass of fly ash increased the compressive strength
of geopolymer when compared to those without borax. The use of borax to replace sodium silicate
as the alkaline activator was reported by Nazari et al. [14]. Borax was used together with NaOH
solution for the alkaline activator, whereas the use of borax was about 35%, by mass, of fly ash. It
was reported that the use of borax in the geopolymer mixture resulted in favorable effect.
The objectives of this paper are to report the occurrence of flash setting in the geopolymer
mixture when using high calcium fly ash and to investigate the effect of borax usage as additive in
geopolymer mixture on its setting time. Effect of the use of borax on the setting time and
compressive strength of geopolymer will be discussed.
Experimental Method
Material. High calcium and low calcium fly ash were obtained from a power plant in Paiton, East
Java, Indonesia. Each fly ash was coded in relation with their measured pH value [3]. Physical
properties of fly ash, i.e. particle size distribution, specific gravity and particle shape, were
evaluated. In addition to the physical examination, x-ray fluorescence (XRF) analysis was also
performed to examine the chemical content of fly ash. Chemical compound and physical properties
of the fly ash can be seen in Table 1.
Table 1. Fly ash chemical and physical properties
Oxide (%, by mass) Fly ash Y11.2
Fly ash Y9.8
SiO
2
39.78 51.12
Al
2
O
3
17.87 18.90
Fe
2
O
3
15.00 17.71
CaO 15.47 5.54
K
2
O 1.32 0.82
MgO 6.45 3.17
SO
3
1.32 0.47
MnO
2
0.18 0.33
LOI 0.49 6.96
SSA (m
2
/kg) 2618 1785
D
v
(50) (µm) 6.98 10.30
D
v
(90) (µm) 60.80 84.30
Specific gravity
2.63 2.36
ASTM Standard C618 [15] classifies fly ash as pozzolanic material based on the total content of
the main chemical compounds of SiO
2
, Fe
2
O
3
and Al
2
O
3
to be more than 70%. Both fly ash samples
used in this study satisfy the standard. Based on the CaO content, fly ash Y11.2 can be classified as
type C (high calcium) fly ash, while fly ash Y9.8 is classified as type F (low calcium) fly ash.
Borax was obtained in anhydrous form from local chemical store. Sand from Lumajang, East
Java, with fineness modulus of 2.52 was used for making the geopolymer mixture. Alkaline
solution was prepared from a combination of sodium hydroxide (NaOH) and sodium silicate
solution.
Mixture proportion and sample preparation. Experimental study was conducted on
geopolymer mortar to measure the compressive strength and on geopolymer paste to measure the
setting time. Samples were prepared with alkaline solution to fly ash ratio of 0.25 and sand to fly
ash ratio of 2.0, by mass. Sodium hydroxide solid was dissolved in water one day prior mixing.
Materials Science Forum Vol. 857 417
Borax was dissolved into the sodium hydroxide solution before mixing it with sodium silicate
solution. The ratio of alkaline solution, which defined as the ratio of sodium silicate liquid to
sodium hydroxide solid, by mass, was set at 2.0. The addition of borax into the geopolymer mixture
was varied from 0, 1, 2, 3, 5 and 7%, by mass of fly ash. Fine aggregate and fly ash was mixed
thoroughly before mixing with the alkaline activator and borax.
Compressive strength and setting time test. Fresh geopolymer mortar was cast in 50 mm cube
mould to prepare for compressive testing. After casting, plastic sheet was wrapped to the mould and
cured at 60°C for 24 hours. After curing, samples were demolded and stored at room temperature
until the day of testing. Compressive strength tests were conducted on mortar cubes aged 7 days.
Three specimens were made for each variation. Setting time of the geopolymer paste was measured
by using Vicat needle apparatus [16]. The initial setting time was determined from needle
penetration of 25 mm, while final set was noted when no penetration was observed any longer.
Setting time of the geopolymer paste was measured in the laboratory condition with no heat curing.
Results and Discussions
Flash setting. The occurrence of flash setting was observed by the rapid hardening of the
geopolymer paste. Black [10] noted that there might be two conditions for the flash setting to occur,
i.e. when the NaOH concentration is very high and when high calcium fly ash is used. Authors’
previous experience with the geopolymer flash setting mostly can be attributed to the high calcium
content in the fly ash. However, there are other factors that could lead to flash setting such as the
reactivity of fly ash related to its particle size, casting temperature, and high volume mixing. Lower
liquid content in the geopolymer mixture could also causing rapid stiffening in the mixture. Sodium
silicate to sodium hydroxide ratio has contributing effect. Higher silicate ratio accelerates the
polymerization reaction, as shown by the increase of compressive strength, and at the same time
also accelerating the early stiffening of the matrix.
In most cases, flash setting can be detected easily, as the fresh mixture would be already too stiff
to be poured into the mould only after a few minutes. Figure 1 shows some defective specimens
with big entrapped air when flash setting occurs. The fresh geopolymer was poured into the mould
in one layer and compacted on the vibrator table. It was realized that the vibration cannot expel the
entrapped air because the matrix was already very stiff, even when the vibration was carried out
immediately after pouring. Evaluating the specimens’ density need to be performed to detect the
internal entrapped void.
Figure 1. Flash setting observed after opening the mould.
Setting time. Setting time of the geopolymer paste was measured using Vicat needle apparatus.
Geopolymer paste was prepared with the mixture proportion exactly the same as the one for making
the geopolymer mortar, with the removal of the sand content. Therefore the geopolymer paste is
slightly soggier. Figure 2(a) and (b) show the graph of Vicat needle penetration into the paste with
time for fly ash Y11.2 and Y9.8, respectively. Control mixture without borax shows fast setting
time, especially on geopolymer paste with fly ash Y11.2. Figure 2(a) indicates some penetration
resistance occurs only within 5 minutes. The addition of borax shows the increase in the setting
time of geopolymer paste, compared to the one of the control mixture. Borax addition is shown to
cause similar prolong initial setting time, irrespective of the borax content. However, the trend is
different with the final setting time. Borax addition of 5% shows a different setting time behavior.
418 Advanced Materials Engineering and Technology IV
The slope of penetration was reduced with the increase of time. This shown that the early reaction
rate of the geopolymer was retarded by the addition of borax. Kusbiantoro et al. confirmed that the
hardening behavior of geopolymer paste was quite abrupt [13], similar to the control mixture in this
research. Adding sucrose only shifted the setting time, without altering the abruptness of setting.
The decrease of slope with the addition of borax in the geopolymer mixture suggests that there
could be a change on the rate of the geopolymerization process. Fly ash with lower calcium content
causes slower setting time, as shown by Figure 2(b).
Figure 2. Setting time measured by Vicat needle penetration for geopolymer paste with: (a) Fly ash
Y11.2 and (b) Fly ash Y9.8
Compressive strength. The compressive strength of geopolymer mortar with fly ash Y11.2 and
Y9.8 is shown in Figure 3. From the control sample without borax, it was shown that geopolymer
mortar with fly ash Y11.2 has higher compressive strength of 85.33 MPa, compared to the one with
fly ash Y9.8 (73.07 MPa). Higher compressive strength due to the use of fly ash with higher CaO
content could be attributed to the presence of hydration reaction, beside the geopolymerization
reaction. With the addition of 1 to 5% borax, the compressive strength is shown to be slightly
increased. The highest compressive strength of 90.53 MPa is observed with the addition of 1%
borax. At 7% borax addition, the compressive strength was slightly reduced to 82.80 MPa.
For geopolymer mortar with fly ash Y9.8, the addition of borax was performed at 0, 1 and 2% by
mass of fly ash. It serves as a control mixture as well. Figure 3 shows that the addition of borax of
2%, increases the compressive strength from 73.07 MPa to 88.67 MPa. This indicates that borax
addition has also positive effect in increasing the compressive strength, in addition to delaying the
setting time of the geopolymer.
Figure 3. Compressive strength of geopolymer mortar on 7
th
day with addition of borax.
Conclusions
• The pH level of fly ash can be used as one indicator to detect the possible occurrence of flash
setting of fly ash-based geopolymer, especially the one using high calcium fly ash. The
occurrence of flash setting in geopolymer can hinder its application in large scale or volume.
0
5
10
15
20
25
30
35
40
45
0 15 30 45 60 75 90 105 120 135
Needle Penetration (mm)
Time (mins)
0% 1%
2% 3%
5% 7%
initial set
final set
0
5
10
15
20
25
30
35
40
45
0 15 30 45 60 75 90 105 120 135
Needle Penetration (mm)
Time (mins)
0% 1%
2%
initial set
final set
0
20
40
60
80
100
120
0 1 2 3 4 5 6 7 8
Compressive strength
(MPa)
Borax addition (% of FA)
Y11.2
Y9.8
Materials Science Forum Vol. 857 419
• The addition of borax in the mixture can prolong the setting time of the fly ash-based
geopolymer, especially on those using high calcium fly ash.
• The compressive strength of geopolymer is slightly increased with the addition of borax.
• Low calcium fly ash-based geopolymer with no tendency of flash setting can also be benefitted
from the addition of borax. The addition of borax at 1 to 3% by mass of low calcium fly ash
improves the uniformity of the mixture and hence increases its compressive strength.
Acknowledgements
The authors gratefully acknowledge The Ministry of Research, Technology and Higher Education,
Indonesia, who provided the research grant.
Reference
[1] D. Cross, J. Stephen, and J. Vollmer, “Structural application of 100 percent fly ash concrete”,
2005 WOCA, Lexington, USA, 6 April (2005).
[2] S. Tsimas and A. Moutsatsou-Tsima, “High-calcium fly ash as the fourth constituent in
concrete: problems, solutions and perspectives”. Cem. & Concrete Composites, 27(2), 231-237.
(2005).
[3] Antoni, R. Gunawan and D. Hardjito, “Rapid Indicators in Detecting Variation of Fly Ash for
Making HVFA Concrete” Appl. Mech. Mater. Vol. 815 pp 153-157 (2015).
[4] P. Chindaprasirt, T. Chareerat, and V. Sirivivatnanon, “Workability and strength of coarse high
calcium fly ash geopolymer”. Cement and Concrete Composites, 29(3), 224-229. (2007).
[5] X. Guo, H. Shi, and W.A. Dick, “Compressive strength and microstructural characteristics of
class C fly ash geopolymer”. Cement and Concrete Composites, 32(2), 142-147. (2010).
[6] P. Topark-Ngarm, P. Chindaprasirt, and V. Sata, “Setting Time, Strength, and Bond of High-
Calcium Fly Ash Geopolymer Concrete”. Journal of Materials in Civil Engineering (2014).
[7] S. Pangdaeng, T. Phoo-ngernkham, V. Sata, and P. Chindaprasirt, “Influence of curing
conditions on properties of high calcium fly ash geopolymer containing Portland cement as
additive”. Materials & Design, 53, 269-274. (2014).
[8] D. Hardjito, C.C. Cheak, and C.H.L. Ing, “Strength and setting times of low calcium fly ash-
based geopolymer mortar”, Mod. Appl. Sci. Vol 2, No 4, (2008).
[9] J. Tailby and K.J.D. MacKenzie, “Structure and mechanical properties of aluminosilicate
geopolymer composites with Portland cement and its constituent minerals”, Cem. & Con. Res.
Vol. 40 pp 787-794, (2010).
[10] J.R. Black, “Mix design process for alkaline-activated class F fly ash geopolymer concrete”,
Final project report, University of New South Wales, Australia (2012).
[11] H.W. Nugteren, V.C.L. Butselaar-Orthilieb, M. Izquierdo, G.J. Witkamp, and M.T. Kreutzer,
“High strength geopolymer from fractionated and pulverized fly ash”, 2009 WOCA. Lexington,
USA, May (2009).
[12] J. Davidovits, Geopolymer chemistry and applications, 2
nd
ed. Saint-Quentin, France: Institute
Géopolymère, (2008).
[13] A. Kusbiantoro, M.S. Ibrahim, K. Muthusamy, and A. Alias, “Development of sucrose and
citric acid as the natural based admixture for fly ash based geopolymer”, Proc. Env. Sci. Vol 17
pp 596-602, (2013).
[14] A. Nazari, A. Maghsoudpour, and J.G. Sanjayan, “Flexural strength of plain and fiber
reinforced boroaluminosilicate geopolymer”, Const. & Build. Materials.Vol. 76 pp 207-213,
(2015).
[15] ASTM C618-08, Standard Specification for Coal Fly Ash and Raw or Calcined Natural
Pozzolan for Use in Concrete, ASTM International, West Conshohocken, PA, (2008)
[16] ASTM C191-04, Standard Test Method for Time of Setting of Hydraulic Cement by Vicat
Needle, ASTM International, West Conshohocken, PA, (2004)
420 Advanced Materials Engineering and Technology IV
