Construction and Building Materials 234 (2020) 117868
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Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Electrical resistivity of fly ash and metakaolin based geopolymers
Jingming Cai a,b, Jinlong Pan a, Xiaopeng Li c,⇑, Jiawei Tan b, Jiabin Li b,*
a
Key Laboratory of Concrete, Prestressed Concrete Structures of Ministry of Education, Southeast University, Nanjing, China
Department of Civil Engineering, KU Leuven, Bruges, Belgium
c
Department of Civil and Environmental Engineering, University of California, Irvine, USA
b
h i g h l i g h t s
The electrical resistivity behavior of fly ash and metakaolin based geopolymer was investigated.
The effects of alkali activator concentration, curing temperature and slag substitution rate were discussed.
The conductive mechanism of geopolymer was discussed with microscopic test methods.
a r t i c l e
i n f o
Article history:
Received 17 June 2019
Received in revised form 11 December 2019
Accepted 12 December 2019
Available online 24 December 2019
Keywords:
Geopolymers
Fly ash
Metakaolin
Electrical resistivity
a b s t r a c t
In this paper, the electrical resistivity behavior of fly ash and metakaolin based geopolymer was investigated with the two-probe measurement method. The effects of alkali activator concentration, curing temperature and slag substitution rate on the electrical resistivity and pore size distribution were discussed.
It was found that the electrical resistivity for fly ash based geopolymer is much higher than that for metakaolin based geopolymers. With the increase of alkali concentration, the electrical resistivity for both fly
ash and metakaolin based geopolymers decreased significantly. The curing temperature has a significant
influence on the electrical resistivity for fly ash based geopolymers, while the influence of curing temperature for metakaolin based geopolymer is negligible with a prolonged curing period. The increase of slag
replacement ratio would increase the electrical resistivity for fly ash based geopolymer.
Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction
Concrete is by far the world’s most versatile and popular construction material. However, it has been reported that the application of concrete for new buildings and infrastructure released
around 8% of global CO2 emissions [1]. There is an urgent need to
reduce the usage of cement and develop more durable and environmentally friendly binders. One of the alternatives for ordinary
concrete is geopolymers. Geopolymers exhibit similar mechanical
properties as normal Portland cement binders but significantly
reduce CO2 emissions [2,3]. Also, the molecular structures of
geopolymers has been reported to be rigid 3D networks consisting
of Si–O–Al and Si–O–Si bonds [4], which grants geopolymers additional advantages such as high thermal stability and corrosion
resistance [5,6].
⇑ Corresponding authors at: The Henry Samueli School of Engineering University
of California, Irvine, CA 92697, USA (X. Li). KU leuven Bruges Campus Spoorwegstraat 12, Bruges 8200, Sint-Michiels, Belgium (J. Li).
E-mail addresses: xiaopel@uci.edu (X. Li), jiabin.li@kuleuven.be (J. Li).
https://doi.org/10.1016/j.conbuildmat.2019.117868
0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
The most widely used solid precursors for geopolymers are fly
ash, metakaolin and slags. Meanwhile, the typical alkali activators
are sodium hydroxide or potassium hydroxide, sodium silicate and
their mixtures. Extensive studies have been performed to investigate the mechanical behaviors of geopolymer materials and their
applications towards structural engineering [7–10]. However, the
investigations on the electrical behaviors of geopolymers are still
limited. Hanjitsuwan et al. analyzed the effects of NaOH concentrations on the electrical properties of high calcium fly ash geopolymer paste [11] and found that its conductivity increased with
NaOH concentration resulting from the increased geopolymerization extend. Payakaniti et al. studied the electrical conductivity
of fly ash based geopolymer reinforced with carbon fiber [12],
the results indicated that the electrical conductivity of fly ash
based geopolymer was highly dependent on the carbon fiber concentration. As a basic physical behavior, the electrical resistivity
behavior of geopolymers with a broader range of parameters still
needs more discussions and investigations, especially while being
compared with the extensive studies conducted on the electrical
resistivity behaviors of cementitious materials in recent decades
[13–15].
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J. Cai et al. / Construction and Building Materials 234 (2020) 117868
Set back this background, the electrical resistivity behavior of
fly ash and metakaolin based geopolymers are investigated in this
paper. The effects of alkali activator concentration, curing temperature and slag substitution rate are discussed. Moreover, the conductivity differences for fly ash and metakaolin based
geopolymers are analyzed.
2. Experiment
2.1. Materials
Both fly ash and metakaolin based geopolymers were investigated in the study. The solid precursors including low calcium
(Class F) fly ash, metakaolin and slag. The macro and micro morphologies of the raw materials are shown in Figs. 1 and 2, respectively. Numerous spherical substances can be found in fly ash and
the crystalline materials were observed in slag and metakaolin. The
particle size distributions of the raw materials were determined
using laser diffraction (Mastersizer 2000, Malvern Instruments),
as shown in Fig. 3. The particle size for metakaolin is concentrated
in 0.5-5lm, while the particle size for fly ash is about 5-50lm,
indicating the metakaolin powder possess a higher fineness than
fly ash. The main chemical compositions for each material, which
were determined by X-ray fluorescence (XRF-1800, Shimadzu),
are shown in Table 1. It can be seen that slag has a much higher
content of CaO than fly ash and metakaolin. A combination of Na2SiO3 (14.7% of Na2O, 29.4% of SiO2 and 55.9% of water) and KOH
solutions was applied as the alkaline activator. KOH solutions with
different concentrations (4 mol/L, 8 mol/L and 12 mol/L) were prepared by dissolving KOH flakes (98% of purity) in water. A constant
Na2SiO3 solution to KOH solution ratio of 1.5 by weight was used in
this paper. Na2SiO3 and KOH activator solutions were mixed and
stored for 24 h before being mixed with solid precursors.
To prepare the geopolymer paste, the solid precursors were
firstly mixed with an electric blender, at the rate of 1400 rpm for
about 5 min. Then, the alkali activator was added into the blender
and mixed with precursors for another 10 min until the mixture
being homogeneous and uniform. Finally, the fresh paste was
casted in the cylinder molds. The cylinder specimens were firstly
kept at room temperature for 24 h before being demoulded. The
specimens were then cured in an automatic oven (Hebei Rongyao
Corporation, China) at specific temperatures (25 °C, 50 °C and
70 °C) for 28 days. Each sample was taken out from the oven and
recorded the electrical resistance every two days. The dimensions
of typical geopolymer specimen used for electrical resistance measurement are shown in Fig. 4(a).
2.3. Test set-up
The electrical resistance of each geopolymer sample was determined by a two-probe measurement method. The schematic diagram of test set-up is shown in Fig. 4(b). The copper sheets were
attached to both ends of the specimen. Silver paint was applied
between the copper sheets and the sample surface to further
enhance the electrical contact. Two copper wires were wrapped
around the copper sheets and connected to the anodic and cathodic
terminal of a digital multimeter (Fluke Technologies, 8808A),
respectively. Due to the polarization effect [18], it has been
reported that the recorded electrical resistance is actually higher
than the true electrical resistance for cementitious material [19].
In this study, it was noticed that the electrical resistance for
geopolymer specimen become stable after 2 h. For the convenience
of comparison, the electrical resistance for all geopolymer specimens were recorded after 2 h as the electrical current was supplied
by the multimeter.
2.2. Sample preparation
3. Results and discussions
A total of 11 series of geopolymer samples with different
parameters were casted and the details are shown in Table 2.
According to the previous research [16,17], the solid-to-liquid
ratios for fly ash and metakaolin based geopolymers were set as
3 and 1, respectively, considering the fact that metakaolin powder
has a higher fineness than fly ash, as shown in Fig. 3 and Table 1.
The sample ‘FA-4-0’ is used for explaining the nomenclature: the
first two letters ‘FA’ denotes the type of solid precursors, while
the following number depicts the KOH concentration for each
specimen. The last number depicts the slag replacement rate,
which is defined as follows:
3.1. Influence of solid precursors
a¼
ms
mf þ ms
where a is the slag replacement ratio, mf and ms are the weight of
fly ash and slag.
Fig. 5 describes the electrical resistivity for geopolymers with
different solid precursors (metakaolin, fly ash and fly ash with
slag). The electrical resistance was recorded every two days. It
can be seen that fly ash based geopolymer showed much higher
electrical resistivity than that for metakaolin based geopolymers.
For example, the electrical resistivity for specimen FA-8-0-50 °C
at 28 days is 98 Xm, while the value for specimen MK-8-0-50 °C
is 19.5 Xm. This phenomenon may be attributed to two reasons.
Firstly, the solid-to-liquid ratio for fly ash and metakaolin based
geopolymers are 3 and 1, respectively, meaning that metakaolin
based geopolymers contain more alkali activator than fly ash based
geopolymers. The alkali activator, which contains numerous conductive K+, OH and Na+ ions, could introduce more electrical network connections and therefore significantly reduce the resistivity
Fig. 1. Raw materials (a) fly ash, (b) slag, (c) metakaolin.
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J. Cai et al. / Construction and Building Materials 234 (2020) 117868
Fig. 2. The micromorphology of raw materials (a) fly ash, (b) slag, (c) metakaolin.
Fig. 3. Particle size distribution curves of raw materials.
of geopolymers. As can be seen in Fig. 3 shown above, the specific
surface area for metakaolin is much higher than that for fly ash.
Thus, the geopolymerization products for metakaolin based
geopolymer is much denser and more compact than fly ash based
geopolymers. The SEM micrograph of specimen MK-8-0-50 °C and
specimen FA-8-0-50 °C at 28 days are shown in Fig. 6. A compact
and continues gel-like structure can be observed for specimen
MK-8-0-50 °C. By contrast, a porous and complex structure as well
as some unreacted fly ash particles can be observed for specimen
MK-8-0-50 °C. The microstructure of cementitious material could
dramatically affect its electrical resistivity, a compact and dense
structure is capable of creating more electronic circuits and thus
resulting a lower electrical resistivity [20]. The pore size distribution with mercury intrusion porosimetry method (Quantachrome,
Boynton Beach, FL, USA) for both metakaolin and fly ash based
geopolymer specimens after 28 days are shown in Fig. 7. It is clear
to see that the pore size for metakaolin based geopolymer was
about 10 nm, while the pore size for fly ash based geopolymer
can be as high as several micrometer, which furtherly proves that
the metakaolin based geopolymer has a more compact structure.
As can be seen in Fig. 5, the electrical resistivity for metakaolin
based geopolymers increased rapidly during the first 16 days and
then maintained unchanged. Since the conductivity of geopolymer
paste was mainly determined by its conductive ion concentration
and micro-structures, it can be inferred that the reaction products
and micro-structures of metakaolin based geopolymer remained
unchanged after 16 days. This phenomenon once again demonstrated that metakaolin possesses high chemical activity and its
geopolymerization reaction could be quickly completed during
the early stage. For fly ash based geopolymer, however, the electrical resistivity increased continuously during the whole curing period, indicating the geopolymerization process was slow and
continuous due to the inactive chemical property of fly ash. With
fly ash partly being replaced by slag, the electrical resistivity for
specimen MK-8-30-50 °C is much higher than that for MK-8-050 °C, as also shown in Fig. 5. According to the previous studies,
slag exhibits a higher chemical activity than fly ash and its main
reaction product is calcium aluminosilicate hydrates (C-A-S-H)
[21]. The addition of slag could accelerate the reaction process
and shorten the initial setting time. However, the rapid reaction
also prevents the formation of more solid and dense spatial network structures. Compared with conventional cement, geopolymer
paste is perceived as more homogenous with the consisting of 3-D
aluminosilicate microstructure. For homogenous materials or
materials composed of single-phase, it has been reported that the
electrical conductivity increased with the increase of compactness
[22,23].
3.2. Influence of alkali concentration
The electrical resistivities for geopolymers with different alkali
concentrations (from 4 to 12 mol/L) are shown in Fig. 8. For metakaolin based geopolymer, the electrical resistivity decreased with
the increase of alkali concentration. This is quite reasonable since
a higher alkali concentration introduces more conductive OH
and K+ ions into the system. Additionally, the proper increase of
alkali concentration shows positive influences on the compressive
strength, Young’s modulus and microstructure of metakaolin based
geopolymers [24]. That is to say, a higher alkali concentration
would result in a denser microstructure, which would benefit the
transportation of electrons and ions. The pore size distributions
for metakaolin based geopolymers with different alkali concentrations are shown in Fig. 9(a), it can be seen that the peak value for
pore size shifted to be smaller with the increase of alkali
concentration.
It can also be noticed that for specimen MK-12-0-50 °C, the
electrical resistivity increased rapidly during the initial 16 days,
while the value kept instant during the last 12 days. The obvious
inflection point, from which point the electrical resistivity
Table 1
Chemical constituents of solid precursors (wt%).
Compositions
CaO
SiO2
Al2O3
Fe2O3
MgO
K2O
Na2O
P2O5
MnO
Metakaolin
Fly ash
Slag
0.13
3.71
34.93
61.45
50.86
40.28
32.45
28.16
10.11
0.89
6.24
0.07
2.08
1.28
8.14
0.81
0.67
1.12
0.77
1.27
1.12
0.07
0.12
0.08
–
0.07
1.14
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J. Cai et al. / Construction and Building Materials 234 (2020) 117868
Table 2
Details of all samples.
Sample ID
Solid precursors
KOH concentrations (mol/L)
a (%)
Solid-to-liquid ratio
FA-4-0
FA-8-0
FA-12-0
FA-16-0
FA-8-30
FA-8-60
FA-8-90
MK-4-0
MK-8-0
MK-12-0
MK-16-0
Fly ash
Fly ash
Fly ash
Fly ash
Fly ash with slag
Fly ash with slag
Fly ash with slag
Metakaolin
Metakaolin
Metakaolin
Metakaolin
4
8
12
16
8
8
8
4
8
12
16
0
0
0
0
30
60
90
0
0
0
0
3
3
3
3
3
3
3
1
1
1
1
Fig. 4. (a) Specimen size; (b) A schematic diagram of the test set-up.
concentration of Al and Si atoms in the alkaline solutions, which
would be converted into geopolymer gel with the geopolymerization process goes on. The pore size distributions for fly ash based
geopolymer specimens with different alkali concentrations at
28 days are shown in Fig. 9(b). With the increase of alkali concentration, the pore size was smaller and distributed much more
evenly. Also, the alkali concentration seems to have a more prominent influence on fly ash based geopolymer while being compared
with the results shown in Fig. 9(a) and (b).
3.3. Influence of curing temperature
Fig. 5. Electrical resistivity for geopolymers with different solid precursors.
increased much more smoothly, could also be observed for other
metakaolin based geopolymer specimens. This may be attributed
to the high chemical activity of metakaolin, the geopolymerization
products and microstructure remained stable very quickly. By contrast, as shown in Fig. 8(b), the electrical resistivity for fly ash
based geopolymer increased continuously during the curing period
and no inflection point was observed. With the increase of alkali
concentration, the electrical resistivity for fly ash based geopolymer decreased significantly, indicating that the increase of alkali
concentration would be beneficial for the geopolymerization process of fly ash based geopolymer. For example, the electrical resistivity for specimen FA-12-0-50 °C and FA-4-0-50 °C at 28 days
were 60 Xm and 115.6 Xm, respectively. This is reasonable since
higher alkali concentration could provide more OH–, thus more
Al and Si atoms will be dissolved from fly ash, followed by a higher
The curing temperature has significant influences on the
strength, pore distribution and microstructure of both fly ash and
metakaolin based geopolymers [25,26]. In this paper, the curing
temperatures was set as 25 °C, 50 °C and 70 °C. For metakaolin
based geopolymer, as can be seen in Fig. 10a, the inflection point
disappeared and the electrical resistivity kept increasing during
the whole stage when the curing temperature was set as 25 °C,
indicating that the reaction products and microstructure were
changing all the time. Compared with Fig. 10(b), it can be found
in Fig. 10(a) that the electrical resistivity for all metakaolin based
geopolymer samples at 28 days are in the same order of magnitude. It has also been reported that the low temperature curing
has negligible influences on the quality and properties of geopolymerization products for metakaolin based geopolymer, the compressive strength for metakaolin based geopolymer with high
curing temperature was even lower than geopolymer with low
curing temperature at the age of 28 days [25].
For fly ash based geopolymer, as can be seen in Fig. 10(b), the
influence of curing temperature was significant. The 28-day electrical resistivity remained at a low level with a 25 °C curing temperature, indicating that numerous conductive OH, K+ and Na+
still existed in the paste. The previous studies found that the compressive strength for fly ash based geopolymer cured at ambient
J. Cai et al. / Construction and Building Materials 234 (2020) 117868
5
Fig. 6. SEM micrograph of metakaolin and fly ash based geopolymers (a) MK-8-0-50 °C.
The pore size distribution for 28-day geopolymers with different curing temperature are shown in Fig. 12. For metakaolin based
geopolymer, all the curves differ slightly and the pore size ranges
from 5 nm to 50 nm, once again demonstrating that the curing
temperature has limited influence on the geopolymerization process of metakaolin based geopolymer with enough curing. By contrast, as shown in Fig. 12(b), the curves for fly ash based
geopolymer with different curing temperatures changed a lot.
With the increase of curing temperature, the pores tend to be smaller and well distributed, indicating the geopolymerization process
of fly ash based geopolymer was very sensitive to curing
temperature.
3.4. Influence of slag replacement rate
Fig. 7. The pore size for fly ash and geopolymer based geopolymers.
temperature was negligible at 28 days [27]. By contrast, when the
curing temperature was set as 70 °C, the electrical resistivity
increased rapidly and an inflection point appeared, demonstrating
the geopolymerization process was accelerated and a stable electrically conductive path formed in the paste. The SEM micrograph
of specimen FA-8-0-25 °C and specimen FA-8-0-70 °C at 28 days
are shown in Fig. 11. A more compact and denser structure can
be observed with the elevated curing temperature, comparing with
numerous unreacted fly ash microspheres existed in fly ash
geopolymer specimen with an ambient curing temperature. This
can be likely explained by the low chemical activity and high activation energy barrier of fly ash [28], thus elevating the curing temperature would effectively accelerate the process of dissolving
aluminum and silicon atoms from inactive glassy phases in fly ash.
The curing temperature has a great influence on the geopolymerization process of fly ash based geopolymer, thus the initial
curing is very essential. It has been reported that the addition of
slag into fly ash based geopolymer could dramatically increase
the mechanical strength with ambient curing temperature [29].
The effects of slag replacement rate on the electrical resistivity of
fly ash based geopolymers are shown in Fig. 13. There is no inflection point observed for all samples, indicating the reaction process
was continuous during the whole curing period. It also can be
found that the electrical resistivity increased with the increase of
slag replacement ratio. For example, the electrical resistivity for
specimen FA-8-90-50 °C is about three times higher than that of
specimen FA-8-0-50 °C. The pore size for fly ash based geopolymers with different slag replacement ratio is shown in Fig. 14. It
can be seen that the specimen FA-8-90-50 °C has denser
microstructures than FA-0-50 °C. However, the electrical resistivity
for specimen FA-8-90-50 °C is higher than FA-0-50 °C, indicating
that a denser microstructure is not necessarily lead to a lower elec-
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J. Cai et al. / Construction and Building Materials 234 (2020) 117868
Fig. 8. Electrical resistivity for geopolymers with different alkali concentrations: (a) metakaolin based geopolymers; (b) fly ash based geopolymers.
Fig. 9. The pore size for geopolymers with different alkali concentration: (a) metakaolin based geopolymers; (b) fly ash based geopolymers.
Fig. 10. Electrical resistivity for geopolymers with different curing temperature: (a) metakaolin based geopolymers; (b) fly ash based geopolymers.
J. Cai et al. / Construction and Building Materials 234 (2020) 117868
7
Fig. 11. SEM micrograph of fly ash based geopolymer at different curing temperature (a) FA-8-0-25 (b) FA-8-0-70 °C.
Fig. 12. The pore size for geopolymers with different curing temperature: (a) metakaolin based geopolymers; (b) fly ash based geopolymers.
Fig. 13. Electrical resistivity for fly ash based geopolymers with different slag
replacement ratio.
Fig. 14. The pore size for fly ash based geopolymers with different slag replacement
ratio.
trical resistivity, which is quite different from other geopolymer
samples made with pure fly ash or metakaolin.
It was hypothesized that both microstructures and conductive
ion concentrations could significantly affect the electrical
resistance of geopolymers. For fly ash based geopolymers with
the addition of slag, it has been reported that slag tends to consume more OH and SiO2
3 with the producing of C-A-S-H gel in
the geopolymer paste due to pozzolanic reactions [30,31]. The
8
J. Cai et al. / Construction and Building Materials 234 (2020) 117868
reaction between alkali activator and slag is much more rapid than
that with fly ash and metakaolin, which would dramatically
decrease the concentration of conductive ions [32]. Thus, even
though adding slag makes microstructure denser, it reduces conducting ions in the system.
4. Conclusions
In this research, the two-probe measurement method was
applied to study the electrical resistivity behaviors of fly ash and
metakaolin based geopolymers. The main conclusions can be
drawn as follows:
1. The electrical resistivity for fly ash based geopolymer is much
higher than that for metakaolin based geopolymer, since metakaolin based geopolymer has a higher solid-to-liquid ratio and
chemical activity.
2. The electrical resistivity for both fly ash and metakaolin based
geopolymers decreased significantly with the increase of alkali
concentration. The higher alkali concentration would result in
denser microstructure for geopolymers, which would benefit
the transportation of electrons and ions.
3. The influence of curing temperature on the electrical resistivity
of metakaolin based geopolymer is insignificant with the prolonged curing period, while fly ash based geopolymer was more
sensitive to the curing temperature.
4. The electrical resistivity increased with the increase of slag
replacement ratio, since the addition of slag would consume
more conductive ions.
CRediT authorship contribution statement
Jingming Cai: Conceptualization, Methodology, Writing - original draft. Jinlong Pan: Funding acquisition. Xiaopeng Li: Conceptualization, Writing - review & editing, Supervision. Jiawei Tan:
Validation, Investigation, Supervision. Jiabin Li: Conceptualization,
Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
This work was financially supported by National Key R&D Program of China (2016YFC0701907), Natural Science Foundation of
China (No. 51778131 and No. 51908117), Fundamental Research
Funds for the Central Universities (2242019R20003) and Jiangsu
Planned Projects for Postdoctoral Research Funds (2019K042).
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