effects of chemical composition of flyash on comp strength of FA cement mortar

Construction and Building Materials 204 (2019) 255–264
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Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Effects of chemical composition of fly ash on compressive strength of fly
ash cement mortar
Young Keun Cho a , Sang Hwa Jung a , Young Cheol Choi b,⇑
a Construction Technology Research Center, Korea Conformity Laboratories, Seoul 08503, South Korea
b Department of Civil and Environmental Engineering, Gachon University, 1342 Seongnamdaero, Sujeong-gu, Seongnam-si, Gyeonggi-do 13120, South Korea
highlights
Fly ash cement mortar was made by replacing 25 wt% of cement with 16 fly ash types.
The compressive strengths of these specimens were evaluated.
The compressive strength increased with the mortar’s age.
The pozzolanic reaction of fly ash was affected by the components of the mortar.
article
info
abstract
Article history:
Received 4 October 2018
Received in revised form 2 January 2019
Accepted 27 January 2019
Keywords:
Fly ash
Amorphous characteristic
Pozzolanic reaction
Compressive strength
Glass structure
The effect of the chemical composition of the amorphous and crystalline phases of fly ash on the
compressive strength of fly ash cement mortar was studied. The fly ash cement mortars were made by
replacing 25 wt% of cement with 16 types of fly ashes, and the specimen’s compressive strengths were
evaluated. The compressive strength increased with the mortar’s age due to differences in pozzolanic
reactivity. The effects of various chemical parameters of the fly ash were analyzed. The pozzolanic reaction
of fly ash was significantly affected by SiO 2 ,Al 2 O 3 , and Fe 2 O 3 components, which form the framework
of the glass phase, and CaO, MgO, Na 2 O, and K 2 O components, which depolymerize the glass structure.
Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction
Fly ash is a by-product of coal-fired thermal power plants, and it
is widely used in the concrete industry as a supplementary cementitious
material (SCM), which is a substitute for cement. Fly ash has
attracted a great deal of attention because of its potential to reduce
CO 2 emissions and improve the durability of concrete. Its ability to
generate less heat during hydration is employed to suppress reactions
in mass concrete and high fluidity concrete, and it is used as a
countermeasure in alkali aggregate reactions [1–5].
The physical and chemical characteristics of fly ash vary greatly
according to the type of coal used at the power plant and the
equipment. These differences have a significant impact on the concrete’s
strength due to the pozzolanic reactivity of fly ash, even
when the fly ash is used as a SCM with a consistent mixture ratio
[4,6]. The differences in compressive strength development are the
⇑ Corresponding author.
E-mail address: zerofe@gachon.ac.kr (Y.C. Choi).
biggest problem of using fly ash as a SCM. In the pozzolanic reaction
of fly ash, the reactivity of the fly ash particle surface is
increased owing to the Na and K ions that are dissolved in the fly
ash, and Ca-Si and Ca-Al hydrates are created when the dissolved
Si and Al ions react with Ca(OH) 2 , which is a product of the cement
hydration process. In addition, the amorphous phase of fly ash is
dissolved by OH during the pozzolanic reaction; hence, a sufficient
concentration of OH is needed during the reaction [7,8]. In
addition, the pore solution used in the cement hydration process
must be kept at 20 °C for at least one week to increase the OH
concentration of the solution to achieve a pH of 13.3 or more.
Hence, the pozzolanic reaction occurs only after 1 week [2].
A number of studies have been conducted to determine the
pozzolanic reactivity of fly ash. The pozzolanic reactivity can be
evaluated by precisely measuring the amount of dissolved silica
from fly ash by using a chemical analyzer. However, since the
silicate forms a gel at a pH higher than 10, it is necessary to
consider the amount of silica consumed in the gel formation [9].
Another metric for evaluating the pozzolanic reactivity of fly ash
https://doi.org/10.1016/j.conbuildmat.2019.01.208
0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

256 Y.K. Cho et al. / Construction and Building Materials 204 (2019) 255–264
is the heat of hydration [10,11], which is expressed as the sum of
simultaneous endothermic and exothermic reactions. In addition,
the method for measuring the consumption of calcium hydroxide,
which is the main reactant of pozzolanic reactions, by XRD and
thermal analysis, is relatively accurate in evaluating the reactivity
of fly ash [12,13]. However, this method requires a minimum of
28 days for sufficient occurrence of the fly ash pozzolanic reaction.
The reaction of fly ash with cement is greatly influenced by the
chemical composition of fly ash [9]. Studies have been carried out
to evaluate the compressive strength development of fly ash
cement with respect to the fly ash chemical composition [6,14].
Otsuka et al. reported that the ratio of the charge balance component
and SiO 2 component in the glassy phase had a large effect on
the pozzolanic reactivity [6]. Ogawa et al. proposed a correlation
between the compressive strength of fly ash cement mortar and
the ratio of SiO 2 /C 3 S [14].
The potential pozzolanic index was proposed to investigate the
pozzolanic reaction of fly ash by Paya et al. [15]. Hubbard et al. [16]
studied the lime reactivity in fly ash caused by a combination of
the solubility of silica in acid and the Blaine specific surface area.
Their work exploit the fact that the silica component of fly ash is
composed of crystalline quartz and amorphous glass and amorphous
glass’ property to dissolve in acid. Ramezanianpour and
Cabrera performed experiments that mixed Ca(OH) 2 and fly ash
in the same ratio and showed that the amount of reacted Ca
(OH) 2 is related to the pozzolanic reaction [17]. Several researchers
proposed the use of the assessed pozzolanic-activity index (API),
which considers the amount of Ca 2+ consumed in a suspension of
cement and fly ash [18,19]. In this method, it is assumed that the
Na + and K + ions in Portland cement are mainly dissolved in the
amorphous phase, and the dissolved Si and Al ions react with the
Ca 2+ from the portlandite (Ca(OH) 2 ). Moon et al. showed a correlation
between the amorphous content of fly ash and its pozzolanic
strength [20]. Methods for determining the amount of reacted fly
ash include selective dissolution methods, methods that measure
the amount of portlandite consumed, and methods that determine
the reactivity of fly ash in a highly diluted solution [21].
The compressive strength of fly ash cement mortar is significantly
affected by the chemical components of the fly ash itself
[22]. The reactivity of the fly ash varies greatly according to the
properties of the amorphous phase in the fly ash, the alkali content,
and the temperature during reaction [2,4,20–23]. This study examined
the relationship between the chemical characteristics of fly
ash and the compressive strength of fly ash cement mortar. Of
these chemical characteristics, both the amount and composition
of the amorphous phase content were specifically analyzed to
determine their effect on the compressive strength of the fly ash
cement mortar.
2. Experiment
2.1. Materials
Ordinary Portland cement (OPC) and 16 different types of fly
ash found in South Korean ready-mixed concrete production plants
(FA1-FA16) were used as raw materials to examine the effects of
the chemical composition of the fly ash on the mechanical properties
of the resulting fly ash cement mortar. The density of the OPC
was 3.15 g/cm 3 , and the fineness was 3460 cm 2 /g. Table 1 shows
the chemical compositions of the fly ashes and the OPC used. All
of the fly ash used was grade F according to ASTM C 618, and it
consisted of 80.6% to 89.7% of SiO 2 +Al 2 O 3 +Fe 2 O 3 and 2.5% to
6.2% of CaO. Standard sand was used for the fine aggregate in
accordance with ISO 679. Table 2 shows the density, Blaine surface
area, and activity index of the fly ash types [24] after 28 and
91 days. The densities of the 16 kinds of fly ash ranged from
2172 kg/m 3 to 2353 kg/m 3 . The Blaine surface areas ranged from
332 m 2 /kg to 455 m 2 /kg. The activity index for the OPC’s compressive
strength was between 73.2% and 91.7% after 28 days, and
between 84.8% and 107.8% after 91 days. The 28-day activity
indexes of FA3 and FA13 were less than 80%, and the 91-day activity
indexes of FA3, FA11, and FA13 were less than 90%. Fig. 1 shows
the particle size distribution (PSD) of 16 different types of fly ash
measured by laser diffraction.
2.2. Mixture proportions and test methods
The mortar was mixed in a 1:3 mass ratio of binder to fine
aggregate, and the water–binder ratio was 0.5. In the fly ash
cement mortar, 25 wt% of the OPC was replaced with fly ash. In
accordance with ISO 679, the mortar was cast in a steel mold after
mixing. Once casting was completed, the mold was cured in a
chamber at 20 °C, at 95% relative humidity. After 24 h of curing,
the mortar was removed from the mold and placed in a water bath
at a constant temperature of 20 °C. Each of the mortar specimens
was 400 400 1600 mm. Compressive strength tests were
performed on the mortar specimens after 28 days and 91 days
in accordance with ISO 679. The loading rate was 2,400
N/s ± 200 N/s.\
An X-ray diffraction (XRD) analysis was performed using a
Rigaku MiniFlex600 diffractometer (40 kV and 20 mA). The scans
ranged from a 2h angle equal to 5° up to 65° with a step size of
Table 1
Chemical composition of OPC and fly ashes (mass%) by ICP.
SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO K 2 O Na 2 O SO 3 LOI Sum
OPC 21.0 5.09 2.99 61.6 2.07 1.03 0.21 2.41 2.66 99.06
FA1 55.4 22.2 6.84 5.12 1.84 1.55 1.26 0.71 3.70 98.62
FA2 59.1 20.0 6.22 3.65 1.71 1.62 0.99 0.36 4.43 98.08
FA3 62.6 20.0 7.13 2.83 1.20 1.20 0.65 0.32 2.62 98.55
FA4 54.0 22.0 6.43 4.76 1.48 1.21 1.34 0.50 6.70 98.42
FA5 62.4 17.7 6.89 4.15 1.55 0.97 1.24 0.34 2.53 97.77
FA6 62.3 19.0 6.30 3.42 1.49 1.62 0.75 0.37 3.55 98.80
FA7 57.7 21.1 6.39 4.26 1.80 1.67 1.06 0.52 3.91 98.41
FA8 53.0 20.7 6.94 6.17 2.31 1.21 2.30 0.51 4.93 98.07
FA9 56.6 20.9 8.09 4.66 1.82 1.20 1.27 0.72 2.61 97.87
FA10 58.3 20.8 6.83 3.44 1.39 1.15 0.94 0.35 5.16 98.36
FA11 60.0 19.8 6.41 3.14 1.32 1.18 0.90 0.49 4.76 98.00
FA12 61.9 18.7 6.15 3.28 1.33 1.19 0.81 0.48 4.43 98.27
FA13 62.3 20.2 6.66 2.54 1.15 1.18 0.64 0.42 3.28 98.37
FA14 52.2 22.4 7.57 5.22 1.93 1.12 1.46 0.82 5.16 97.88
FA15 57.5 20.5 7.16 5.07 1.72 1.43 0.78 0.71 2.75 97.62
FA16 52.4 23.0 8.85 5.51 2.06 0.79 1.26 0.47 2.94 97.28

Y.K. Cho et al. / Construction and Building Materials 204 (2019) 255–264 257
Table 2
Physical properties of fly ashes.
Density (kg/m 3 ) Blaine (m 2 /kg) Activity Index (%)
28 days 91 days
FA1 2209 415 87.6 100.5
FA2 2172 390 86.4 90.6
FA3 2230 382 76.3 85.3
FA4 2201 380 81.1 91.5
FA5 2278 378 83.3 93.1
FA6 2243 332 83.4 93.4
FA7 2202 417 87.5 91.9
FA8 2250 455 91.7 107.8
FA9 2353 384 87.8 96.4
FA10 2144 353 82.2 90.3
FA11 2197 396 86.7 86.3
FA12 2187 441 82.1 90.1
FA13 2234 415 73.2 84.8
FA14 2300 310 81.6 99.6
FA15 2280 372 82.3 91.5
FA16 2291 384 87.3 99.4
Average 2236 388 83.8 93.3
Fig. 1. Particle size distributions of fly ashes.
0.02 2h. 10% Al 2 O 3 was used as the internal reference material for
the XRD quantitative analysis [25,26]. The crystalline mineralogical
compositions of the fly ashes were analyzed using the Rietveld
method [27,28], which is the most widely used technique for
determining the mineralogical composition of building materials
[25]. The HighScore Plus (PANalytical) software program was used
to determine the crystalline content in the fly ash. The amount of
the amorphous phase of the fly ash was determined using Eq.
(1), which is shown below
Amorphous content ð%
Þ ¼ 100
3. Results and discussion
3.1. Amorphous characteristic of fly ashes
The chemical characteristics of fly ash vary greatly depending
on the equipment used in the thermal power plant, operating
conditions, and the type of coal used [6,20,29]. Fig. 2 shows the
XRD patterns of the fly ash specimens used in this study. Quartz
X
Crystalline LOI ð1Þ
and mullite were observed to be the main crystalline phases in
the fly ashes. A hematite diffraction peak was observed for FA9.
The hump exhibited by amorphous materials is seen between

258 Y.K. Cho et al. / Construction and Building Materials 204 (2019) 255–264
greatly affected by the amorphous phase content [20–22]. Table 4
shows the composition of the crystalline and amorphous phases of
the 16 kinds of fly ash. Among the crystalline phases, the SiO 2 content
ranged from 11.6% to 20.8% and the Al 2 O 3 content ranged from
4.1% to 9.8%. Among the amorphous phases, the SiO 2 content was
the lowest in FA14 at 36.9% and the highest in FA13 at 46.3%.
The Al 2 O 3 content was the lowest in FA11 at 12.3% and highest
in FA1 at 16.4%, as shown in Fig. 3.
3.2. Compressive strength of fly ash cement mortars
The compressive strengths of the fly ash cement mortars are
shown in Fig. 4 and Table 5. The compressive strength of the mortar
specimens increased with the age of the fly ash mortar. However,
the compressive strength varied according to the type of fly
ash at all ages. The 28-day strength of the fly ash cement mortar
was lowest in FA13 at 41.1 MPa and highest in FA8 at 51.5 MPa.
Similarly, the 91-day strength was lowest in FA13 and highest in
FA8.
As the pozzolanic reaction of the fly ash progresses, additional
reaction products such as C-S-H and C-A-H are created in the
cement matrix, making its structure more dense; hence, the compressive
strength of the mortar increases [2,5,20]. The 16 types of
fly ash were mixed in the same mixing ratio, and the results
showed that the variation in the compressive strength of the
FA1-FA16 mortars increased as the mortars aged. After 28 days,
the difference between the lowest and highest strength was
10.4 MPa, but after 91 days, this difference increased to 14.8 MPa.
These results are caused by the differences in the pozzolanic reactivity
of the fly ash.
3.3. Analysis of pozzolanic reaction
Fig. 2. X-ray diffraction patterns of fly ashes.
15°–35°. Table 3 shows the fly ash mineral composition determined
using the Rietveld quantitative analysis method. As shown,
mostly quartz and mullite were observed, with trace amounts of
hematite. The crystalline and amorphous phase properties of the
different fly ashes were diverse, exhibiting quartz compositions
of 8.7%–18.9% (an average of 14.4%) and mullite compositions of
5.3%–12.7% (an average of 8.6%). The specimens were comprised
of 18.8%–27.4% (an average of 23.1%) of crystalline phases and
68.1% to 77.6% (an average of 73.0%) of amorphous phases. The
quartz content was found to be lowest in FA16 and highest in
FA6. The mullite content was lowest in FA5 and highest in FA16.
The amount of crystalline phases was the lowest in FA4 and highest
in FA12. The amount of amorphous phases was the lowest at
68.1% in FA12, which had the most crystalline phases. FA15 had
the highest amount of amorphous phases at 77.6% due to the difference
in the loss on ignition (LOI).
Generally, the amorphous phase of fly ash is considered the
reactive phase. Therefore, the pozzolanic reactivity of fly ash is
In the pozzolanic reaction, the Ca(OH) 2 generated by the hydration
of the cement reacts with pozzolanic material, i.e., fly ash. This
reaction creates additional cement hydrates such as calcium silicate
hydrate (C-S-H). These products make the hardened body
more dense and improve the strength and durability of the body.
The internal part of the concrete is strongly alkaline; therefore,
components of fly ash such as SiO 2 and Al 2 O 3 are dissolved in it.
These dissolved components react with the Ca in the cement and
trigger pozzolanic reactions [2,4]. The pozzolanic reactivity is significantly
affected by the chemical characteristics of fly ash [22].
This reactivity is influenced by the irregular glass structure of fly
ash. Therefore, the glass content and the overall composition of
fly ash play an important part in determining the hydraulic properties
of fly ash [29]. Sakai et al. [4] reported that the pozzolanic
activity increases with the amorphous phase content of fly ash.
Hence, it is important to analyze the amount and composition of
the glass phase of fly ash in the pozzolanic reaction. As mentioned
above, the chemical composition determines the structure of glass
materials and plays an important role in determining the pozzolanic
reactivity of fly ash.
The basicity of ground granulated blast furnace slag (GGBFS)
(Table 6, No. 5) is used as a hydraulic index. In many countries,
basicity is used as a standard of quality of GGBFS. For example,
in Europe, Japan, and South Korea, the quality standards state that
the basicity must be greater than 1.0, 1.4, and 1.6, respectively [30].
The hydraulic modulus (Table. 6, No. 6) is the ratio of the basic
component, CaO, to the total amount of the acidic components,
SiO 2 ,Al 2 O 3 , and Fe 2 O 3 . It is used as an index for evaluating the
hydration of cement. The hydraulic modulus is normally limited
to between 1.7 and 2.3. As the value of the hydraulic modulus
increases, the early age strength increases, and a large amount of
hydration heat is generated [31]. Otsuka et al. [6] reported that
the (CaO + MgO + R 2 O)/SiO 2 (Table. 6, No. 7) value has a greater

Y.K. Cho et al. / Construction and Building Materials 204 (2019) 255–264 259
Table 3
Mineralogical composition of fly ashes.
Crystalline (%) Amorphous (%)
Quartz Mullite Hematite Sum
FA1 12.1 7.6 – 19.7 76.7
FA2 16.8 8.1 – 24.9 70.7
FA3 16.7 9.0 – 25.7 71.7
FA4 10.4 8.4 – 18.8 74.4
FA5 17.4 5.3 – 22.7 74.7
FA6 18.9 8.3 – 27.2 69.2
FA7 15.8 9.2 – 25.0 71.1
FA8 14.3 6.1 – 20.4 74.6
FA9 12.7 7.2 0.56 20.4 77.0
FA10 15.8 9.2 – 25.0 69.8
FA11 14.2 9.8 – 24.0 71.2
FA12 18.1 9.3 – 27.4 68.1
FA13 13.9 9.2 – 23.1 73.6
FA14 12.9 10.8 – 23.7 71.2
FA15 12.4 7.2 – 19.6 77.6
FA16 8.7 12.7 – 21.4 75.7
Average 14.4 8.6 0.56 23.1 73.0
Table 4
Chemical compositions of crystalline and amorphous phases of fly ashes.
Crystalline (%) Amorphous (%)
SiO 2 Al 2 O 3 Fe 2 O 3 SiO 2 Al 2 O 3 Fe 2 O 3 Others
FA1 13.82 5.83 – 41.58 16.37 6.84 11.86
FA2 18.62 6.27 – 40.48 13.73 6.22 10.25
FA3 18.69 6.94 – 43.91 13.06 7.13 7.65
FA4 12.37 6.52 – 41.63 15.48 6.43 10.87
FA5 18.66 4.13 – 43.74 13.58 6.89 10.48
FA6 20.79 6.44 – 41.51 12.56 6.30 8.85
FA7 17.90 7.13 – 39.80 13.97 6.39 10.90
FA8 15.72 4.72 – 37.28 15.98 6.94 14.43
FA9 14.31 5.58 0.56 42.29 15.32 7.53 11.80
FA10 17.90 7.13 – 40.40 13.67 6.83 8.91
FA11 16.45 7.55 – 43.55 12.25 6.41 9.03
FA12 20.24 7.21 – 41.66 11.49 6.15 8.82
FA13 15.99 7.12 – 46.31 13.08 6.66 7.56
FA14 15.34 8.32 – 36.86 14.08 7.57 12.67
FA15 14.09 5.58 – 43.41 14.92 7.16 12.09
FA16 11.56 9.79 – 40.84 13.21 8.85 12.81
Fig. 3. Amorphous contents of fly ashes.

260 Y.K. Cho et al. / Construction and Building Materials 204 (2019) 255–264
Fig. 4. Compressive strength of fly ash mortars.
Table 5
Compressive strength test results.
28 days 91 days
Mean Max. Min. S.D. Mean Max. Min. S.D.
FA1 49.2 54.2 44.3 3.95 64.8 70.5 61.3 2.92
FA2 48.5 51.3 44.3 2.19 58.4 64.1 54.5 3.99
FA3 42.8 45.3 41.1 1.39 55.0 59.8 51.1 3.74
FA4 45.5 48.0 43.2 1.72 59.0 61.6 55.3 2.48
FA5 46.7 50.9 44.2 2.31 60.1 62.6 59.0 1.33
FA6 46.8 50.5 43.6 2.47 60.3 63.0 54.8 3.03
FA7 49.1 50.1 47.6 1.10 59.3 62.4 56.4 1.82
FA8 51.5 54.9 49.3 2.17 69.5 73.4 66.6 2.96
FA9 49.2 52.9 45.4 3.21 62.2 63.6 59.8 1.31
FA10 46.1 49.4 42.0 2.63 58.2 62.0 55.7 2.22
FA11 48.6 51.5 45.5 2.08 55.7 59.8 53.2 2.10
FA12 46.1 49.3 41.9 2.77 58.1 61.2 53.0 2.93
FA13 41.1 42.0 39.8 0.87 54.7 59.2 49.9 2.71
FA14 45.8 49.7 43.9 2.31 64.2 67.0 59.3 3.04
FA15 46.2 48.9 42.1 2.84 59.0 64.3 54.7 3.11
FA16 49.0 51.8 45.5 2.00 64.1 69.6 57.3 4.43
Average 47.0 50.0 44.0 60.2 64.0 56.4
S.D. 2.52 3.07 2.33 3.85 3.97 4.01
effect on the pozzolanic reactivity than the basicity of the fly ashes,
which have almost the same glass content and specific surface
area. (CaO + MgO + R 2 O)/SiO 2 is the ratio of the network modifier
components (Na, K, Ca, Mg) to SiO 2 . Network modifiers can result
in considerable changes in glass structures. These components
are used to create non-bridging oxygen and for charge balancing,
and the SiO 2 forms the framework of the glass phase. The Si atoms
react with four O atoms to form a continuous three-dimensional
network in which SiO 4 tetrahedrons are the main building blocks
[32]. If alkali/alkali earth metals (R 2 O and RO) are introduced in
amorphous silica, they will be the network modifiers.
The R + ions bond with O and act as a bridge between two Si
atoms, creating non-bridging O. This reduces the polymerization
of the silicate framework, and as a result, its reactivity increases
[33]. In certain circumstances, Al can react to form a tetrahedron
like Si, and hence, it can be used as a network former. However,
because it has a charge of 3 + , Al can only engage in four-fold coordination
and act as a charge balancer when R + ions are present
[34]. The reactivity of fly ash increases with an increase in the
amount of R 2 O (Na 2 O and K 2 O), which changes the glass network
structure [20,33,34].
As shown in Table 6, various chemical parameters were used to
compare the amorphous characteristics of fly ash and the compressive
strengths. The coefficient of correlation between the shortterm
compressive strength (28 days) of fly ash cement mortar
and the chemical characteristics of fly ash was low, i.e., 0.159–
0.497, but that for the long-term compressive strength (91 days)
was high, i.e., 0.383–0.872. These results prove that the chemical
characteristics of fly ash have a greater effect on the long-term
compressive strength than the short-term compressive strength.
Fig. 5 shows the results of a comparison of the chemical characteristics
of the fly ash amorphous phase components and their
effect on the compressive strength. The SiO 2 and Al 2 O 3 contents
of fly ash did not show a high correlation with the compressive

Y.K. Cho et al. / Construction and Building Materials 204 (2019) 255–264 261
Table 6
Relationship between compressive strength and amorphous content.
Compressive Strength = A CP + B
CP (amorphous content) A B Coefficient of correlation, R 2
No. 1 SiO 2 28 days 0.62 72.88 0.333
91 days 1.19 109.76 0.527
No. 2 Al 2 O 3 28 days 0.75 36.51 0.159
91 days 1.79 35.31 0.383
No. 3 CaO/SiO 2 28 days 56.00 41.28 0.418
91 days 121.88 47.69 0.852
No. 4 Na 2 O/SiO 2 28 days 141.33 43.19 0.400
91 days 298.48 52.09 0.766
No. 5 Basicity : (CaO + MgO + Al 2 O 3 )/SiO 2 [19] 28 days 20.44 37.23 0.401
91 days 44.18 39.02 0.805
No. 6 CaO/(SiO 2 +Al 2 O 3 +Fe 2 O 3 ) [19] 28 days 98.47 40.37 0.434
91 days 208.57 46.08 0.837
No. 7 (CaO + MgO + R 2 O)/SiO 2 [6] 28 days 35.91 39.84 0.497
91 days 72.58 45.67 0.872
No. 8 Basicity (SiO 2 +Al 2 O 3 ) [14] 28 days 0.36 37.51 0.342
91 days 0.78 39.42 0.700
No. 9 NMC/T 28 days 65.86 38.33 0.533
91 days 127.91 43.29 0.864
strength, regardless of age. This shows that the pozzolanic reactivity
of fly ash cannot be evaluated based on the amorphous SiO 2 and
Al 2 O 3 content alone. However, amorphous CaO/SiO 2 ,Na 2 O/SiO 2 ,
(CaO + MgO + Al 2 O 3 )/SiO 2 (basicity), CaO/(SiO 2 +Al 2 O 3 +Fe 2 O 3 )
(hydraulic modulus), (CaO + MgO + R 2 O)/SiO 2 , and basicity
(SiO 2 +Al 2 O 3 ) showed a high correlation with the compressive
strength after 91 days; the coefficient of correlation was 0.7.
(CaO + MgO + R 2 O)/SiO 2 showed the highest correlation with the
compressive strength after 91 days (R 2 = 0.872). Fig. 6 shows a
comparison of compressive strength and the chemical parameters
determined using the chemical components of fly ash. After
91 days, CaO/(SiO 2 +Al 2 O 3 +Fe 2 O 3 ) and (CaO + MgO + R 2 O)/SiO 2
had slightly lower coefficients of correlation compared to those
for the amorphous components. The coefficients of correlation
were 0.806 and 0.846, respectively, which indicated a very high
correlation with the compressive strength. However, when the
basicity and basicity (SiO 2 +Al 2 O 3 ) were calculated using chemical
components, their correlation with the compressive strength
after 91 days was lower than that for the amorphous components.
These results show that the ratio of the amount of Si, Al, and Fe
to the amount of Ca, Mg, Na, and K is an important determinant of
the pozzolanic reactivity of fly ash. Si, Al, and Fe form the framework
of the glass network, while Ca, Mg, Na, and K are network
modifiers that balance charges that are insufficient for elements
such as Al to form tetrahedral structures. The equation below
was used to analyze fly ash pozzolanic reactivity.
NMC
T
¼ ðNa 2O þ K 2 O þ CaO þ MgOÞ
ðSiO 2 þ Al 2 O 3 þ Fe 2 O 3 Þ
The results also show that the highest correlation with the compressive
strength after 28 days was R 2 = 0.533 for amorphous
NMC/T and R 2 = 0.501 for NMC/T as shown in Fig. 7. There was a
high correlation with the compressive strength after 91 days, but
almost no difference in the values calculated using chemical components
(R 2 = 0.855) and NMC/T (R 2 = 0.864).
4. Conclusions
The following conclusions were obtained based on a comparison
of the strength and chemical characteristics of 16 types of
fly ash specimens with various chemical parameters.
ð2Þ
(1) Variations in the compressive strength of the fly ash
cement mortar are greater in the long term (91 days) than
the short term (28 days) due to differences in pozzolanic
reactivity.
(2) The chemical parameters have a higher correlation with the
91-day strength than with the 28-day strength of fly ash
cement mortars, as the pozzolanic reaction progressed further
after 91 days. Therefore, the chemical characteristics
of fly ash have a large effect on pozzolanic reactivity.
(3) Chemical parameters determined using the chemical composition
of the fly ash such as the hydraulic modulus, M,
and NMC/T have a high correlation (R 2 = 0.8 or more) with
the compressive strength after 91 days.
(4) Chemical parameters determined using the amorphous
chemical composition of the fly ash have higher correlations
with the compressive strength of the fly ash cement mortars
than the chemical parameters determined with crystalline
components. For basicity and basicity (SiO 2 +Al 2 O 3 ) in
particular, there is a significant difference when the
chemical parameters are determined using only
amorphous components and when crystalline components
are included.
(5) CaO/SiO 2 , (CaO + MgO + R 2 O)/SiO 2 , and NMC/T have the
highest correlation with the compressive strength of the
fly ash mortar. The pozzolanic reactivity of fly ash is significantly
affected by the ratio of components that form the
framework of the glass network (SiO 2 , Al 2 O 3 , and Fe 2 O 3 )
and network modifier components (CaO, MgO, Na 2 O, and
K 2 O).
(6) In the future, the use of blended cement is expected to
increase significantly in the concrete field to reduce greenhouse
gases. Although fly ash is very useful as a cement
admixture, there is a drawback that the performance of
the blended cement varies greatly depending on the quality
of the produced fly ash. This study investigated the effect of
chemical composition of the amorphous and crystalline
phases of fly ash on the mechanical properties of fly ash
cement mortar. The results of this study can be used in
the future as a basis to predict the quality of fly ash, which
will increase the utilization of fly ash as a cement
admixture.

262 Y.K. Cho et al. / Construction and Building Materials 204 (2019) 255–264
Fig. 5. Effect of amorphous chemical parameters on compressive strength of fly ash mortars.

Y.K. Cho et al. / Construction and Building Materials 204 (2019) 255–264 263
Fig. 6. Effect of chemical parameters on compressive strength of fly ash mortars.
Fig. 7. Effect of NMC/T on compressive strength of fly ash mortars.
Conflict of interest
None.
Acknowledgments
This research was supported by the Korea Institute of Energy
Technology Evaluation and Planning (KETEP) and the Ministry of
Trade, Industry & Energy (MOTIE) of the Republic of Korea (No.
20155020301020). This research was also supported by Korea
Environment Industry & Technology Institute (KEITI) through
Public Technology Program based on Environmental Policy Project,
funded by Korea Ministry of Environment (MOE) (No.
2016000700001).
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