Journal of Water Resources & Environmental Engineering - No. 87 (12/2023)
9
Mechanical-microstructural characteristics
of concrete containing high volumes of coal bottom ash
Nguyen Van Dung
1*
, Mai Thi Hong
1
, Nguyen Thi Thanh
1
, Nguyen Vu Linh
1
Abstract: This paper aims
to study the effect of coal bottom ash (CBA) as a fine aggregate substitution
at high levels on the mechanical properties and microstructure characteristics of concrete. CBA that
sourced from a local coal-fired power plant in Vietnam was used to replace t
he natural sand (NS) in the
concrete mixtures at different levels of 0, 30, 50, 70, and 100%. The concrete samples were prepared in
the laboratory and checked for properties through the tests of compressive strength (CS), ultrasonic
pulse velocity (UPV), w
ater absorption (WA), porosity, electrical surface resistivity (ESR), and thermal
conductivity (TC). Notably, the scanning electron microscope was also used to characterize the
microstructure of the hardened concrete. Test results show that the replacement
of NS by CBA affected
the concrete’s properties significantly. However, all of the concrete samples were classified as good
quality with the UPV, CS, WA, porosity, ESR, and TC values fell within the ranges of 4176 –
4636 m/s,
11.3 – 25.9 MPa, 2.89 – 7.02%, 5.11 – 10.19%, 5.21 – 10.55 kΩ.cm, and 1.17 –
1.72 W/mK,
respectively. Therefore, the proper quantity of CBA will be suggested depending on the requirement for
the quality of the concrete for a specific application.
Keywords: Concrete, coal bottom ash, mechanical property, durability, microstructure.
1. Introduction
*
Recently, the rapid development of the
construction industry requires a large quantity of
concrete, which is a well-known and major
construction material so far. In which, natural
aggregate such as river sand or crushed sand is one
of the main components that occupy a large
proportion of the concrete mixture. Hence, the
consumption of a large quantity of concrete requires
a considerable amount of NS every year, leading to
the depletion of this type of natural aggregate as
announcing by the local Government in Vietnam. In
addition, the over-exploitation of NS has also been
caused by many environmental issues, e.g., water
pollution, erosion, and landside, etc.
Besides, the rapid development of other
industrial activities generates a large number of
1
Hong Duc University
* Corresponding author; Email: nguyenvandung@hdu.edu.vn
Received 20
th
Jun. 2023
Accepted 28
th
Aug. 2023
Available online 31
st
Dec. 2023
different types of solid waste. In which, CBA is
one of the solid wastes that generated with a
considerable quantity by the coal-fired power
plants. In Vietnam, the Government statistic
points out that about 15 million tons of CBA are
generated in 2020. The CBA quantity will be
jumped to about 17 and 21 million tons in 2025
and 2030, respectively. It is important to remark
that the generation of such a large quantity of
CBA will cause serious pollution to the
environment if there are no sufficient treatment
methods. Thus, one of the possible ways to treat
the CBA is by turning it into construction
materials. By the way, more CBA is consumed
sufficiently.
Therefore, this paper is conducted with the
purpose of using CBA sourced from a local coal-
fired power plant in Vietnam as fine aggregate to
partially and fully replace NS in the production of
concrete. The potential of utilization and
application of CBA in concrete is evaluated
through the study of the effect of CBA on both the
Journal of Water Resources & Environmental Engineering - No. 87 (12/2023)
10
mechanical properties and microstructure
characteristics of concrete. The following steps are
covered in this experimental works:
(1) Check for characteristics of raw
materials.
(2) Design the proportions of concrete
ingredients.
(3) Cast the concrete samples for testing.
(4) Perform the test methods to evaluate
mechanical properties and microstructure of
concrete samples.
(5) Analyze the results and withdraw the
conclusions.
2. Experimental details
2.1 Materials
In this study, cement PCB 40 sourced from
Nghi Son company was used as a binder
material for making concrete. Natural stone
(D
max
of 12.5 mm, density of 2690 kg/m
3
, and
WA of 0.1%) and blended NS (density of 2620
kg/m
3
, WA of 1.1%) and CBA (density of 1987
kg/m
3
, WA of 23.2%, loss on ignition of
15.1%) at different proportions were used as
coarse and fine aggregates in concrete
mixtures, respectively. This study used CBA, a
by-product of a coal-fired power plant in
Northern Vietnam, to partially or fully replace
NS in concrete. As determined by a sieve
analysis (Table 1), the fineness modulus of NS
and CBA was 2.83 and 1.97, respectively.
Type-G superplasticizer (SP) sourced from
China with a specific gravity of 1.15 was used
to get the desired workability of fresh concrete
mixtures. Tap water was used for mixing.
Table 1. Sieve analysis and fineness modulus
(FM) of NS and CBA
NS CBA
Sieve size,
mm
Percentage of
passing, (%)
Sieve size,
mm
Percentage of
passing, (%)
5.0 95.4 5.0 80.6
2.5 88.6 2.5 75.8
1.25 75.6 1.25 71.6
0.63 43.4 0.63 65.8
0.315 8.0 0.315 59.7
0.14 1.3 0.14 50.0
FM = 2.83 FM = 1.97
2.2. Mixture proportions
The mixture proportions for the preparation
of concrete specimens are presented in Table 2.
In which, the NS was replaced by CBA at
different levels of 0% (M42-00), 30% (M42-
30), 50% (M42-50), 70% (M42-70), and 100%
(M42-100) and the ratio of water-to-cement was
kept at 0.42 for all concrete mixtures. Similarly,
the same aggregate-to-binder ratio and SP
dosage of 4.1 and 1%, respectively, were
applied for all concrete mixtures.
Table 2. Mixture proportions of concrete specimens
Sample codes Ingredients
(kg/m
3
) M42-00 M42-30 M42-50 M42-70 M42-00
Cement 428.6 428.6 428.6 428.6 428.6
NS 991.3 693.9 495.7 297.4 0
CBA 0 297.4 495.7 693.9 991.3
Natural stone 751.1 751.1 751.1 751.1 751.1
Water 175.7 175.7 175.7 175.7 175.7
SP 4.3 4.3 4.3 4.3 4.3
2.3. Samples preparation and test methods
Cylindrical concrete samples of Ø10×20 cm
were prepared for the test programs. The mixing
procedures could be described as follows:
Journal of Water Resources & Environmental Engineering - No. 87 (12/2023)
11
Cement was firstly mixed with a part of water and
a small amount of SP for 2 minutes to get a
viscous paste. Fine aggregate, including NS and
CBA, was then added to the mixture and mixing
continued for an additional 2 minutes. Finally,
natural stone was added to the mixing bowl,
followed by the rest of the water and SP. Mixing
was allowed to continue for another 2 minutes to
obtain a uniform mixture. After mixing, the
concrete samples were prepared and cured in
saturated lime-water until the testing time.
The concrete samples were subjected to the
tests of CS (ASTM C39), UPV (ASTM C597),
ESR (using a four-point Wenner array
equipment), TC (using a portable Isomet 2104
heat transfer analysis equipment), and WA and
porosity (ASTM C642). In addition, the
microstructure of concretes was also observed
through the scanning electron microscope of
ZEISS at 5 kV and 25 pA chamber pressure.
The CS, UPV, and TC tests were conducted
at 3, 7, 14, and 28 days, whereas the tests of TC,
porosity, and microstructure observation were
performed at 28 days. The presented result of
each test was the average value of three
measurements.
3. Results and discussion
3.1. Compressive strength
The growth in the CS of concrete samples is
displayed in Fig. 1. As can be seen, the CS of all
concrete samples developed over the tested
time. After 28 days, the CS values of concrete
containing 0, 30, 50, 70, and 100% CBA were
25.9, 25.3, 22.3, 16.0, and 11.3 MPa,
respectively.
Fig. 1 shows that that the replacement of NS
by CBA caused the decline in the CS of the
concrete samples, and the CS loss was
significant at high replacement levels (over
50%). After 28 days of curing, the CS values of
concrete samples incorporating 30, 50, 70, and
100% CBA were about 2.4, 13.7, 38.1, and
56.2% lower than that of the blain concrete
without CBA. It is believed that the inclusion of
CBA with low density and highly porous
particles resulted in a reduction in the CS of
concrete. A similar finding was previously
reported by Bai et al. (Bai et al., 2005), Kurama
and Kaya (Kurama and Kaya, (2008) and Kou
and Poon (Kou and Poon, 2009).
Fig 1. CS development of concretes
On the other hand, Fig. 1 exhibits that the CS
loss was less significant at the replacement
levels of NS by CBA by ≤ 50%, and the CS
values were dropped sharply at the replacement
levels of above 50%. From 14-day to 28-day
old, the highest strength increment rate of
approximately 15%, which was about 4% higher
than that of blain concrete, was observed at the
concrete samples containing 30% CBA and the
increasing trend in the strength of this concrete
mixture was expected furtherly. The lowest 28-
day strength value of 11.3 MPa was measured at
the concrete samples containing 100% BA. This
relatively low strength is due to the inclusion of
an extremely high quantity of BA in the
concrete mixture.
3.2. Ultrasonic pulse velocity
UPV is an in-situ test, which can be used to
indirectly assess the strength and quality of
concrete by measuring the velocity of an
ultrasonic pulse passing through the concrete
specimens. The UPV test results of all
Journal of Water Resources & Environmental Engineering - No. 87 (12/2023)
12
concrete samples are presented in Fig. 2. It
could be observed that the UPV values of
concretes had a similar development trend as
their CS (Fig. 1). At 28 days, the UPV values
of concrete samples with 0, 30, 50, 70, and
100% CBA were 4636, 4518, 4449, 4353, and
4176 m/s, respectively. As a result, the
inclusion of CBA in the concrete mixtures
caused a reduction in UPV.
In addition, the more the CBA content, the
lower the UPV values of concrete were. It is
believed that the inclusion of highly porous
CBA particles as compared to NS introduced
more voids or spaces within the concrete
structure, leading to the reduction in UPV
values. This is supported by Singh and Siddique
(Singh and Siddique, 2015). Although the UPV
of concretes reduced when NS was replaced by
CBA, all concrete samples registered a UPV
value of above 4100 m/s, which is the minimum
classification level of good quality concrete
(Carcano and Moreno, 2008).
Fig 2. UPV of concretes
It is found that UPV and CS of concretes had
a close relationship, which was described by the
equation of y = 26.76x + 3886.58 with R
2
=
0.91, as presented in Fig. 3. This figure also
showed that the higher the CS gained, the higher
the UPV values of the concrete were. A similar
finding was reported by Irrigaray et al.
(Irrigaray et al., 2016).
Fig 3. The relationship between UPV
and CS of concretes
3.3. Water absorption and porosity
The WA and porosity of concrete samples at
28 days are shown in Fig. 4.
Fig 4. TC and porosity of concretes
WA and porosity of all concrete samples
ranged from 2.89% to 7.02% and from 5.11% to
10.19%, respectively. It can be seen that WA
increased proportionally with porosity. At the
replacement levels of 30, 50, 70, and 100% of NS
by CBA, the WA of concretes increased about 29,
45, 94, and 143% as compared to that of concrete
without CBA. Whereas, the porosity of the
concretes increased about 19, 30, 52, and 99% at
the same replacement levels. The replacement of
NS by CBA with porous particles (Fig. 8a) caused
Journal of Water Resources & Environmental Engineering - No. 87 (12/2023)
13
the increase in void volume within the concrete
structure, leading to an increase in porosity and
thus increasing TC of the concrete.
3.4. Electrical surface resistivity
ESR has been used to access the durability of
concrete. The measurement of the ESR of all
concrete samples at 28 days was conducted with
the results, as presented in Fig. 5.
Fig 5. ESR of concretes
It can be seen from the experimental data that
the incorporation of CBA increased the ESR of
the concrete in comparison to that of blain
concrete. Moreover, the ESR values increased
with the CBA content in the concrete mixtures.
The concrete samples containing 30, 50, 70, and
100% CBA had the ESR values of approximately
33, 66, 74, and 102% higher than that of plain
concrete. The increased ESR of concretes may be
attributed to the partial involvement of CBA (only
some fine and active particles) in the pozzolanic
reaction and thus created C-S-H gel as it has a
high percentage of silica and alumina sources
(Muthusamy at al, 2020).
In addition, the ESR value directly reflects the
ability to against the chemical attack of concrete.
So that a higher ESR value indicated a better
resistance to chemical attack. This supported to
results of this study that concrete containing CBA
had better resistance to the chemical attack in
comparison with the plain concrete. This finding
is in line with the results of Singh and Siddique
(Singh and Siddique, 2014).
3.5. Thermal conductivity
Fig. 6 presents the TC values of all concrete
samples prepared for this study. After 28 days of
curing, concretes had the TC values in the range
of 1.17 – 1.72 W/mK. A similar development
pattern to CS development was observed with a
TC as the TC of concrete increased with curing
time, and the inclusion of CBA contributed to
reducing the TC of the concretes as expected
significantly. Kim et al (Kim et al., 2003)
previously pointed out that TC was related to the
porosity of concrete. Thus, the presence of
porous CBA (Fig. 8a) increased porosity of
concretes as mentioned above (Fig. 4), resulting
in the reduction of TC of concretes.
Fig 6. TC of concretes
Fig 7. The relationship between TC and
porosity of concretes