J. Build. Mater. Struct. (2021) 8: 82-92 Original Article
DOI : 10.34118/jbms.v8i1.1208
ISSN 2353-0057, EISSN : 2600-6936
Use of bottom ash as part replacement of sand for making concrete
blocks
Satish Sharma, VV Arora, PN Ojha, Brijesh Singh*, Vikas Patel, Adarsh Kumar NS
National Council for Cement and Building Materials, India.
* Corresponding Author: brijeshsehwagiitr96@gmail.com
Received: 17-02-2021
Accepted: 19-06-2021
Abstract. Coal-based thermal power plants all over the world facing serious problems of
handling and disposal of the ash produced. The productive use of coal Bottom Ash (BA) is the
best way to alleviate the problems associated with its disposal. This paper covers the studies
on laboratory scale evaluation of vibro compaction concrete blocks using BA I, BA II & BA III
collected from three different location of Coal Fed Thermal Power Station. In the present
investigation laboratory investigation have been carried to utilize BA as part replacement of
sand in concrete. This study cover manufacture of concrete blocks without flyash & with BA
using for making solid block as per specification laid down in IS:2185 using vibro
compaction machine. Three different sources of BA were used in concrete mix each @ 30%,
40% & 50% replacement by weight of sand were adopted in making concrete blocks.
Comparative study of compressive strength of concrete at different age of curing, wet
density, drying shrinkage is reported in this study. Wet density is found to be lower in blocks
containing BA & dry shrinkage values are found well within the limits of specifications.
Concrete Blocks having BA @ 30% by weight of sand are found suitable for use in the
manufacture of concrete blocks.
Key words: Bottom Ash, Mix Proportion, Vibro Compaction Technique, Concrete Blocks.
1. Introduction
Bottom Ash (BA) is the coarser material, which drops into the bottom of the furnace in latest
large thermal power plants and constitute about 20% of gross ash content of the coal fed in the
boilers. It consists of noncombustible materials, and is the residual part from the incineration of
household and similar waste. Raw BA is a granular material that consists of a mix of inert
materials such as sand, stone, glass, porcelain, metals and ash from burnt materials. Aggarwal et
al., (2007) have investigated the replacement of BA upto 50% replacement by using the
superplasticizer. The study of replacement of BA is done by using high range water reducing
admixture because as the replacement of fine aggregate increases the water demand rapidly
which also enhances due to high specific surface area BA due to high fine content. Hence by the
all mechanical properties get affected due to less cementitious material available in per cubic
meter content in early age of concrete mix. Detailed studies are reported in references. BA is a
combination of heavier particulate matter and molten slag, which forms on the walls and the
bottom of the combustion chamber of power station boiler fired with pulverized fuel. In
appearance it usually ranges from a highly verified, glossy and heavy material to a lightweight,
open textured and more friable type. Sometimes it is found mixed with fly ash in stockpiles. Its
precise nature depends on the boiler plant and the coal type but higher fines of fly ash in BA are
generally a part of coal ash. This study utilized BA of Unchahar Super Thermal Power Plant
(NTPC) currently there are two boiler systems – wet and dry.
Each system produces a uniquely different BA – wet BA and dry BA. Wet BA has a relatively low
melting point and coalesces into large molten masses known as slag. Its particles usually are
angular to sub angular in shape, have a smooth surface texture and looked much like crushed
glass. Dry BA has quite angular particles and a highly porous surface texture, usually has the
appearance similar to fine sand and lighter in weight per unit volume and colour than bottom
slag (Bai and Barheer, 2001; Syahrul et al., 2010).
mailto:brijeshsehwagiitr96@gmail.com
Sharma Satish et al., J. Build. Mater. Struct. (2021) 8: 82-9é 83
The compressive strength of concrete mixes made with various percentage of washed
Bottom Ash as sand replacement inclusive of control sample (fully natural sand) was determined
at 3, 7, 28, and 60 days of curing (Syahrul et al., 2010). Study by Syahrul et al., (2010) indicated
that the compressive strength of concrete mixes of sand replacement is much lower than control
sample at all tested day. According to Purushothaman and Senthamarai (2013), BA added
Concrete mixes showed enhanced compressive strength than the conventional concrete and
shows uniformly higher compressive strengths at almost all ages. The ample gain in strength is
thought to be due to very high pozzolanic reactivity of the two mineral admixtures silica fume
and BA. Tang et al. (2013) found that the compressive and flexural strength of concrete drain
with the augmentation of the BA fines at the identical curing age, particularly after 3 and 7 days.
Study done by Remya Raju and Aboobacker (2014) indicated that compressive strength reduced
marginally on the inclusion of BA in concrete and there was decrease in slump values of
mixtures with the increase in level of replacement of sand by BA. Compressive strength, split
tensile strength and flexural strength increased up to 20% replacement of BA. Based on the
review of existing literature, it can be inferred that there is a potential for use of BA as
replacement of fine aggregate in concrete. Compared with fly ash, BA usually has no
cementitious properties, very little pozzolanic activity, coarse particle size, higher carbon
content and higher water demand. Therefore, previous studies are mainly focused on using it in
blocks, highway sub-base materials, structural filling materials etc. However, due to the leakage
of some heavy metal elements from the above mentioned large volume construction work, only
part of BA is used in these fields (Ghafoori, 1992; Ghafoori and Cai, 1998; Cheriaf et al., 1999).
This has led to little recycling and large dumping of BA in landfill sites, which has gradually
become a serious environmental problem in both developed and developing countries.
Furthermore, owing to the construction boom in both developed and developing countries, there
is a critical shortage of good quality sand in many areas throughout the world. Within the
European Union, an aggregate tax has been introduced in order to conserve the natural
aggregate. Therefore, there is a need for reducing the use of natural aggregate by recycling
industrial by products.
Aydin and Arel (2017) investigated about the effects of high volume of fly ash in cement mix for
low strength applications. Supplementary cementitious material in the mix was optimized and
physical and mechanical properties were evaluated and predictive model was developed. Oruji
et al. (2017) had explored the re-usability of coal bottom ash as cement replacement in mortar.
The bottom ash was made pulverized and the effect on workability and setting time were
studied. Kurda et al. (2018) had reviewed all earlier works done by the other researchers on the
environmental impact and toxicity characteristics of recycled concrete aggregates (rca), fly ash
(fa), cement production as well as their substitution aspect. The analysis with respect to abiotic
depletion potential, ozone depletion potential, photochemical ozone creation, acidification
potential, eutrophication potential, toxicity, leachability etc. were considered. It was revealed
that environmental impact and cost of concrete reduced considerably with such incorporations,
and also requirement of landfill space reduced drastically.
Bottom ash is a by-product from the combustion of coal. Recent installation of coal power plant
in Norachcholai area produces bottom ash and they have no proper usage. This bottom ash has
low specific gravity compared with sand. Therefore, bottom ash can be used as a raw material to
manufacture light-weight masonry blocks (Abeykoon, 2012).
This paper was carried out by using BA from a thermal power plant, to replace part of sand in
concrete. The main purpose was to study the possibility of using BA to make a lightweight, high
performance and environmentally friendly concrete. The effect of BA on the workability,
compressive strength and durability of concrete are reported in these paper so that the research
going on BA found some measuring points as:
84 Sharma Satish et al., J. Build. Mater. Struct. (2021) 8: 82-92
i) The particle size distribution of the BA from the power station in is close to natural sand
which makes it possible to replace the natural sand in concrete with the BA.
ii) When BA sand was used in concrete, the workability of fresh concrete improved; when
used in medium to high strength concrete even for the same slump value the concrete
with BA sand was easier to be cast.
iii) The replacement of sand with BA sand causes decrease of compressive strength at all
ages and at all W/C. However at later ages, especially after 28 days, the compressive
strength of concrete with BA sand was found to increase at a higher rate than that of the
control concrete (without BA sand).
iv) The durability of concrete was detrimentally affected by increasing the replacement of
natural sand with the BA sand. Further study on BA to replace the natural sand would
be necessary in order to evaluate the beneficial effect of the BA sand on durability of
concrete.
v) The initial results of this research suggest that BA sand can be used to replace the
natural sand. Other aspects which require further investigation include the effect of ions
& heavy metals present in BA on the long-term performance of concrete.
2. Materials used
The physical and chemical characteristics of 43 grade Ordinary Portland Cement is given in
Table-1. The results of cement are satisfying the codal provision laid down in IS: 269. Coarse
aggregates with a maximum nominal size of 10 mm and natural riverbed sand confirming to
Zone II as per Indian Standard IS: 383-2016 was used as fine aggregate for concrete (Table 2 and
Table 3). The physical characteristics of BA used in the study is given in Table-4. Different tests
(Physical & Chemical) were carried out on the bottom ash collected from Coal Fed Thermal
Power Plant of NTPC Unchahar.
Table 1. Physical and chemical characteristics of 43 grade ordinary Portland cement
Sl. No. Test Results Obtained
1 Blain’s Fineness, m2/kg 299.5
2 Setting Time, minutes: Initial & Final 126 & 206
3 Soundness : Lechatlier Exp., mm
Autoclave Exp., %
1.00
0.082
4 Compressive Strength, N/mm2
3 days
7 days
28 days
32
40
48
5 Chemical Analysis, % :
Loss on Ignition
Silica (SiO2)
Iron Oxide (Fe2O3)
Aluminium (Al2O3)
Calcium Oxide (CaO)
Sulphate (SO3)
Magnesium Oxide (MgO)
2.05
22.45
5.18
5.44
59.69
1.26
2.80
6 Alkalies (%):
Na2O & K2O
0.57 & 0.36
Sharma Satish et al., J. Build. Mater. Struct. (2021) 8: 82-9é 85
Table 2. Physical characteristics of fine aggregate
Sl. No. Parameter/Tests Test Value
1 Silt content 2.33%
2 Clay lumps Nil
3 Organic impurities Nil
4 Soundness 2.27%
5 Sp. Gravity 2.67
6 Water absorption 0.90%
7 Sieve analysis
Sieve Size Percent Passing
10 mm 100.00
4.75 mm 90.60
2.36 mm 78.65
1.18 mm 54.60
600 micron 40.15
300 micron 23.15
150 micron 10.35
Pan Nil
Table 3. Physical characteristics of coarse aggregate
Sl No. Parameter/tests Test Results
1 Specific gravity 2.69
2 Water absorption 0.40%
3 Crushing value 24.73%
4 Impact value 18.34
5 Abrasion value 19.04
6 Soundness 0.30%
7 Flakiness Index (< 10 mm) 29%
8 Elongation Index (<10 mm) 32.76%
9 Sieve analysis
Sieve Size Percent Passing
10 mm 96.06%
4.75mm 16.40%
2.36mm Nil
Figure-1 shows the gradation curve of BA I, II & III and fine aggregate. All three type of BA are
found to be very finer than the sand grading. In all three types the 600µ and lower particle size
greater than the usually present in coarse sand used in this study. Also compare with the grading
of plaster sand & masonry sand the grading of BA is not comparable because BA has very fine
particle 600µ and down size. BA and boiler slag are made essentially out of silica, alumina, and
iron, with smaller percentages of calcium, magnesium, sulfates, and other compounds. The
composition of the above particles is controlled principally by the source of the coal and not by
the type of furnace. BA or boiler slag derived from lignite or sub-bituminous coals have a higher
percentage of calcium than that from anthracite or bituminous coals. Due to the salt content and,
in some cases, the low pH of BA and boiler slag, these materials could display corrosive
properties. When BA or boiler slag is used in an embankment, backfill, sub-base, or even
possibly in a base course, the potential for corrosion of metal structures that may come in
contact with the material is of high concern and should be investigated prior to use so that it
does not pose any problem. Water meeting the requirements of Indian Standard IS: 456-2000
has been used.
86 Sharma Satish et al., J. Build. Mater. Struct. (2021) 8: 82-92
Table 4. Characteristics of bottom ash (BA)
Sl No. Test BA I BA II BA III
Physical Test
1 Water absorption (%) 2.90 3.80 4.20
2 Sp. gravity 2.11 2.12 2.19
3 Lime reactivity (Kg/cm
2
) 19.2 16.0 21.2
4 Sieve analysis
4.75 100.00 100.00 100.00
2.36 100.00 98.75 99.00
1.18 95.70 94.62 98.00
600micron 92.40 90.12 95.63
300micron 72.50 66.74 82.00
150micron 41.10 33.61 46.00
Pan 0.00 0.00 0.00
Chemical Test
1 Loss on Ignition 2.80 1.76 2.63
2 Silica (SiO2) 62.45 58.75 58.98
3 Iron oxide 7.16 14.41 7.81
4 Aluminum oxide 22.17 19.69 24.81
5 Calcium oxide (CaO) 1.34 1.28 1.29
6 Magnesium oxide (MgO) 0.48 0.58 0.46
BA I=Bottom Ash Stake I
BA II=Bottom Ash Stake II
BA III=Bottom Ash Stake III
Fig. 1. Gradation Curve for Fine Aggregate and Bottom Ash: BA-I, II & B-III
0
10
20
30
40
50
60
70
80
90
100
0.1110
P
e
rc
e
n
ta
g
e
P
a
ss
in
g
(
%
)
Sieve Size (mm)
BA I
BA II
BA III
FA
Sharma Satish et al., J. Build. Mater. Struct. (2021) 8: 82-9é 87
3. Selection of Mix Proportion with Different W/C Ratio
Different trial mixes (Table-5) were made using:
1. Direct replacement of bottom ash (by weight of coarse sand in percentage)
2. Replacement in equal volume of coarse sand and bottom ash
Table 5. Matrix for Selection of W/C ratio & Ratio of concrete mix
W/C
Ratio
Ratio of Cement : Sand : Aggregate
1:3:6 1:3.125:6.25 1:2.5:5
0.45
Seems more water
required for casting to
block in vibro machine
Blocks but did not give clear
dimensions, so the mix was
rejected
Block successfully but
did not give cost-
effective option
0.50
Less sand & less water
content observed
Block caste successfully
with economy in view
Block successfully
0.55
Less sand content
observed
Block caste successfully for
study of required properties
Block caste successfully
but higher cement
content did not allow
further work
Hence, ratio of 1:3.125:6.25 is selected for mix proportion and W/C ratio 0.50 is selected on
the basis of strength. Method of mix design by replacing the ratio of specific gravity was found
suitable than direct replacement of sand, but fines of BA still gave higher specific surface area
thus increasing the water demand. Water demand also was found higher due to higher water
absorption by BA. Optimised mix proportion for required strength without using BA (Table-6):
Table 6. Optimised mix proportion for required strength without using Bottom Ash (BA)
Ingredient in
Mix
Cement Coarse Sand-Natural
10 mm Aggregate-
Crushed Stone
Free W/C
Mix proportion 1 Part 3.125 Part 6.25 Part 0.5
Batch Mix Proportion used in Laboratory (W/C = 0.5%) using BA to replace the sand content
given in Table-7:
Table 7. Mix proportions of concrete blocks
Ingredient
Mix
Proportions
Without
Bottom Ash
Mix Proportions using Bottom Ash
@30%
Replacement as
Fine Aggregate
@40%
Replacement as
Fine Aggregate
@50%
Replacement as
Fine Aggregate
Quantity (Kg) Quantity (Kg) Quantity (Kg) Quantity (Kg)
Cement, Kg 188.24 181.17 177.5 170.8
Coarse Sand, Kg/cum 0.37 0.25 0.21 0.17
Bottom Ash, kg/cum Nil 135.16 176.77 212.64
Coarse Aggregate
(<10mm)
0.74 0.57 0.55 0.53
Water, Ltr 94.11 90.5 90.5 85.4
Wet Density of
Concrete, Kg/cum
2000 1925 1886 1815
Weight of Concrete
Block, Kg (390x175x90
mm)
12.28 11.82 11.57 11.14
The study indicated that for various percentages of replacement of sand with bottom ash the
workability of concrete decreased with the increase in bottom ash content. The workability of
the concrete was correlated to its slump value and it was observed that up to 50% replacement
88 Sharma Satish et al., J. Build. Mater. Struct. (2021) 8: 82-92
of Coal Bottom ash, the slump was found to be in the range 60-150 mm. The decrease in
workability can be attributed to the fact that the Coal Bottom Ash is irregular in shape, vesicular
in texture and porous such that friction between particles is high. Thus, with increasing Coal
Bottom Ash content, there is an increase in water demand to achieve similar workability as for
the control mix.
3.1. Procedure using Vibro Compaction Technique for Making Concrete Blocks
Vibro Compaction method is probably the most extensively used technique for compacting
concrete. In this method, internal friction between aggregate particles is eliminated for a short
time and concrete mixtures behave like liquid and gravitational force will come into effect.
Following procedure was adopted in making concrete blocks:
Step I Take all the material as per batch given in Table-7.
Step II Make a dummy trial batch (around 10% quantity with same proportion) and remove
from concrete mixture so that a thin layer of mix can adhere with surface of mixture
for simulating the continuous batch mix.
Step III Now add all the quantity (excepts aggregate) and mix it for 60 seconds. Now add
aggregate and mix the total batch for 120 seconds.
Step IV Now remove the batch to a non-water absorbing surface and fill the mould to cast
the block in vibro compaction machine.
Step V When blocks are filled to total mould volume, vibrate for 60 seconds and remove
from mould.
Step VI Lift the caste block for drying (in shaded area) for 24 hours.
Step VII Cure the block 3 day with gunny bags/in moist environment and the cure 7 day in
submersed condition & there after cure the bock stacks in air by sprinkling water
4. Results & Discussion
4.1 Compressive Strength
The compressive strength of concrete blocks made with BA I, II and III are given in Table 8. From
this table it can be concluded that 30% replacement of BA I, II & III gives results of compressive
strength almost similar to control concrete.
Table 8. Compressive strength of concrete blocks obtained.
Compressive Strength (N/mm2) at
Different Age (Day)
Control Mix
Percentage Replacement in the Ratio
of Specific of BA/Sand
30% 40% 50%
Using BA–I for NTPC,
Unchahar (Source I)
3 7.01 6.89 2.34 1.92
7 10.02 9.88 2.27 2.11
28 11.24 10.67 3.47 3.50
56 11.74 11.25 4.12 3.85
Using BA–II for NTPC,
Unchahar (Source II)
3 7.01 6.85 1.57 0.99
7 10.02 9.96 2.89 2.53
28 11.24 10.86 4.02 3.38
56 11.74 11.12 4.44 3.69
Using BA–III for NTPC,
Unchahar (Source III)
3 7.01 6.95 1.35 0.96
7 10.02 10.12 2.47 2.46
28 11.24 10.50 4.26 3.98
56 11.74 10.76 4.36 4.23
Sharma Satish et al., J. Build. Mater. Struct. (2021) 8: 82-9é 89
Fig. 2. Compressive Strength of control mix and mix with different replacement percentages of Bottom Ash at
different ages
From Figure-2 above it can be see that the compressive strength of concrete blocks using BA
decreases at early age but slightly increases further at 56 day & 90 day. The results of higher
replacement of BA beyond 30% indicates that there is further no substantial increase in
compressive strength.
4.2. Density & Drying Shrinkage of Concrete Block
The density of concrete block decreases with increase in percentage of BA (Table 9 & Figure-3).
The density of concrete block also increases when the finer particle from BA is removed. Drying
shrinkage is found to be slightly higher in BA samples prepared by cutting the concrete blocks
samples from concrete blocks with 30%, 40% & 50% of BA as compared to control concrete
blocks without BA (Tabel-10 and Figure-4). The density of concrete block can be increased by
using state-of-the-art block making machine or use of Superplasticiser (High range water
reducer agent) which in turn will improve the compressive strength and density.
Table 9. Density of concrete blocks (in Kg/m3)
Bottom Ash Replacement Percentage
30% 40% 50% Control Mix
BA-I 1925 1886 1815 2000
BA-II 1915 1868 1794 2000
BA-III 1754 1854 1787 2000
0
2
4
6
8
10
12
14
C
o
n
tr
o
l
M
ix
3
0
%
4
0
%
5
0
%
C
o
n
tr
o
l
M
ix
3
0
%
4
0
%
5
0
%
C
o
n
tr
o
l
M
ix
3
0
%
4
0
%
5
0
%
C
o
n
tr
o
l
M
ix
3
0
%
4
0
%
5
0
%
3 Days 7 Days 28 Days 56 Days
C
o
m
p
re
ss
iv
e
S
tr
e
n
g
th
(
N
/m
m
2
)
Percentage replacement at different ages
BA–I BA–II BA–III
90 Sharma Satish et al., J. Build. Mater. Struct. (2021) 8: 82-92
Fig. 3. Density of control mix and mix with different replacement percentages of Bottom Ash at different ages
Table 10. Drying shrinkage of concrete blocks (%)
Bottom Ash Replacement Percentage
30% 40% 50% Control Mix
BA-I 0.0692 0.0692 0.0817 0.0413%
BA-II 0.0712 0.0714 0.0720 0.0413 %
BA-III 0.0746 0.0770 0.0890 0.0413 %
Fig. 4. Drying Shrinkage of control mix and mix with different replacement percentages of Bottom Ash at
different ages
0
500
1000
1500
2000
2500
Control Mix 30% Replacement 40% Replacement 50% Replacement
D
E
N
S
IT
Y
O
F
C
O
N
C
R
E
T
E
B
LO
C
K
S
(
in
K
g
/m
3
)
Different percentages of replacement of Bottom Ash
BA-I BA-II BA-III
0.000%
0.010%
0.020%
0.030%
0.040%
0.050%
0.060%
0.070%
0.080%
0.090%
0.100%
Control Mix 30% Replacement 40% Replacement 50% Replacement
D
R
Y
IN
G
S
H
R
IN
K
A
G
E
O
F
C
O
N
C
R
E
T
E
B
LO
C
K
S
(
%
)
Different percentages of replacement of Bottom Ash
BA-I BA-II BA-III
Sharma Satish et al., J. Build. Mater. Struct. (2021) 8: 82-9é 91
Due to lower density & higher water absorption higher percentage of fines in all the three
sources of BA, the coal BA as alternative to sand is not found to be techno-economically viable
alternative. It can be said that replacement of sand with BA up to 30% is technically
suitable/feasible and meets the requirement of IS-2185. Although using 40% & 50% of BA from
the three sources the compressive strength obtained in concrete blocks is lower than 10 N/mm2
but certainly by use of vibro-compaction technique, all the concrete blocks are found technically
suitable as far as the requirement of meeting the compressive strength as laid down in IS: 2185
is concerned. Higher compacting effort using of modern machinery with use of Superplasticiser
will definitely improve the strength & density of concrete blocks
5. Conclusion and Recommendations
- Some of the properties of BA such a higher fine particles and higher water absorption are
in general found economically unviable for replacing sand due to the decrease in
strength associated with the increased fine content in the concrete.
- Density of concrete blocks is also found lower using 30%, 40% & 50% BA as compared
to the concrete blocks without BA irrespective of the source of BA.
- Drying shrinkage is found to be slightly higher in BA samples prepared by cutting the
concrete blocks samples from concrete blocks with 30%, 40% & 50% of BA as compared
to control concrete blocks without BA.
- Due to lower density & higher drying shrinkage values, concrete blocks, corresponding
compressive strength of concrete blocks using 30%, 40% & 50% is also found lower at
all ages i.e. 3, 7, 28, 56 & 90 day. This is true for all the three sources of fly ash
investigated. 30% replacement of BA I, II & III gives results of compressive strength
almost similar to control concrete.
- Due to lower density & higher water absorption higher percentage of fines in all the
three sources of BA, the coal BA as alternative to sand is not found to be techno-
economically viable alternative. It can be said that replacement of sand with BA upto
30% is technically suitable/feasible and meets the requirement of IS-2185. Although
using 40% & 50% of BA from the three sources the compressive strength obtained in
concrete blocks is lower than 10 N/mm2 but certainly by use of vibro compaction
technique, all the concrete blocks are found technically suitable as far as the requirement
of meeting the compressive strength as laid down in IS: 2185 is concerned.
- To improve the quality of BA to make the same as techno economically viable alternative,
sieved BA after removed of after 150µ and 300 down size shall be preferable for usage in
concrete blocks.
- From the grading it is seen that the BA can also be used in mortar making for use in
masonry and plastering work as the grading of bottom is found suitable for these works.
The grading requirement of fine sand for use in mortar making and after partly removal
of 150µ & 300µ sieve.
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