Environmentally friendly unburnt bricks using raw rice husk and bottom ash as fine aggregates: Physical and mechanical properties
Journal of Science and Technology in Civil Engineering, NUCE 2021. 15 (1): 110–120
ENVIRONMENTALLY FRIENDLY UNBURNT BRICKS
USING RAW RICE HUSK AND BOTTOM ASH AS FINE
AGGREGATES: PHYSICAL AND MECHANICAL
PROPERTIES
Ngo Si Huya, Nguyen Ngoc Tanb,∗, Mai Thi Ngoc Hanga, Le Ngoc Quangc
aDepartment of Engineering and Technology, Hong Duc University,
565 Quang Trung street, Dong Ve ward, Thanh Hoa city, Vietnam
bFaculty of Building and Industrial Construction, National University of Civil Engineering,
55 Giai Phong road, Hai Ba Trung district, Hanoi, Vietnam
cUrban Management Office of Thanh Hoa City, Nguyen Hoang Avenue,
Dong Hai ward, Thanh Hoa city, Vietnam
Article history:
Received 30/11/2020, Revised 07/01/2020, Accepted 08/01/2020
Abstract
In order to reduce the serious impacts of industrial and agricultural wastes on the environment, raw rice husk
and bottom ash were used as fine aggregates, while fly ash was utilized as a binder material in the produc-
tion of unburnt building bricks. Two group mixtures were designed with water-to-binder (W/B) ratios of 0.30
and 0.35. The rice husk was used to replace 0%, 3%, 6%, and 9% of bottom ash content by mass. An experi-
mental program was carried out on the brick samples at different ages from 3-day to 28-day to determine the
main physical and mechanical properties of brick, such as unit weight, compressive strength, water absorption,
ultrasonic pulse velocity and thermal conductivity. The microstructure of brick material was captured using
scanning electron microscope technique. The experimental results allow to identify the effects of rice husk,
bottom ash content as well as W/B ratio on the properties of bricks. Brick samples produced in this study had a
proper compressive strength meeting the practice requirement and were classified as Grade M3.5 and 5.0 based
on TCVN 6477:2016. At the use of 9% rice husk, the unit weight and thermal conductivity of bricks were really
low (1.06÷1.08 T/m3 and 0.201÷0.216 W/m.K), they are conformed to be used in temporary construction and
insulation structures.
Keywords: rice husk; bottom ash; fly ash; unburnt brick; ultrasonic pulse velocity; thermal conductivity.
1. Introduction
From past to present, brick is one of the common construction building materials not only do-
mestically but also worldwide. As estimated in previous studies, 42 billion and 1.391 trillion units of
them were fired-clay bricks, referred to as traditional bricks. In order to produce a huge quantity of
fired-clay bricks as mentioned above, a lot of fuel and natural resources were demanded. The produc-
tion process of traditional bricks also released a large amount of toxic gases into the air, especially
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carbon dioxide, causing environmental pollution. Therefore, the Prime Minister of Vietnam has is-
sued the Directive on enhancing the use of unburnt construction materials and limiting the production
of fired-clay bricks since 2012. Especially, the reuse of solid wastes from industry to produce unfired
building bricks has been encouraging.
In Vietnam, the common unfired building bricks are produced from cement, sand or crushed
stone as fine aggregates, and water. Their unit weight and compressive strength are typical around
causes the depletion of natural resources and put the negative impacts on the environment. Recently,
the over extraction of river sand is another issue related to aquatic life, water sources, erosion and land
loss. While the crushing of quarried stones also has some problems associated with the noise pollution
and damages the natural environment. On the other hand, many types of industrial wastes such as fly
ash, bottom ash, copper mining tailings, gold mill tailings, red mud are emitted to the environment
due to the growth of population and a higher demand of human. These industrial wastes have seriously
impacts on the environment and human life. Turning such wastes into green construction materials is
an effective solution not only for the environment but also for the economic benefit.
Among many types of solid industrial wastes, the thermal power plant ashes such as fly ash and
bottom ash were widely used in the production of unburnt bricks. While fly ash was used as a binder
material [4–10], bottom ash was utilized as fine aggregate [3, 11–14]. Although the test results were
different among these studies, but most of them showed the high possible using both fly ash and
bottom ash in the manufacturing of unfired bricks. Besides fly ash and bottom ash, other industrial
recycled in the production of unburnt bricks. The recycling of industrial and agricultural wastes in
the production of unburnt bricks is a trend in the 4.0 revolution of the construction industry that is
attracted to many researchers.
Rice husk is an agricultural waste, which is a major by-product of the rice milling process. As
estimated in 2010, an approximation of 149 million tons of rice husk was generated in over the world
quently, a huge amount of rice husk is emitted. A part of it is used as fuel by famers, another part
is used in agricultural land reclamation. However, most of them are dumped into the rivers, causing
some serious problems related to environmental pollution [1]. Therefore, turning such waste into the
construction materials has received many attentions from researchers. While some studies investi-
gated the use of rice husk as a component in fired – clay bricks [18–21], others used rice husk ash in
the production of unburnt bricks [1, 22]. It is noticed that ground rice husk ash can be used as binder
of test results indicated that both the unit weight and compressive strength of bricks reduced, but the
water absorption of bricks increased with the increase in rice husk content. With the use of 10% rice
husk by weight, the unit weight and compressive strength of bricks could decline to around 1.15 T/m3
S¸ims¸ek [21] investigated the use of rice husk as 5%, 10%, and 15% by volume of fired-clay brick. Test
result showed that the unit weight, compressive strength, water absorption, and thermal conductivity
of bricks ranged 1.28÷1.67 T/m3, 2.0÷9.0 MPa, 21.9÷37.8%, and 0.173÷0494 W/m.K, respectively.
The utilization of rice husk in the production of cement brick has been investigated by Abdullah and
The compressive strength and water absorption of bricks were respective 2.0÷6.0 MPa and 10÷50%.
Most of the previous studies proved that rice husk and rice husk ash can be used in the production
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of fired-clay bricks and unfired bricks. Although the compressive strength decreased, water absorption
increased, but the unit weight of bricks significantly declined to incorporate rice husk. Therefore, rice
husk shows a great potential in the manufacture of lightweight bricks. However, the use of raw rice
husk in the production of unfired building bricks is still limited. In this study, raw rice husk and
bottom ash are used as fine aggregates, while fly ash is used as a binder material in the production of
environmentally friendly unfired building bricks. The effect of raw rice husk content on the properties
of green building bricks is investigated.
2. Experimental program
2.1. Materials
In this study, the main compositions used to produce the unburnt brick samples includes cement,
fly ash, bottom ash, and rice husk. The type-PC40 cement was taken from the Nghi Son factory
with a specific gravity of 3.12. Both fly ash and bottom ash were sourced from Nghi Son thermal
power plant in Thanh Hoa province with their respective densities of 2.16 T/m3 and 1.99 T/m3. The
density, fineness modulus, moisture content, and water absorption of bottom ash were 1.08 T/m3,
1.97, 17.06% and 23.15%, respectively. Rice husk was taken from a rice mill in Thanh Hoa city with
its characteristics as the density of 1.068 T/m3, the bulk density of 0.098 T/m3, the fineness modulus
of 3.38, and the water absorption of 3.7%. The low value of rice husk’s bulk density was proved
bottom ash and rice husk.
Table 1. Physical and chemical properties of raw materials
Chemical compositions (% by weight)
Density
(g/cm3)
LOI*
(%)
Materials
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Others
Cement
Fly ash
3.12
2.16
0.41
15.85
21.23
51.50
5.50
20.20
4.90
7.07
61.02
1.99
2.97
1.23
1.47
-
2.50
2.16
LOI* = Loss on ignition.
Table 2. Sieve analysis and fineness modulus of bottom ash and rice husk
Sieve size (mm)
5.0
Bottom ash 80.6 75.8 71.6 65.8
Rice husk 69.6 34.4 34 31.6
2.5
1.25 0.63 0.315 0.14 FM*
59.7
31.2
50
1.97
Percentage of passing (%)
FM* = Fineness modulus
2.2. Mixture proportions
30.8 3.38
The brick mixtures were designed with two water-to-binder (W/B) ratios of 0.30 and 0.35, denoted as
M30 and M35, respectively. For each W/B ratio, the rice husk was used to replace 3%, 6%, and 9% of
bottom ash content by mass. RH denotes rice husk, and the numbers 0, 3, 6, and 9 after it denote the
percentage of rice husk to replace bottom ash in brick mixtures. Two control mixtures were designed
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Table 3. Mix proportions
Mix proportions (kg/m3)
W/B
Mix
notation
Cement
Fly ash
Bottom ash
Rice husk
Water
M30RH0
M30RH3
M30RH6
M30RH9
180.0
177.4
174.9
172.4
420.0
413.9
408.0
402.3
1128.3
1078.7
1030.4
983.5
0.0
180.0
177.4
174.9
172.4
33.4
65.8
97.3
0.30
0.35
M35RH0
M35RH3
M35RH6
M35RH9
154.3
151.9
149.6
147.4
360.0
354.5
349.1
343.9
1199.9
1146.1
1093.8
1043.1
0.0
35.4
69.8
103.2
180.0
177.2
174.6
172.0
without rice husk, referred to as M30RH0 and M35RH0. The content of cement and fly ash were
respective 30% and 70% by weight of the total binder. Except cement was used with low content,
other ingredients were industrial and agricultural wastes. In which, rice husk was utilized to reduce
brick’s weight. The purpose of these designed mixtures is to investigate the effect of rice husk content
on the properties of unfired building bricks.
2.3. Sample preparation
ash were firstly mixed by the use of the laboratory mixing pan in two minutes. After that, bottom ash
and rice husk were added and mixed for another two minutes. Then, water was gradually poured, and
(a) Rice husk
(b) Bottom ash
Figure 1. Manufacturation of brick samples in the laboratory
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(c) Test samples
Huy, N. S., et al. / Journal of Science and Technology in Civil Engineering
the mixer continuously ran at moderate speed until a homogeneous mixture was obtained. Finally,
the mixture was poured into a steel mold with dimensions of 160×85×40 mm and produced under
forming pressure of around 0.5 MPa. The brick samples were immediately removed from the mold
2.4. Test program
compressive strength, unit weight, and water absorption were tested. Other properties consisting of
ultrasonic pulse velocity and thermal conductivity were also tested. It is noticed that the compressive
strength and ultrasonic pulse velocity of brick samples were tested at 3, 7, 14, and 28 days of age,
while other testes were conducted at 28 days of age. The reported values in this study are average
of three measurements. The compressive strength, unit weight and water absorption were conducted
in accordance with TCVN 6477:2016. Ultrasonic pulse velocity and thermal conductivity values of
brick samples were directly measured by testing equipments such as MATEST C369 and ISOMET-
2014, respectively. The ultrasonic pulse velocity was conducted on natural-dry samples according to
microstructure of bricks was also observed by using the scanning electron microscope provided by
SEISS producer. All the compressive strength values presented herein were multiplied with 0.73,
which is a coefficient factor reflecting the shape effect of the brick sample as stipulated in TCVN
6477:2016.
3. Results and discussion
3.1. Unit weight
1.6
The correlation between rice husk content and
M30
1.5
1.4
1.3
1.2
1.1
1
M35
observed, when rice husk content changed from
0% to 6%, the unit weight of bricks slightly re-
duced from 1.56 T/m3 to 1.49 T/m3 and from
1.49 T/m3 to 1.31 T/m3 corresponding to W/B ra-
tios of 0.30 and 0.35. Since rice husk content in-
creased up to 9%, the unit weight of bricks sig-
nificantly dropped to 1.06÷1.08 T/m3. These val-
ues are notably smaller than that of current cement
It is noticed that the bulk density of rice husk
(0.098 T/m3) is really low due to the existence of
gaps and voids among them. When the rice husk
content is low, these gaps and voids were easily
6
3
9
0
Rice husk content (%)
Figure 2. Effect of rice husk content on unit
weight of brick samples
filled with cementitious paste. However, when its content increased, it is very difficult to compact and
fulfill these gaps and voids by paste. This phenomenon is proved during the casting process of brick
samples, and explained for the significant reduction in unit weight of brick samples with high rice
husk content. This result shows a potential to apply rice husk in the production of lightweight bricks.
On the other hand, the unit weight of bricks was slightly reduced with increasing the W/B ratio.
This is explained that when the W/B ratio increased, then the water content increased and the binder
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content decreased. The density of water (1.0 T/m3) is smaller than that of cement (3.12 T/m3) and
fly ash (2.16 T/m3). Another possible reason is due to the water evaporation during the hardening
process. They leads to the reduction in unit weight of bricks as increasing W/B ratio.
3.2. Compressive strength
Compressive strength is a basic characteristic of bricks, then the grade of bricks is classified based
on this property. The popular bricks used in practice for normal walls have a grade of from M3.5 to
M7.5 corresponding to the compressive strength of from 3.5 MPa to 7.5 MPa. For lightweight bricks,
the common compressive strength is around 3.5 MPa to 5.0 MPa. The development of compressive
strength of brick samples with a W/B ratio of 0.3 is higher than that of the samples with a W/B ratio
of 0.35. This is due to the effect of the W/B ratio on the strength development of cement hydration
products. The compressive strength of M30 and M35 bricks felt within the range 4.22÷9.62 MPa and
3.42÷5.75 MPa, respectively. These values are similar to compressive strength results from previous
(2.0÷6.0 MPa).
10
10
M30RH0
M30RH3
M30RH6
M30RH9
M35RH0
M35RH3
M35RH6
M35RH9
8
8
6
6
4
4
2
2
0
0
0
7
14
21
28
0
7
14
21
28
Curing time (days)
Curing time (days)
(a)
(b)
Figure 3. Compressive strength of (a) M30 and (b) M35 brick samples
With an increase in the rice husk content, the compressive strength of bricks reduced. As afore-
mentioned, the bulk density of rice husk is really slight due to the voids among them, leading the
less compaction during the manufacturing of brick samples. This is the main cause leading to the
reduction in compressive strength of bricks with the presence of rice husk. However, the compressive
strength of bricks is still higher than 5.0 MPa (referred to as Grade M5.0) if the replacement of 3%
bottom ash by rice husk (M30RH3 and M35RH3). When increasing the rice husk content to 6% and
9%, the compressive strength of bricks is still higher than 3.42 MPa, could be classified as Grade
M3.5. It demonstrated that rice husk can be utilized as fine aggregate in the production of unfired
building bricks, at least considered as lightweight bricks. These rice husk bricks can be suitably used
in temporary construction and non-load bearing walls due to the lightweight.
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3.3. Water absorption
40
35
30
25
20
15
10
5
The effect of rice husk content on water ab-
The water absorption value is often related to
the compactness and density of the brick sample,
so it is also related to the compressive strength
bricks slightly increased since the rice husk con-
tent increased from 0% to 6% or the W/B ratio
increased. Its value significantly increased when
the rice husk content increased to 9%. These find-
ings are associated with the change of brick’s unit
weight as aforementioned. The higher water ab-
sorption of bricks is attributed to the voids among
rice husk particles. The water absorption of bricks
in this study ranged from 12.4% to 35.3%. Al-
M30
M35
0
6
3
9
0
Rice husk content (%)
Figure 4. Effect of rice husk content on water
absorption of brick samples
though these values are really high in comparison with that of normal cement bricks. However, these
results are similar to those results from previous studies, felt within the range 12.7% ÷ 37.8% [19–21].
It is noticed that the water-resistance of bricks in practice will be enhanced by a layer of mortar.
3.4. Ultrasonic pulse velocity
Ultrasonic pulse velocity (UPV in km/s) test is a non-destructive method, its result is related to
samples corresponding to W/B ratios of 0.3 and 0.35, which were measured from 3-day to 28-day of
bricks. The UPV of the M30 brick sample is higher than that of the corresponding M35 brick sample.
This is due to the higher density of the M30 brick sample compared with the M35 brick sample. For
the same W/B ratio, the UPV values of bricks declined with increasing rice husk content. At 28-day
age, the UPV values of M30 brick samples were 2.89, 2.44, 1.94, and 1.25 km/s corresponding to rice
3
2.5
2
3
2.5
2
1.5
1
1.5
1
M30RH0
M30RH3
M30RH6
M30RH9
M35RH0
M35RH3
M35RH6
M35RH9
0.5
0
0.5
0
0
7
14
21
28
0
7
14
21
28
Curing time (days)
Curing time (days)
(a)
(b)
Figure 5. Ultrasonic pulse velocity of (a) M30 and (b) M35 brick samples
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husk content of 0%, 3%, 6%, and 9%, respectively. Those were 2.61, 2.24, 1.66, and 1.18 km/s for
the corresponding M35 brick samples. As mentioned above, the density of bricks reduced when rice
husk content increased, resulting in reduction in UPV values. The low UPV value of brick samples
is attributed to the voids among rice husk particles. It is demonstrated in previous studies [3, 29]
that the UPV values and the compressive strength has a relationship. The bricks produced by the
use of thermal power plant ashes had UPV values of 1.10÷1.77 km/s coressponding to compressive
higher, the UPV values of bricks ranged from 1.18 km/s to 1.94 km/s, similar to those values from
previous studies [3, 30]. It is noticed that the compressive strength and unit weight of these bricks
were around 3.42÷4.81 MPa and 1.06÷1.49 T/m3, considered as lightweight bricks. While other
brick samples with rice husk content of 0% and 3% had compressive strength of 5.33÷9.62 MPa,
unit weight of 1.46÷1.56 T/m3, resulting in UPV values of above 2.2 km/s. They may be considered
as normal unfired building bricks. Depending on the actual requirements, the rice husk content is
properly selected.
3.5. Thermal conductivity
0.7
The lightweight bricks are usualy used in heat
M30
isolation structures, then the thermal conductivity
test was conducted to assess the insulation capac-
0.6
M35
0.5
ity of these bricks in this study. The effect of rice
husk content on the thermal conductivity of bricks
0.4
0.3
the thermal conductivity of bricks decreased with
0.2
increasing rice husk content and W/B ratio. It
was stated that the thermal conductivity of a brick
0.1
0
sample is related to its unit weight and porosity
unit weight of bricks declined. Consequently, their
thermal conductivity values reduced. The similar
trend is observed when W/B ratio increased. In-
corporating rice husk, the thermal conductivity of
6
3
9
0
Rice husk content (%)
Figure 6. Effect of rice husk content on thermal
conductivity of brick samples
bricks felt within the range of 0.201÷0.454 W/m.K. This range is similar to the result from Go¨rhan
and S¸ims¸ek’s study [21]. It is noticed that Go¨rhan and S¸ims¸ek used rice husk in the production of
fired-clay lightweight bricks. Their thermal conductivity ranged from 0.173 to 0.494 W/m.K. It is
worth noting that the brick samples with 9% rice husk had a very low thermal conductivity values
(0.201÷0.216 W/m.K). As the results, with low unit weight and low thermal conductivity, these rice
husk bricks in this study is also suitably used in isolation structures.
3.6. SEM observation
The scanning electron microscope (SEM) was used to observe the microstructure of bricks, espe-
cially the connection between rice husk and paste. The SEM micrographs of unfired building bricks
incorporating rice husk are shown in Figs. 7 and 8. These micrographs were taken on different samples
with magnifications (denoted Mag) of 300, 500, and 1000 times. It is interesting that the connection
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voids were detected around the connection between the belly of rice husk and paste as shown in Fig. 8.
The spikes on the back of rice husk help them bond well with the cementitious paste. Therefore, when
the rice husk content level is low (around 3%), the compressive strength of bricks is still good (higher
than 5.0 MPa). Whereas, the gaps in the belly of rice husk are responsible for the low unit weight,
UPV, thermal conductivity, compressive strength, and high water absorption. Consequently, if the rice
husk content is high (6% and 9%), the quality of brick is significantly reduced. These findings are
related to all test results presented above.
(a) Mag = 300 X
(b) Mag = 500 X
(c) Mag = 1000 X
Figure 7. The connection between back of rice husk and paste
(a) Mag = 300 X
(b) Mag = 500 X
(c) Mag = 1000 X
Figure 8. The connection between belly of rice husk and paste
4. Conclusions
In this study, raw rice husk and bottom ash were used as fine aggregates in the production of
environmentally friendly unburnt bricks. In which, rice husk was used to substitute 0%, 3%, 6%, and
9% of bottom ash content by weight. The brief conclusions can be drawn based on the experimental
results as follows:
- As the increase in rice husk content, the unit weight, compressive strength, UPV, the thermal
conductivity of bricks decreased, while water absorption of bricks increased. However, all bricks
produced in this study can be classified as Grade M3.5 or M5.0.
- The unit weight and thermal conductivity of bricks are significantly dropped (1.06÷1.08 T/m3
and 0.201÷0.216 W/m.K), when the rice husk content increased up to 9%. The tested bricks are
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suitably used in temporary construction and isolation structures. Depending on the actual requirement,
the amount of rice husk is properly selected.
- The back of the rice husk showed a good connection with cementitious paste, while the belly of
the rice husk exhibited a poor connection. Many voids around the connection between belly of rice
husk and cementitious paste were detected under the SEM image. This finding explained the poor
properties of bricks containing high amount of rice husk.
- This study showed that industrial wastes from thermal power plants (fly ash and bottom ash) and
agricultural wastes (rice husk) located in Vietnam can potentially be used in the production of unfired
bricks, especially the use of rice husk for lightweight bricks.
Acknowledgment
The experimental works were carried out at the construction material laboratory of the Depart-
ment of Engineering and Technology, Hong Duc University. The author would like to thank Mai Thi
Hong and Le Thi Thanh Tam, lecturers of Hong Duc University for their assistance in the experimental
work.
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[31] Sutcu, M., Akkurt, S. (2009). The use of recycled paper processing residues in making porous brick with
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