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.
https://doi.org/10.31814/stce.nuce2021-15(1)-10 © 2021 National University of Civil Engineering
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
bricks were annually consumed in Vietnam [1] and in the world [2], respectively. However, most 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
∗Corresponding author. E-mail address: tannn@nuce.edu.vn (Tan, N. N.)
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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
2.0÷2.2 T/m3 and 3.5÷7.5 MPa, respectively [3]. It is noticed that the production of cement also
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 botto ... ed in Fig. 6. It can be s en in Fig. 6, the thermal conductivity of 
bricks decreased with increasing rice husk content and /B ratio. It was stated that 
the thermal conductivity of a brick sample is related to its unit weight and porosity [3, 
31]. When the rice husk content increased, the unit weight of bricks declined. 
Consequently, their thermal conductivity values reduced. The similar trend is 
observed when W/B ratio increased. Incorporating rice husk, the thermal conductivity 
of bricks felt within the range of 0.201÷0.454 W/m.K. This range is similar to the 
result from Görhan and Şimşek’s study [21]. It is noticed that Görhan and Şimşek 
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. 
0 7 14 21 28
Curing time (days)
0
0.5
1
1.5
2
2.5
3
U
ltr
as
on
ic
 p
ul
se
 v
el
oc
ity
 (k
m
/s
)
M30RH0
M30RH3
M30RH6
M30RH9
0 7 14 21 28
Curing time (days)
0
0.5
1
1.5
2
2.5
3
U
ltr
as
on
ic
 p
ul
se
 v
el
oc
ity
 (k
m
/s
)
M35RH0
M35RH3
M35RH6
M35RH9
(b)
Figure 5. Ultrasonic pulse velocity of (a) 30 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
strength of 1.18÷7.7 MPa [3]. While the bricks used 5% rice husk ash and 5% surgarcane bagasse
ash had UPV values of 1.16÷1.64 km/s [30]. In present study, at the rice husk content of 6% or
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
Journal of Science and Technology in Civil Engineering NUCE 2021 
Figure 6. Effect of rice husk content on thermal conductivity of brick samples 
3.6. SEM observation 
 The scanning electron microscope (SEM) was used to observe the microstructure 
of bricks, especially 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 
between the back of rice husk and cementitious paste is greatly good as shown in Fig. 
7, while many 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 
Rice husk content (%)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Th
er
m
al
 c
on
du
ct
iv
ity
 (W
/m
K
) M30
M35
0 3 6 9
Figure 6. Effect of rice husk content on thermal
conductivity of brick samples
The lightweight bricks are usualy used in heat
isolation structures, then the thermal conductivity
test was conducted to assess the insulation capac-
ity of these bricks in this study. The effect of rice
husk content on the thermal conductivity of bricks
is illustrated in Fig. 6. It can be seen in Fig. 6,
the thermal conductivity of bricks decreased with
increasing rice husk content and W/B ratio. It
was stated that the thermal conductivity of a brick
sample is related to its unit weight and porosity
[3, 31]. When the rice husk content increased, the
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
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 ri e 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 us d to obser the micr structure 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
between the back of rice husk and cementitious paste is greatly good as shown in Fig. 7, while many
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Huy, N. S., et al. / Journal of Science and Technology in Civil Engineering
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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