Các đặc tính kỹ thuật của bê tông bọt sử dụng xi măng, tro bay và xỉ lò cao nghiền mịn làm chất kết dính

n this study. The ratio of foam to 
water is 1/30 as suggested from producer. The 
superplasticizer (SP) with density of 1.05 T/m
3
 is 
utilized to reduce the water content in all concrete 
mixture. 
Cement, fly ash, and GGBFS are used as 
binder materials in this study. In which, cement is 
Nghi Son Type - PCB40, fly ash is sourced from 
Nghi Son coal power plant, and GGBFS is Hoa 
VẬT LIỆU XÂY DỰNG - MƠI TRƯỜNG 
Tạp chí KHCN Xây dựng - số 2/2021 43 
Phat Type - S95. Physical and chemical properties 
of all these binders are shown in Table 1. Specific 
gravity of fly ash is the lowest among these binders, 
followed by GGBFS and cement. The particle 
shape of them, which are observed by using 
scanning electron microscopy (SEM), are shown in 
Fig. 1. The shape of fly ash particles is spherical 
with different sizes, while the shape of cement and 
GGBFS are irregular. As seen in Fig. 1b, there are 
some unburnt impurities in SEM image of fly ash, 
this explains for a higher loss on ignition of fly ash 
(6.91%) compared with other binder materials in 
Table 1. This may affect the properties of foam 
concrete as presented later. 
Natural river sand with particle size from 0.15 mm 
to 0.63 mm and density of 2.68 T/m
3
 is used as fine 
aggregate. As aforementioned, properties of foam 
lightweight concrete strongly depend on quality of raw 
materials. In the beginning, natural sand with the size 
from 0.15 mm to 5.0 mm was used, however the 
volume of foam concrete is significant change during 
the forming process and it is difficult to fabricate the 
samples. The use of small sand makes the volume of 
foam concrete is more stable, therefore the sand with 
the size of 0.15 ÷ 0.63 mm is used in this study. 
Foam EABASSOC with density of 1.02 T/m
3
, which is 
original from England and is supplied by Thang Tien 
Company, is used in this study. The ratio of foam to 
water is 1/30 as suggested from producer. The 
superplasticizer (SP) with density of 1.05 T/m
3
 is 
utilized to reduce the water content in all concrete 
mixture.
Table 1. Physical and chemical analysis of binder materials 
Items Cement Fly ash GGBFS 
Physical properties Specific gravity 3.12 2.16 2.82 
Chemical compositions (%) 
SiO2 22.30 55.73 36.87 
Al2O3 6.68 21.67 12.38 
Fe2O3 4.73 6.58 - 
CaO 55.45 1.06 30.73 
MgO 2.40 2.17 14.8 
Loss on ignition 0.45 6.91 0.38 
(a) (b) (c) 
Figure 1. SEM images of (a) cement, (b) fly ash, and (c) GGBFS 
2.2 Mixture proportions 
Six foam concrete mixtures were designed with 
a constant water-to-binder ratio (W/B) of 0.20. 
Proportions of each raw material are showed in 
Table 2. In which, GGBFS is used as 30% amount 
of total binder materials by weight for all mixtures, 
while fly ash is used as 10 and 20% amount of total 
binder materials in mixtures M3, M5, M6, and 
mixtures M1, M2, and M4, respectively. In order to 
make foam concrete samples with various density, 
the foam content is used as from 22.4% to 60.8% 
total volume of samples. It is noticed that an 
electronic device was attached to the foam 
generator machine to control the amount of foam 
produced over the time. However, it is hard to 
control the exact the amount of foam produced, 
hence these values presented in Table 2 are 
assumed to equal the total amount of void in the 
samples. In the practice, the exact amount of foam 
used is higher than these values presented in Table 
2 due to the foam bubbles broken in the air during 
the experiment. The effect of foam content on the 
properties of foam concrete such as compressive 
strength, water absorption, ultrasonic pulse velocity, 
and thermal conductivity is investigated in this study. 
VẬT LIỆU XÂY DỰNG - MƠI TRƯỜNG 
44 Tạp chí KHCN Xây dựng - số 2/2021 
It is noticed that the water-to-binder ratio and 
superplasticizer were selected by trials from 
experiment, so that the mortar paste from cement-fly 
ash-GGBFS and sand has a sufficient workability. 
The plasticity of mortar paste is an important factor 
affecting the success of sample formation. 
Table 2. Mixture proportions 
Mixture ID. W/B 
Proportion ingredients (kg/m
3
) Foam 
(m
3
) 
Cement Fly ash GGBFS Sand Water SP 
 M1 
0.20 
594.4 237.8 356.7 292.7 237.8 1.7 0.224 
M2 499.9 199.9 299.9 249.9 199.9 1.4 0.347 
M3 568.4 94.7 284.2 236.8 189.5 1.3 0.395 
M4 375.8 150.3 225.5 187.9 150.3 1.1 0.509 
M5 394.8 65.8 197.4 164.5 131.6 0.9 0.580 
M6 368.3 61.4 184.2 153.5 122.8 0.9 0.608 
2.3 Specimen preparation and test programs 
Based on Table 2, all materials were prepared 
with their corresponding proportion for mixture. The 
superplasticizer and water were mixed together. Dry 
materials (Fig. 2a) including cement, fly ash, 
GGBFS, and sand were mixed first in three minutes. 
After that the mixture of water and SP were added 
and mixed until achieving homogeneous paste with 
sufficient workability. The foam was created using 
the Foam Master I machine provided by Thang Tien 
Company. Foam was slowly poured into the mortar 
paste with the proportion increase from mixture M1 
to mixture M6 in order to produce concrete samples 
with different density. The mixer was continuously 
run until a uniform mixture was obtained. The steel 
mold with dimension of 100 × 100 × 100 mm was 
used to fabricate the samples. After 24 hours, the 
specimens were demolded and stored at room 
condition until the testing days. The specimens after 
demolding in the laboratory are illustrated in Fig. 2c. 
The wet unit weight of foam concrete was 
immediately tested after the uniform mixture of 
mortar paste and foam was obtained. Compressive 
strength, ultrasonic pulse velocity, and thermal 
conductivity tests were conducted at 7, 14, and 28 
days, while the water absorption, dry unit weight, 
and microstructure of concrete were tested at 28 
days. Each measurement was conducted in three 
samples and the average values are reported herein. 
The microstructure of foam concrete is examined 
using the scanning electron microscopy of Hong 
Duc University. 
(a) (b) (c) 
Figure 2. Specimen preparation (a) drying materials, (b) foam, and (c) concrete samples 
3. Results and Discussion 
3.1 Unit weight and water absorption 
Table 3 shows the wet and dry unit weight of all 
foam concrete mixtures corresponding to the foam 
content. It is clearly seen that as increasing foam 
content results in decreasing both wet and dry unit 
weight of foam concrete. This finding is related to 
the amount of air bubbles existed inside the foam 
concrete, leading to the reduction in concrete 
density. The dry unit weight of foam concrete 
produced in this study reduces from 1553 kg/m
3
 to 
VẬT LIỆU XÂY DỰNG - MƠI TRƯỜNG 
Tạp chí KHCN Xây dựng - số 2/2021 45 
849 kg/m
3
 corresponding to foam content change 
from 22.4% to 60.8% by volume of samples. This 
range is similar to density of foam concrete from 
previous studies [2, 7]. This finding also means that 
with the use of at least 40% foam by volume of 
samples, the density of samples is lower than 1000 
kg/m
3
. Fig. 3 shows the relationship between dry 
unit weight and foam content, which can be 
described by linear equation as follows (Eq. 1). 
18.78 1990y x   (1) 
Opposite trend is observed for water absorption. 
According to Table 3, the water absorption of foam 
concrete increases with increasing foam content. 
The water absorption value changes from 4.0% to 
28.1%, similar to experimental result from Abbas 
and Dunya’s study (from 1% to 26%) [6]. The effect 
of foam content on the water absorption of foam 
concrete is shown in Fig. 4, and Eq. (2) is used to 
describe their relationship. 
0.050.98 xy e
(2) 
Table 3. Unit weight of concrete 
Mixture ID. 
 Foam 
(% by volume) 
Wet unit weight (kg/m
3
) Dry unit weight (kg/m
3
) 
Water absorption 
(%) 
 M1 22.4 1726 1553 4.0 
M2 34.7 1451 1337 4.6 
M3 39.5 1375 1301 5.7 
M4 50.9 1091 986 9.8 
M5 58.0 955 914 17.5 
M6 60.8 891 849 28.1 
 Figure 3. Relationship between dry unit weight and foam content 
 Figure 4. Relationship between water absorption and foam content 
3.2 Compressive strength 
Compressive strength is an important property 
of foam concrete, deciding where it can be used for. 
The compressive strength of all foam concrete in 
this study are presented in Table 4. When the foam 
content changes from 22.4% to 60.8%, the 28-days 
compressive strength of foam concrete decreases 
from 32.5 MPa to 1.9 MPa. It is noticed that the 
10 20 30 40 50 60 70
Foam content (%)
400
600
800
1000
1200
1400
1600
1800
D
ry
 u
n
it
 w
e
ig
h
t 
(k
g
/m
3
)
y=-18.78x+1990
R2=0.98
10 20 30 40 50 60 70
Foam content (%)
0
5
10
15
20
25
30
W
a
te
r 
a
b
s
o
rp
ti
o
n
 (
%
)
y=0.98e0.05x
R2=0.90
VẬT LIỆU XÂY DỰNG - MƠI TRƯỜNG 
46 Tạp chí KHCN Xây dựng - số 2/2021 
compressive strength of foam concrete is 
associated with its dry unit weight [3, 5, 6]. Based on 
previous studies, the compressive strength of foam 
concrete was ranged from 1.07 MPa to 29 MPa 
corresponding to its density from 850 kg/m
3
 to 1600 
kg/
3
 [2, 5]. In the present study, the concrete with 
density of 849 kg/m
3
 (Mixture M6) and 1600 kg/m
3
(Mixture M1) has 28-days compressive strength of 
1.9 MPa and 32.5 MPa, respectively, similar to 
those results from previous studies [2, 5]. The 
reduction in compressive strength is related to the 
air bubbles in foam concrete, which also causes the 
decrease in density of foam concrete as 
aforementioned, this will be clarified later by using 
scanning electron microscopy. Mixtures M1 and M2 
with compressive strength of above 20 MPa, which 
can be used in bearing structure. On the other hand, 
remain mixtures (M3 to M6) with low compressive 
strength can be used as bricks, retaining walls, and 
roof tiles. A linear relationship between the 
compressive strength and foam content (Eq. 3) is 
obtained by linear regression as shown in Fig. 5. 
0.80 48.78y x   (3) 
Table 4. Compressive strength 
Days 
Compressive strength (MPa) 
M1 M2 M3 M4 M5 M6 
 7 18.8 11.0 9.6 3.4 3.0 1.1 
14 22.2 11.7 11.0 3.7 3.2 1.5 
28 32.5 20.3 16.1 3.9 3.8 1.9 
 Figure 5. Relationship between compressive strength and foam content 
3.3 Ultrasonic pulse velocity 
The ultrasonic pulse velocity test is non-
destructive method, which is used to assess the 
relative quality of concrete. Concrete with high value 
of ultrasonic pulse velocity often shows the high 
quality with high strength and high density. The 
value of ultrasonic pulse velocity is also used to 
classify the concrete as mentioned in previous study 
[11]. Table 5 shows the ultrasonic pulse velocity 
values of all foam concrete samples in this study. It 
is clear to see that the concrete with high density 
(low foam content) shows a higher ultrasonic pulse 
velocity value than the concrete with low density 
(high foam content), except Mixtures M4 and M5 at 
7-days age (this could be an error during the 
measurement). It is also noticed that the 
compressive strength and density of foam concrete 
has a close relationship. The higher density, the 
higher compressive strength is. Therefore, the 
ultrasonic pulse velocity value of foam concrete has 
an association with its compressive strength. For 
mixtures M1 to M3 with compressive strength of 
above 15 MPa, their ultrasonic pulse velocity values 
are higher than 3100 m/s. On the contrary, 
ultrasonic pulse velocity values of mixtures M4 to 
M6 with compressive strength of lower than 4 MPa 
are lower than 2500 m/s. Equation (4) shows the 
linear relationship between 28-days ultrasonic pulse 
velocity value of concrete and foam content. 
49.45 5088.3y x  
(4) 
10 20 30 40 50 60 70
Foam content (%)
0
5
10
15
20
25
30
35
C
o
m
p
re
s
s
iv
e
 s
tr
e
n
g
th
 (
M
P
a
)
y=-0.80x+48.78
R2=0.96
VẬT LIỆU XÂY DỰNG - MƠI TRƯỜNG 
Tạp chí KHCN Xây dựng - số 2/2021 47 
Table 5. Ultrasonic pulse velocity 
Days 
Ultrasonic pulse velocity (m/s) 
M1 M2 M3 M4 M5 M6 
 7 3507 2879 2946 2137 2295 1842 
14 3745 3117 3089 2416 2382 1895 
28 4130 3175 3129 2476 2433 2024 
 Figure 6. Relationship between ultrasonic pulse velocity and foam content 
3.4 Thermal conductivity 
Thermal conductivity test is used to assess the 
heat insulation capacity of concrete. The concrete 
with low thermal conductivity value is often utilized 
in the thermal isolated structure. Table 6 shows the 
thermal conductivity values of all foam concrete 
investigated in this study. As well as compressive 
strength, the thermal conductivity of foam concrete 
increases with the time. At 28 days, these values fall 
in the range from 0.263 to 1.410 W/m.K. In general, 
the thermal conductivity value of foam concrete 
decreases with increasing foam content, except 
mixtures M2 and M3. It is noticed that mixture M2 
was designed with 20% fly ash as total binder 
weight, while mixture M3 was designed with only 
10% fly ash. The proportion of mixture may affect to 
the thermal conductivity value of foam concrete, 
which needs to clarify in the future research. Similar 
to dry unit weight, compressive strength, and 
ultrasonic pulse velocity, a negative linear equation 
as shown in Eq. (5) is used to illustrate the 
correlation between thermal conductivity value and 
foam content. The thermal conductivity test was also 
used in previous studies for foam concrete [6, 12]. 
Test results exhibited that the general range of 
thermal conductivity value was from 0.1 to 0.48 
W/m.K. It is noticed that most foam concrete in the 
present study has a thermal conductivity value of 
from 0.263 to 0.679 W/m.K, except mixture M1, 
which has high compressive strength like normal 
concrete. With a low thermal conductivity value, 
these foam concrete in this study can be applied in 
thermal isolated structure such as roof tiles and 
thermal insulation walls.
0.027 1.799y x  
(5) 
Table 6. Thermal conductivity 
Days 
Thermal conductivity (W/m.K) 
M1 M2 M3 M4 M5 M6 
 7 1.334 0.591 0.653 0.365 0.265 0.256 
14 1.347 0.598 0.674 0.370 0.277 0.257 
28 1.410 0.611 0.679 0.388 0.282 0.263 
10 20 30 40 50 60 70
Foam content (%)
0
500
1000
1500
2000
2500
3000
3500
4000
4500
U
lt
ra
s
o
n
ic
 p
u
ls
e
 v
e
lo
c
it
y
 (
m
/s
)
y=-49.45x+5088.3
R2=0.96
VẬT LIỆU XÂY DỰNG - MƠI TRƯỜNG 
48 Tạp chí KHCN Xây dựng - số 2/2021 
 Figure 7. Relationship between thermal conductivity and foam content 
3.5 Scanning electron microscopy observation 
Figure 8 shows the SEM images of all foam 
concrete with magnification of 500 times. For 
three first mixtures (M1 to M3) with 
compressive strength of above 15 MPa, some 
air bubbles are observed in the microstructure 
of concrete. The number of air bubbles 
increases in the last three mixtures (M4 to M6), 
these mixtures has a low compressive strength 
as presented above (lower than 4.0 MPa). It is 
noticed that the former mixtures were designed 
with low foam content, therefore the air volume 
in these mixture is less than the later mixtures. 
For mixture M6, many bubbles connect to each 
other to create the large air bubble. This 
explains why this mixture has really low dry 
unit weight (849 kg/m
3
), low compressive 
strength (1.9 MPa) and high water absorption 
(28.1%). The air bubbles inside concrete is 
contributable to the reduction in density, 
compressive strength, ultrasonic pulse velocity, 
and thermal conductivity, but increases water 
absorption of foam concrete. These SEM 
images are related to those findings about the 
effect of foam content on the properties of 
foam concrete as aforementioned. 
(a) (b) (c) 
(d) (e) (f) 
Figure 8. SEM micrographs of (a) M1, (b) M2, (c) M3, (d) M4, (e) M5, and (f) M6 
10 20 30 40 50 60 70
Foam content (%)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
T
h
e
rm
a
l 
c
o
n
d
u
c
ti
v
it
y
 (
W
/m
.K
)
y=-0.027x+1.799
R2=0.86
VẬT LIỆU XÂY DỰNG - MƠI TRƯỜNG 
Tạp chí KHCN Xây dựng - số 2/2021 49 
4. Conclusions 
This study uses the ternary binders of cement, 
fly ash, and GGBFS in the production of foam 
concrete. The effect of foam content on the 
properties of foam concrete is also investigated in 
this study. Based on the experimental program, 
some brief conclusions may be drawn as follows. 
1) The properties of foam concrete are strongly 
depended on the foam content. As increase in foam 
content, unit weigh, compressive strength, ultrasonic 
pulse velocity, and thermal conductivity of concrete 
reduce, but water absorption of concrete increases. 
2) The concrete with low foam content of 22.4% 
and 34.7% by total sample volume has a 
compressive strength of above 20 MPa, which can 
be used in bearing structure. If increasing the foam 
content to over 35%, its compressive strength 
significantly reduces and it can be just used as 
thermal isolated bricks and roof tiles. 
3) The number of air bubbles inside concrete 
increases with increasing foam content, significantly 
affecting the properties of foam concrete. 
Acknowledgment: 
This research is funded by Hong Duc University 
under grant number 574/HD-DHHD. 
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Ngày nhận bài:23/6/2021. 
Ngày nhận bài sửa: 20/7/2021. 
Ngày chấp nhận đăng: 21/7/2021.