Effect of fly ash content on the compressive strengthdevelopment of concrete

VẬT LIỆU XÂY DỰNG – MÔI TRƯỜNG 
Tạp chí KHCN Xây dựng – số 2/2017 31 
EFFECT OF FLY ASH CONTENT ON THE COMPRESSIVE 
STRENGTHDEVELOPMENT OF CONCRETE 
Dr. NGO SI HUY, MEng.LE THI THANH TAM 
Hong Duc University 
Dr.HUYNH TRONG PHUOC 
Can Tho University 
Abstract: The production and use of ordinary 
Portland cement in concrete havea significant effect 
on the surrounding environment by generating a 
large quantity of carbon dioxide and depletingthe 
natural resource. The objective of this research is to 
partially replace ordinary Portland cement in 
concrete mixture with fly ash, which isa by-
productfrom thermal power plant. The effect of fly 
ash content on compressive strength development 
of concrete is investigated. Three mixtures were 
designed with 10%, 20%, and 30% fly ash 
replacement for cement compared with a control 
mixture. Test results indicate that the workability of 
fresh concrete increases and the unit weight of 
concrete reduces with increasing fly ash content. 
The compressive strength of concrete with 10% fly 
ash is the highest, while that ofconcrete with 30% fly 
ash is the worst. Concrete with 20% fly ash has lower 
compressive strength than control concrete before 28 
days; after 56 days it gets higher. 
Keywords:Ordinary Portland cement, fly ash, 
workability, concrete mass, compressive strength. 
Tóm tắt:Quá trình sản xuất và sử dụng xi măng 
ảnh hưởng lớn đến môi trường xung quanh bởi hàm 
lượng khí thải CO2 và làm cạn kiệt nguồn tài nguyên 
thiên nhiên. Mục đích của nghiên cứu này là thay 
thế một phần xi măng bởi tro bay, một dạng phế thải 
của nhà máy nhiệt điện. Sự ảnh hưởng của hàm 
lượng tro bay lên sự phát triển cường độ chịu nén 
của bê tông được nghiên cứu trong bài báo này. Ba 
hỗn hợp bê tông thiết kế với 10%, 20% và 30% xi 
măng được thay thế bởi tro bay so sánh với hỗn hợp 
bê tông không sử dụng tro bay. Kết quả thí nghiệm 
cho thấy rằng, độ linh động của bê tông tươi tăng và 
khối lượng thể tích của bê tông giảm khi tăng hàm 
lượng tro bay. Hỗn hợp bê tông sử dụng 10% tro 
bay có cường độ nén cao nhất, trong khi hỗn hợp 
bê tông chứa 30% tro bay có cường độ nén thấp 
nhất. Cường độ nén của hỗn hợp bê tông sử dụng 
20% tro bay thấp hơn so với cường độ nén của hỗn 
hợp bê tông không tro bay ở thời điểm trước 28 
ngày tuổi, và cao hơn sau 56 ngày tuổi. 
Từ khóa: Xi măng, tro bay, độ linh động của bê 
tông, khối lượng bê tông, cường độ chịu nén. 
1. Introduction 
Portland cement concrete is a popular 
construction material in the world. Unfortunately, the 
production of and use ofordinary Portland cement 
releases a large amount of carbon dioxide (CO2), 
which is a major contributor to the greenhouse effect 
and the global warming of the planet. Generally, the 
production of each ton of cement releases 
approximately 0.7 ton of CO2 to the environment [1], 
accounting for around 8% of global CO2 emissions 
[2]. Furthermore, cement production process causes 
a depletion of thenatural resource. Therefore, with 
concerning the global sustainable development, it is 
necessary to use supplementary cementitious 
materials (SCM) as a partial or full replacement of 
ordinary Portland cement in concrete. The most 
available SCM world-wide is fly ash, a by-product 
from thermal power plant. 
The effect of fly ash on hardened properties of 
concrete, especially on compressive strength has 
received much attention from researchers; however, 
results are largely different. Naik and Ramme (1990) 
indicated that fly ash could be used to replace up to 
40% cement with improved compressive strength [3]. 
Siddique (2003) showed that the use of fly ashas 
replacement of 40-60% cement in concrete 
decreased its 28-day compressive strength; 
however, its 91-day and 360-day compressive 
strengths were acontinuous and significant 
improvement [4]. Oner et al. (2003) [5], Mohamed 
(2011) [6], and Marthong and Agrawal (2012) [7] 
found out that the optimum amount of fly ash to 
replace a part of cement were 40%, 30%, and 20% 
in their studies, respectively. However, Kayali and 
VẬT LIỆU XÂY DỰNG – MÔI TRƯỜNG 
32 Tạp chí KHCN Xây dựng – số 2/2017 
Ahmed (2013) reported that replacing a part of 
cement with fly ash resulted in a reduction in 
compressive strength of concrete [8]. Recent years, 
Wankhede and Fulari (2014) have shown that 
concrete with 10% and 20% replacement of cement 
with fly ash showed better compressive strength at 
28 days than that of normal concrete without fly ash; 
but in the case of 30% replacement, thecompressive 
strength of concrete decreased [9]. On the contrary, 
Bansal et al. (2015) [10] have reported that 10% 
replacement of cement with fly ash led to a 
reduction in compressive strengthof concrete; while 
20% and 30% replacement resulted in an increase 
in compressive strength. All previous studies 
mentioned above have different results because fly 
ash used in each research possessed different 
physical and chemical properties. It is interesting to 
note that the properties of fly ash concrete are 
strongly dependent on the characteristic of fly ash 
used [11]. 
The primary aim of this research is to investigate 
the effect of raw fly ash content, which is taken from 
Nghi Son coal power plant as a local material, on 
compressive strength development of concrete. Its 
effect on fresh concrete properties is also 
investigated. 
2. Experimental program 
2.1. Material properties 
Ordinary Portland cement used in this research 
was Nghi Son Type-PC40 with a compressive 
strength value of 45 MPa. Fly ash was taken from 
Nghi Son coal power plant. The chemical and 
physical characteristic of cement and fly ash are 
given in Table 1. According to ASTM C618 (2005) 
[12] and TCVN 10302 (2014) [13], fly ash used in 
this research is classified as class-F. It is noted 
that the loss on ignition of fly ash is 15.75% over 
the requirement of 6% and 12% that stipulated by 
ASTM C618 (2005) [12] and TCVN 10302 (2014) 
[13], respectively. That is because fly ash used 
herein is raw material, which is not selected as 
compared with fly ash used in previous studies [3-
5], where the loss on ignition is lower than 2%. 
This means the quality of fly ash used in this study 
is worse than that used in previous studies [3-5]. 
The fine aggregate used was natural sand with 
particle size from 0.15 mm to 5 mm, fineness modulus 
of 2.67, density of 2.62 T/m3, dry rodded weight of 
1.43 T/m3, moisture content of 5.65%, and water 
absorption capacity of 1.4%. The coarse aggregate 
used was stone with the nominal maximum size of 
12.5 mm, density of 2.69 T/m3, dry rodded weight of 
1.41 T/m3, moisture content of 0.05%, and water 
absorption capacity of 0.68%. Figure 1 shows the 
gradation curves for sand and crushed stone. 
Compared with ASTM C33 [14], only the gradation 
curve of sand is conformed to the requirement for fine 
aggregate. That curve of crushed stone has violated 
the requirement for the coarse aggregate. However, 
they are existed as local construction materials and 
does not affect so much to the objective of this 
research because they are used the same for all 
mixtures. The superplasticizer (SP) of Sikament R7 
with a specific gravity of 1.15 is used to reduce water 
dosage and ensure the desired workability. 
Table 1. Physical and chemical analysis of cement and fly ash 
Items Cement Fly ash 
Physical properties Specific gravity 3.12 2.16 
Chemical compositions (%) 
SiO2 22.38 48.38 
Al2O3 5.31 20.42 
Fe2O3 4.03 4.79 
CaO 55.93 2.80 
MgO 2.80 1.41 
Loss on ignition 1.98 15.76 
VẬT LIỆU XÂY DỰNG – MÔI TRƯỜNG 
Tạp chí KHCN Xây dựng – số 2/2017 33 
(a) (b) 
Figure 1. Gradation curve for (a) sand and (b) stone 
2.2 Mixture proportions 
Table 2. Concrete mixture proportions 
Mixture ID. 
Fly ash 
(%) w/b 
Concrete proportion ingredients (kg/m3) 
Cement Fly ash Sand Stone Water SP 
A 0 
0.4 
459.3 0.0 867.5 909.4 179.6 4.6 
B 10 410.7 45.6 861.9 903.5 178.5 4.6 
C 20 362.7 90.7 856.3 897.6 177.3 4.5 
D 30 315.3 135.1 850.8 891.9 176.2 4.5 
Four concrete mixtures were designed in 
according with ACI 211.1 [15] with a constant water-
to-binder (w/b) ratio of 0.4. The proportion of 
concrete ingredients is shown in Table 2. Mixture A 
is a control mixture without fly ash. While 10%, 20%, 
and 30% amount of cement were replaced by fly 
ash in mixtures B, C, and D, respectively. The 
purpose of these designed mixtures is to investigate 
the effect of fly ash content on properties of 
concrete, including concrete unit weight, workability, 
and compressive strength. 
2.3 Specimens preparation and test programs 
The concrete ingredients were mixed in a 
laboratory mixer. The binder materials (cement and 
fly ash) were first mixed with a part of water for a 
couple of minutes. A portion of SP was then added 
gradually to the mixture and mixedfor another 3 
minutes to achieve a homogeneous paste. Then, 
the sand was added to the paste and the mixer was 
allowed to run additional 1 minute then addingthe 
stone, followed by the rest of the mixing water and 
SP. The mixer was run for a further 3 minutes in 
order to obtain a uniform mixture. 
(a) (b) 
Figure 2. Concrete specimens(a) after demolding; and (b) curing in water 
It is noted that this study just only focused on 
investigating the possibility of using raw fly ash in 
the production of concrete samples without 
reinforcement and on evaluating the effect of raw fly 
ash content on the compressive strength 
development of the concrete. Thus, the effect of raw 
fly ash with high loss on ignition on reinforcement 
corrosion will be considered in further research, as 
well as the application of this type of concrete in any 
specific area (structural or non-structural elements) 
will not be discussed in this study. 
Cylindrical concrete specimens with 10 cm in 
diameter and 20 cm in length were prepared in the 
0 1 2 3 4 5
Seive size (mm)
0
20
40
60
80
100
P
er
ce
nt
 p
as
si
ng
 (%
)
Sand
5 6 7 8 9 10 11 12 13
Seive size (mm)
0
20
40
60
80
100
Pe
rc
en
t p
as
si
ng
 (%
)
Stone
VẬT LIỆU XÂY DỰNG – MÔI TRƯỜNG 
34 Tạp chí KHCN Xây dựng – số 2/2017 
laboratory. After one day of casting, they were 
demolded (as shown in Figure 2a) and immersed in 
saturated lime-water (as shown in Figure 2b) at a 
room temperature until the testing age. 
Fresh concrete properties including slump and 
unit weight were determined. The compressive 
strength of hardened concrete was measured using 
a controlled compression machine with a loading 
capacity of 3,000 kN at 3, 7, 14, 28, 56, and 91 days. 
The reported value of compressive strength is the 
average value of three concrete specimens. The 
measurement of slump and compressive strength of 
concrete specimens were performed in accordance 
with ASTM C143 [16] and ASTM C39 [17], 
respectively.It is noted that the compressive 
strength values presented herein were converted to 
equivalent values of cylindrical specimen with 15 cm 
in diameter and 30 cm in length based on TCVN 
3118 (1993) [18]. 
3. Results and Discussion 
3.1 Fresh concrete properties 
Workability and unit weight of fresh concreteare 
given in Table 3. The unit weight decreased with 
increasing fly ash content in theconcrete mixture. 
Since replaced 30% cement by fly ash, concrete unit 
weight reduced to approximate 3%. This is due to 
the low specific gravity of fly ash in comparison with 
that of ordinary Portland cement (Table 1). Thus, 
with the same amount, the volume of fly ash is more 
than that of cement. This leads to a reduction in 
mass of fly ash concrete specimen as increasing fly 
ash replacement level. 
On the other hand, workability of fresh concrete 
increased with increasing of fly ash content. Mixture 
A (without fly ash) and Mixture B (10% fly ash) had 
the same slump value of 20 mm. Further replacing 
cement with fly ash resulted in increasing workability 
of fresh concrete. When fly ash content increased to 
20% (Mixture C), the slump slightly increased to 35 
mm. The slump of fresh concrete significantly 
increased to 70 mm since 30% cement was 
replaced by fly ash (mixture D). This is mainly due to 
the spherical shape of fly ash particles and its 
dispersive ability. Generally, cement particles have 
irregular polygonal shape, while fly ash particles 
have spherical shape with various sizes [19]. The 
spherical shape leads to reduce the friction at the 
aggregate-paste interface, thus increases the 
workability of concrete. Moreover, the paste volume 
of fly ash is greater than that of cement because the 
specific gravity of fly ash is lower than that of 
cement (Table 1). The increase of the paste volume 
leads to the increase of plasticity and cohesion, then 
increase the workability of concrete. This finding is 
in good agreement with previous studies [3,7,20]. 
Table 3. Fresh concrete properties 
MixtureID. Fly ash (%) Slump (mm) Unit weight (T/m3) 
A 0 20 2.55 
B 10 20 2.52 
C 20 35 2.51 
D 30 70 2.48 
3.2 Compressive strength development of 
concrete 
The compressive strength development of 
concrete versus age is presented in Figure 3. As a 
result, concrete with 10% fly ash (Mixture B) showed 
the highest compressive strength, while concrete 
with 30% fly ash (Mixture D) showed the lowest 
compressive strength. Additionally, concrete with 
20% fly ash (Mixture C) had lower compressive 
strength than control concrete (Mixture A) before 28-
day ages, after 56-day ages it got higher.At 3 day 
ages, Mixtures A and B (with low fly ash content) 
had higher compressive strength than Mixtures C 
and D (with high fly ash content). The low 
compressive strength at the early age and the 
increased strength at the later age of fly ash 
concrete are associated with the continuous 
pozzolanic reaction of fly ash in concrete, which only 
starts significantly after one or more weeks [21]. 
The use of fly ash with optimum dosage 
increased the compressive strength was proved in 
previous studies [5,22,23]. The main products of 
cement hydration are calcium silicate hydrate (C-S-
H) gel and calcium hydroxide (Ca(OH)2) (see 
equation (1)). While C-S-H is the main carrier of 
strength in hardened concrete, Ca(OH)2 has 
VẬT LIỆU XÂY DỰNG – MÔI TRƯỜNG 
Tạp chí KHCN Xây dựng – số 2/2017 35 
anegative effect on quality of the hardened concrete 
because of its solubility in water to form cavities and 
its low strength. When fly ash is added, Ca(OH)2 is 
transformed into thesecondary C-S-Hgel as a result 
of pozzolanic reaction (see equation (2)). However, 
if fly ash dosage is added over the optimum value, 
all of it does not enter into the reaction, it acts as 
fine aggregate in the mixture rather than a 
cementitious additive. In other word, the fly ash is 
not used in efficiency. 
Cement hydration: 3 2 2 2 ( , )Cement C S C S H O C S H Ca OH (1) 
Pozzolanic reaction: 22Ca OH SiO C S H (2) 
As can be seen from Figure 4, the quantity 
of fly ash used in this study can be replaced 
upto 20% cement. This amount is lower than 
that in previous published studies (from 40% to 
60%) [3-5]. That is because fly ash used in this 
study is araw material with low quality as 
compared with fly ash used in previous studies 
[3-5]. It means that the optimum fly ash content 
used in concrete as cement replacement is 
dependent on its quality. 
Figure 3. Compressive strength development of hardened concrete 
4. Conclusions 
This paper investigates the effect of using raw 
fly ash taken from Nghi Son coal power plant on the 
properties of concrete. Based on the above 
experimental results, the following conclusions may 
be drawn: 
1) Increasing fly ash content as an ordinary 
Portland cement replacement in the concrete 
mixture resulted in improving the workability of 
fresh concrete and decreasing its unit weight. 
Since 30% weight of cement was replaced by fly 
ash, the unit weight reduced to around 3% and 
workability of concrete increased from 20 mm to 
70 mm. 
2) Concrete with 10% fly ash achieved the highest 
compressive strength, while concrete with30% fly 
ash has the lowest compressive strength among 
all tested concrete. 
3) At the early age, concrete with 20% fly ash 
exhibited lower compressive strength than 
control concrete. However, itgot higher at the 
later age of concrete. This phenomenon is 
mainly associated with the continuous pozzolanic 
reaction of fly ash in concrete. 
4) Fly ash from this source can be used to 
replacefor ordinary Portland cement in concrete 
mixture upto 20% with improved compressive 
strength. 
REFERENCES 
[1] World Business Council for Sustainable 
Development (2009), Cement industry energy and 
CO2 performance: Getting the numbers 
right, Visited 
17.06.2016. 
[2] PBL Netherlands Environmental Assessment 
Agency (2015),“Trend in global CO2 emissions: 2015 
Report”. 
0 10 20 30 40 50 60 70 80 90 100
Age (Days)
10
15
20
25
30
35
40
45
50
C
om
pr
es
si
ve
 s
tre
ng
th
 (M
P
a)
A - 0% FA
B - 10% FA
C - 20% FA
D - 30% FA
VẬT LIỆU XÂY DỰNG – MÔI TRƯỜNG 
36 Tạp chí KHCN Xây dựng – số 2/2017 
[3] Nail T. R., and Ramme B. W. (1990), “Effect of high-
lime fly ash content on water demand, time of set, 
and compressive strength of concrete”, ACI 
Materials Journal, Vol. 87, No. 6, pp. 619-626. 
[4] Siddique R. (2003), “Performance characteristics of 
high-volume class F fly ash concrete”, Cement and 
Concrete Research, Vol. 34, pp. 487-493. 
[5] Oner A., Akyuz S., and Yildiz R. (2004), “An 
experimental study on strength development of 
concrete containing fly ash and optimum usage of fly 
ash in concrete”, Cement and Concrete Research, 
Vol. 35, pp. 1165-1171. 
[6] Mohamed H. A. (2011), “Effect of fly ash and silica 
fume on compressive strength of self-compacting 
concrete under different curing conditions”, Ain 
Shams Engineering Journal, Vol. 2, pp. 79-86. 
[7] Marthong C., and Agrawal T. P. (2012), “Effect of fly 
ash additive on concrete properties”, International 
Journal of Engineering Research and Applications, 
Vol. 2, No. 4, pp. 1986-1991. 
[8] Kayali O., and Ahmed M. S. (2013), “Assessment of 
high volume replacement fly ash concrete – concept 
of performance index”, Construction and Building 
Materials, Vol. 39, pp. 71-76. 
[9] Wankhede P. R., and Fulari V. A. (2014), “Effect of 
fly ash on properties of concrete”, International 
Journal of Emerging Technology and Advanced 
Engineering, Vol. 4, No. 7, pp. 284-289. 
[10] Bansal R., Singh V., and Pareek R. K. (2015), 
“Effect on compressive strength with partial 
replacement of fly ash”, International Journal 
ofEmerging Technologies, Vol. 6, No. 1, pp. 1-6. 
[11] Bilodeau A., and Malhotra M. (2000), “High-volume 
fly ash system: Concrete solution for sustainable 
development”, ACI Material Journal, Vol. 97, No. 1, 
pp. 41-47. 
[12] ASTM C618 (2005):“Standard specification for coal 
fly ash and raw or calcined natural pozzolan for use 
in concrete”. 
[13] TCVN 10302 (2014): “Activity admixture – fly ash for 
concrete, mortar and cement”,Vietnam Ministry of 
Science and Technology. 
[14] ASTM C33 (2003): “Standard specification for 
concrete aggregate”. 
[15] ACI 211.1 (1991):“Standard practice for selecting 
proportions for normal, heavyweight, and mass 
concrete”. 
[16] ASTM C143 (2015):“Standard test method for slump 
of hydraulic-cement concrete”. 
[17] ASTM C39 (2012):“Standard test method for 
compressive strength of cylindrical concrete 
specimens”. 
[18] TCVN 3118 (1993): “Heavyweight concrete – 
Method for determination of compressive strength”, 
Vietnam Ministry of Science and Technology. 
[19] Papadakis V. G. (1999), “Effect of fly ash on 
Portland cement systems – Part I. Low-calcium fly 
ash”, Cement and Concrete Research, Vol. 29, No. 
11, pp. 1727-1736. 
[20] Khatib J. M. (2008), “Performance of self-
compacting concrete containing fly ash”, 
Construction and Building Materials, Vol. 22, No. 9, 
pp. 1963-1971. 
[21] Fraay A. L. A., Bijen J. M., and De Haan Y. M. 
(1989), “The reaction of fly ash in concrete a critical 
examination”, Cement and Concrete Research, Vol. 
19, No. 2, pp. 235-246. 
[22] Memon A. H., Radin S. S., Zain M. F. M., and 
TrottierJ. F. (2002), “Effects of mineral and chemical 
admixtures on high-strength concrete in seawater”, 
Cement and Concrete Research, Vol. 32, No. 3, pp. 
373-377. 
[23] Papadakis V. G., and Tsimas S. (2002), 
“Supplementary cementing materials in concrete – 
Part I. Efficiency and design”, Cement and Concrete 
Research, Vol. 32, No. 10, pp. 1525-1532. 
Ngày nhận bài: 29/5/2017. 
Ngày nhận bài sửa lần cuối:4/7/2017.