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AN INNOVATIVE M E T H O D IN P R O D U C I N G
HIGH EARLY STRENGTH PFA CONCRETE
C S Poon
S C Kou
L Lam
Hong Kong Polytechnic University
Hong Kong
ZSLin
Wuhan University of Technology
China
ABSTRACT. The paper introduces a new method (patent pending) in producing high early
strength PFA concrete by accelerating the hydration of the PFA particles. The method
involves the incorporation of a small amount of plaster stone (a kind of anhydrite gypsum), a
natural mineral widely available in some parts of China. The formulated concrete mixes
were allowed to cure in an accelerated environment for a short period of time before normal
water curing. Significant (up to 100%) strength improvement compared to the control mixes
was observed as early as after 3 days' curing. The results showed that the addition of 10%
anhydrite gypsum, by the total weight of cement and fly ash, increased the 3-day compressive
strength by up to 100%) for the mix with high fly ash contents. It also increased the strength at
later ages for these concretes. The results also indicated that the activating effect of anhydrite
gypsum was insignificant to the concrete with low fly ash contents.
Keywords: Fly ash, Concrete, Curing, Steam, Gypsum, Anhydrite, Compressive Strength
D r C S Poon is an Associate Professor at the Department of Civil and Structural Engineering
at The Hong Kong Polytechnic University.
M r S C K o u was a Research Assistant at the Department of of Civil and Structural
Engineering, The Hong Kong Polytechnic University. He is currently a Lecturer at the
Wuhan University of Technology.
M r L L a m is a PhD candidate at the Department of Civil and Structural Engineering, The
Hong Kong Polytechnic University.
Professor Z S L i n is Professor in cement materials at the Wuhan University of Technology.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
132 P o o n , K o u , L a m , L i n
INTRODUCTION
Fly ashes produced by coal-fired power plants are generally suitable for use as a partial
replacement of cement in mortar and concrete. However, the different nature of coal and
combustion processes produce fly ashes with different chemical and physical characteristics.
Studies on the properties of these materials and their influence on mortar and concrete
properties are necessary for their effective use. Recently, fly ash is being increasingly used in
concrete because it lowers the cost and improves the properties of concrete. However, it is
generally regarded that the replacement of Portland cement with fly ash, especially in high
volume, decreases the early strength of concrete.
In general, fly ash improves the properties of concrete in three ways: (l)by reduction of water
requirement for a given slump; (2) by increase of volume of paste thereby improving the
workability and (3) by a pozzolanic reaction between fly ash and Ca(OH) [1]. As pozzolanic
reactions occur at later ages, fly ash contributes little to the early strength of concrete.
2
Different methods have been used to accelerate the pozzolanic reaction between fly ash and
Ca(OH) to increase the early strength of the concrete containing fly ash. Increasing the
fineness of fly ash apparently increases the pozzolanic reactivity of fly ash [2-4]. Elevating
the curing temperature is also beneficial to the early strength development of fly ash concrete
[5-6]. Some studies indicated that the addition of chemical activators can effectively
accelerate or improve the pozzolanic activity of natural pozzolans [7-10]. It was reported that
addition of 3% to 6% gypsum to a low alkali low C3A cement blended with varying amount
(30%-60%) of class F fly ash [11] resulted in a distinct increase in strength, when compared
with the concrete blends without the addition gypsum. Although a few studies were carried
out on the chemical activation of the reactivity of fly ash [12-14], there is little information
available on using anhydrite gypsum as an activating agent.
2
EXPERIMENTAL DETAILS
Material Properties
The materials used to prepare the concrete mixtures were ASTM Type I Portland cement,
ASTM Class F fly ash (low calcium), a crushed granite aggregate, and natural river sand.
Natural anhydrite gypsum from Jiao Cheng, Shanxi, China was used as an activating agent.
The gypsum was ground in a mortar to achieve the desired fineness before use. The chemical
and physical properties of the cement, fly ash and anhydrite gypsum are given in Tables 1
and 2.
Table 1 Chemical composition of cement, fly ash and anhydrite gypsum
MATERIALS
LOSS ON
IGNITION (%)
Si0
(%)
Fe 0
(%)
Cement
Fly Ash
2.97
3.90
19.61
56.79
3.32
5.31
Anhydrite
Gypsum
7.53
1.88
0.02
2
2
3
CaO
(%)
MgO
(%)
S0
(%)
7.33
28.21
63.15
<3
2.54
5.21
2.13
0.68
0.04
37.94
1.57
51.02
A1 0
(%)
2
3
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3
High Early Strength Concrete
133
Table 2 Physical properties of cement, fly ash and anhydrite gypsum
PROPERTIES
CEMENT
ANHYDRITE GYPSUM
FLY ASH
Density
3.16
2.91
2.31
Specific Surface
(cm /g)
3519.5
12503.2
3960
2
Concrete Mix Design
Three series of concrete mixtures were prepared. Series 1 was prepared with 15 to 55% fly
ash (based on the total weight of the cement and fly ash in the concrete), without the addition
of anhydrite gypsum. Series 2 was with 15 to 55% fly ash, and an addition of 10% anhydrite
gypsum. Series 3 was the concrete mixtures containing a fixed fly ash content (35%), and
varied (6% to 14%) anhydrite gypsum contents. All the above percentages were based on the
total weight of the cementitious materials (cement + fly ash) in the concrete mixture. All the
concrete mixtures were designed to have the same cementitious materials content of 500
kg/m , a water content of 150 kg/m , and a sand content of 725 kg/m . The water-tocementitious materials ratio was 0.3. Superplasticizer was used to achieve an adequate
workability of the concretes. The details of the concrete mix design are given in Tables 3.
3
3
3
Specimen P r e p a r a t i o n , C u r i n g a n d Testing
The concrete mixtures were prepared in the laboratory using a pan mixer. 100 x 100 x 100
mm cubes were cast in steel moulds and compacted by a vibrating table. The cubes were
removed from the moulds at 24 hours after casting. The cubes were subjected to a period of
steam curing. The conditions of the steam curing step had been optimized in a preliminary
test in which a 6-hour steam curing at 65 °C produced the best results for the early and later
compressive strength for the concrete mix with 35% fly ash with 10% anhydrite gypsum.
After curing in a steam bath for 6 hours, the cubes was cooled to room temperature, and were
then transferred to a water curing tank at 27 °C, in which the cubes were allowed to continue
to cure until the testing age.
Compression strength test was performed on the cubes using a Denison compression
machine, at the ages of 3, 7, 28 and 90 days.
RESULTS AND DISCUSSION
In this section, the average values of the test results are presented. Each value was obtained
from three measurements.
C o n c r e t e Mixes w i t h o u t Addition of A n h y d r i t e G y p s u m
The results of the compression strength test of the concrete mixes without the addition of
anhydrite gypsum are shown in Table 4. In a previous study by the authors [15], the strength
development of the fly ash concretes prepared with the same materials, the same water/binder
ratio, and the same fly ash replacement levals, as the concrete mixes in the present
experiment, but without the initial steam curing was studied.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
134 P o o n , K o u , L a m , L i n
The results are given in Table 5 for reference. When comparing Table 4 and Table 5, it can be
noticed that at the age of 3 days, the initial steam curing step resulted in about 15% increase
in compressive strength, but at the later ages, the beneficial effect of the initial steam curing
step was not so significant.
Table 3 Mix proportion of concrete
SERIES
MIX
WATER
(kg/m )
3
CEMENTITIOUS
MATERIALS*
(kg/m )
3
FLY ASH
CONTENT
(%)
ADDITION
OF
ANHYDRITE
GYPSUM
CRUSHED
GRANITE
(kg/m )
SAND
(kg/m )
725
725
3
3
(%)
1
2
3
500
500
150
150
PCI
PC2
15
25
0
0
1020
970
PC3
150
500
35
0
922
725
PC4
PC 5
CI
C2
C3
C4
C5
Yl
Y2
Y3
Y4
Y5
150
150
150
150
150
150
150
150
150
150
150
150
500
500
500
500
500
500
500
500
500
500
500
500
45
55
15
25
35
45
55
35
35
35
35
35
0
0
10
10
10
10
10
6
8
10
12
14
873
824
977
920
880
831
782
870
888
880
871
863
725
725
725
725
725
725
725
725
725
725
725
725
•Cementitious materials = cement + fly ash
Table 4 Compressive strength of the fly ash concrete mixes without addition of anhydrite
gypsum (cured in steam at 65 °C for 6 hours before water curing)
MIX
PCI
PC2
PC3
PC4
PC5
FLY ASH
(%)
ANHYDRITE
GYPSUM
(%)
3 days
7 days
28 days
90 days
15
25
35
45
55
0
0
0
0
0
59.08
54.24
46.43
39.42
25.75
59.08
54.24
46.43
39.42
25.75
88.47
86.66
78.93
70.79
62.28
107.60
110.30
101.47
94.47
85.20
COMPRESSIVE STRENGTH f (MPa)
c
C o n c r e t e Mixes w i t h 1 0 % A n h y d r i t e G y p s u m
The results of the compression strength test of the concrete mixes with the addition of 10%
anhydrite gypsum are shown in Table 6. Comparing with Table 4, it can be noticed that the
addition of 10% anhydrite gypsum resulted in significant compressive strength increase. The
extent of the increase is dependent on the percentages of fly ash replacement and curing ages.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
High Early Strength Concrete
135
At 3 days, the addition of 10% anhydrite gypsum resulted in 115% increase in compressive
strength for the mix with 55% fly ash.
However, the increase was only 23% for the mix with 15% fly ash. As the curing age
increased, the strength enhanchment effect of anhydrite gypsum became smaller. At 90 days,
the addition of anhydrite gypsum resulted in 14% increase in compressive strength for the
mix with 55% fly ash, but only a few percents for the mixes with lower levels of fly ash
replacement. The percentages of the strength increase are plotted against the curing age in
Figure 1.
Table 5 Compressive strength of the fly ash concrete mixes
(cured in water at 27 °C) [15]
W A T
]
RATI 0
0.3
0.3
0.3
0.3
0.3
F L
^%)
0
15
25
45
55
S H
COMPRESSIVE (MPa) STRENGTH
3 days
64.9
52.1
48.0
34.0
22.3
7 days
75.5
66.4
65.7
49.2
36.4
28 days
86.8
86.0
85.4
71.8
54.7
90 days
95.7
107.6
110.3
94.5
85.2
Figure 1 Compressive strength increase due to the addition of 10% anhydrite gypsum
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136 P o o n , K o u , L a m , L i n
The above results have shown that anhydrite gypsum has an activating effect on the reaction
of the fly ash in concrete. With an addition of 10% anhydrite gypsum and an initial curing at
65 °C, the compressive strength of the mix with 55% fly ash reached 55 MPa at 3 days and 97
MPa at 90 days which was almost the same as that of the plain cement concrete mixes [15] in
which fly ash, anhydrite gypsum and initial steam curing were not used. These results
indicate a new approach for improving the strength properties of high volume fly ash
concrete.
It should be noted that the anhydrite gypsum used in this study had a very small particle size
(12503 cm /g in specific surface area, see Table 2). It may also serve as a binder material
with micro filler effects. As the anhydrite gypsum was used as an addition to the
cementitious materials rather than a replacement, it lowered the actual water/binder ratio,
which may also result in higher compressive strength.
2
O p t i m i z a t i o n of A n h y d r i t e G y p s u m C o n t e n t a n d S t e a m C u r i n g Conditions
The effects of anhydrite gypsum content and steam curing conditions (curing temperature and
curing time) on compressive strength were investigated with the mix with 35% fly ash
replacement. Table 7 shows that 10% anhydrite gypsum resulted in higher compressive
strength than lower or higher levels of addition of anhydrite gypsum. Table 8 shows that the
3-day compressive strength of the specimens with the initial curing at 65 °C was about 50%
higher than that of 45 °C. Table 9 shows that at 65 °C, a period of 6 hours initial curing period
is sufficient. From Tables 8 and 9, it can also be noticed that an initial curing at the
temperature higher than room temperature is necessary and essential when anhydrite gypsum
is used.
Table 6 Compressive strength of the fly ash concrete mixes with addition of 10
anhydrite gypsum (cured in steam at 65 °C for 6 hours before water curing)
M
I
X
CI
C2
C3
C4
C5
ASH(Vo)
15
25
35
45
55
GYPSUM (%)
10
10
10
10
10
COMPRESSIVE STRENGTH (MPa)
90 days
28 days
7 days
117.33
106.13
83.49
121.23
102.97
80.70
109.20
91.40
75.07
101.63
83.30
68.70
97.37
78.86
63.39
/c
3 days
72.62
67.07
63.87
60.42
55.29
SUMMARY
An attempt has been made to increase the early compressive strength of fly ash concrete. The
experimental results showed that anhydrite gypsum has an activating effect on the pozzolanic
reaction of the fly ash in concrete. An addition of 10% anhydrite gypsum increased the 3-day
compressive strength by about 100% for the concrete with high fly ash contents. It also
increased the strength at the later ages for these concretes.
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High Early Strength Concrete
137
Table 7 Effect of anhydrite gypsum content on compressive strength (cured in steam at
65 °C for 6 hours before water curing)
FLY ASH
ANHYDRITE
(%)
GYPSUM (%)
35
35
35
35
35
6
8
10
12
14
Yl
Y2
Y3
Y4
Y5
COMPRESSIVE STRENGTH (MPa)
3 days
7 days
28 days
90 days
57.93
59.92
63.87
55.68
54.35
68.72
70.07
75.07
67.84
64.34
79.56
85.28
91.40
86.45
82.39
99.69
101.06
109.90
109.45
103.65
Table 8 Effect of initial steam curing temperature on compressive strength
(cured in steam for 6 hours before water curing)
FLY ASH
(%)
35
35
35
35
35
ANHYDRITE
GYPSUM (%)
10
10
10
10
10
STEAM CURING
TEMPERATURE
(°C)
COMPRESSIVE STRENGTH
(MPa)
3 days
42.14
56.31
63.87
67.46
67.40
45
55
65
75
85
7 days
54.80
61.59
75.07
78.80
79.74
28 days
90 days
72.46
85.50
91.40
91.92
93.43
97.56
105.75
109.90
107.69
108.22
Table 9 Effect of steam curing time on compressive strength
(cured in steam at 65 °C before water curing)
FLY ASH
(%)
35
35
35
35
35
ANHYDRITE
GYPSUM (%)
10
10
10
10
10
STEAM CURING
TIME (HOUR)
4
6
8
10
12
COMPRESSIVE STRENGTH
(MPa)
3 days
7 days
54.66
63.87
64.79
65.59
66.79
70.38
75.07
76.05
76.32
78.42
28 days 90 days
84.32
91.59
91.89
91.40
93.18
102.46
109.90
108.74
108.54
110.25
In these cases, the initial curing at the temperature higher than room temperature is necessary
and essential. However, the activating effect of anhydrite gypsum is insignificant to the
concrete with low fly ash content.
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138 Poon, Kou, Lam, Lin
The results of present study indicate a new approach for improving the compressive strength
of high volume fly ash concrete. More microstructure work is being carried out to devise the
mechanism for strength enhancement. Also as other properties such as durability of the
concrete containing anhydrite gypsum are not clear, further investigation is needed.
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