High volume replacement of cement by processed fly ash

Authors
Prof C B Shah
Professor, CEPT UniversityParth Thaker
Assistant professor, CEPT UniversityMs. Pavni D Pandya
Assistant professor, CEPT University

Abstract

The pilot study was aimed at finding the trends due to replacement of Portland Cement by processed fly ash of compressive strength of standard 70.7 mm mortar cubes at the ages of 3,7,28,56 and 90 days. The percentage replacements were 40, 45, 50 & 55 respectively. Test showed that although the initial strengths were lower than that for only opc, for mix at later age (90 days), the results were close to that of opc.

Body

 
Fly ash is a known material, as a by product of thermal power plant using coal. However few know that it is a by product collected at the electro static precipitator (esp) placed at designed height in chimney flue, and is distinctly different from bottom ash which is collected at the grate of the boiler. Fly ash consists of oxides, the principal ones being SIO₂ & Al₂O₃ and unburnt carbon particles and others. It is established by research that amorphous portion of silica combines with hydration products of ordinary Portland cement under favorable thermal and humidity conditions which are available within the mass of concrete and mortar. Through pozzolan actions, more stable mono calcium silicate (CS) products are formed. Thus the utilization of fly ash in concrete is useful to:
v Reduce the quantity of expensive (energy & cost) cement,
v Reduce environmental pollution,
v Reduce leaching of lime from concrete
v  Make cement concrete structures more dense and thus improve their durability.
IS 3812 grades fly ash in two grades depending upon its chemical composition. Recently IS allows up to 30% replacement of cement by Grade I esp fly ash in structural concrete and the (cement + fly ash) is considered a cementing material or binder in IS 456-2005.
In many countries, fly ash is further processed to reduce the undesirable elements and improve the proportion of amorphous silica. This type of fly ash is used in production high volume fly ash concrete in some countries.     A replacement of cement to the extent of 55% by the processed fly ash is reported with good results.
The Programme
A programme was undertaken to study the TRENDS of rates of gain in compressive strength at ages of 3, 7, 28, 56 and 90 days. As study was viewed as preliminary one, The strength of mortar cubes as per IS 4031 was measured.
Materials
v Ordinary Portland cement,53 grade(opc)          IS 12269
v Water Reducer IS 9103: 1999
v Standard sand in three size IS 650: 1991
v Processed fly ash(pfa) Properties (as per producer) is given below
Test
Unit
Typical Test
Result
Blaine – Specific Surface
m2/kg
340
Lime Reactivity
N/mm2
5.48
Autoclave Expansion
%
0.029
Compressive Strength At 28 days
% OPC
96.14
Loss on Ignition
%
1.60
SiO₂ + Al₂O₃ + Fe₂O₃
%
92.49
SiO₂
%
57.30
MgO
%
2.13
SO₃
%
1.06
Na₂O
%
0.73
Total Chlorides
%
0.029
 
Test data
Standard constituency of OPC as per IS 4031 was found as 29.5% of 70.7 mm mortar cubes were prepared using the water corresponding to the standard constituency. Standard sand in three sizes in proportion according to IS 4031 was used. The flow was measured for cement mortar without any replacement. The flow measured 160mm.
OPC was then replaced by 40, 45, 50, 55% processed fly ash. Quantity of water was maintained constant. The consistency of mortar mixes with fly ash was maintained at 160mm flow by addition of water reducer as shown in Table 1 below.
Table – 1 Shows % of Water Reducer
Cement to fly ash %
% Water Reducer
100:00
00
60:40
1.26
55:45
1.40
50:50
1.46
45:55
1.53
Fifteen specimen mortar cubes were cast for testing twice each in compression at the ages of 3, 7, 28, 56 and 90 days. After casting, the specimens were kept indoors, covered with moist gunny bags for 24 hours. These were then demoulded and transferred to water tank till the time of test. Tank water was replaced by fresh water each week.
Test Result of compressive strength
The test results are shown in Table 2 and Table 3 below. Test results in Table 3 shows the observed strength expressed as percentage of that observed at 28 days for each mix. The measured values N/mm² are also shown in the Table 3 for reference. Table 2 shows percent compressive strength at the ages of test with that for OPC with no replacement taken as 100% at each age of test.
Discussion
It is seen from the test results that water needed to maintain constant consistency increases with increase in replacement of OPC
 (Water binder ratio remaining constant). This may be due to very fine particle sizes of pfa than that of OPC.
 Values of percent compressive strength w.r.t. that at 28 days for all mixes are less than that for the mix containing only OPC and the strength at any age is lower with increase in percent replacement. However absolute gain in strength of mixes with fly ash replacing cements increases with percent of replacement.
From Table 2, it is observed that, the mixes containing pfa replacing OPC exhibit Lower strength than that due to 100% OPC. The gaps in absolute strength of OPC only mix and other mixes become narrower with the age.
It is also seen that at the ages of 56 and 90 days the gain in strength of sample OPC 53 grade used in the programme after 28 days was only 0.32% and 1.789% while other mixes exhibited the strength gain from about 10% to about 14% at 56 days and 23% to 30% at 90 days.
Since the pozzolana activity is slow compared to the hydration process of cement, slow gain in strength is to be expected.
From Table – 3 it is observed that at the age of 90 days all the blended mixes show more than 20% gain in strength whereas the OPC only mix show less than 5% gain. From Table – 2 it is seen that none of the mixes except probably that having 40% OPC replacement exhibits higher strength than that of OPC only mix at 90 days. However, all the blended mixes attained the strengths close to that of OPC at   28 days. With rate of gain of strength observed in this study, it is reasonable to expect that all the mixes would achieve strength equal to that of the OPC mixes at about 150 days to 180 days.
Conclusion
The study showed that replacement of OPC 53 grade cement by pfa up to 55% (may be up to 60%) would be possible.
A study on concrete mixes with replacements up to 60% and testing the specimen’s up to age of 180 days is therefore proposed.

 

Table: 2 Shows Results of Compressive Strength (N/mm2)
Cement: processed Fly Ash Proportion (%)
 
3 Days
7 Days
28 Days
56 Days
90 Days
%
%
%
%
%
100 : 00
 
1
24.89
26.49
36.44
36.80
36.09
2
23.82
28.09
37.16
36.98
38.40
3
22.93
29.33
36.09
36.27
37.16
Average Strength
 
23.88
100%
27.97
100%
36.56
100%
36.68
100%
37.21
100%
60 : 40
 
1
14.04
19.02
30.04
34.13
2
13.87
19.20
28.80
35.91
3
13.33
18.67
29.16
35.38
Average Strength
 
13.75
57.57%
18.96
67.80%
29.33
80.23%
35.14
95.80%
38.10*
102.38%*
55 : 45
 
1
10.13
16.36
28.27
32.00
34.67
2
10.84
17.24
28.98
30.93
35.38
3
10.84
16.00
27.73
30.93
37.33
Average Strength
 
10.61
44.42%
16.53
59.11%
28.33
77.47%
31.29
85.30%
35.79
96.18%
50 : 50
 
1
10.31
15.64
25.78
30.40
31.47
2
9.96
14.40
26.84
29.87
32.71
3
9.24
14.58
25.42
29.33
32.00
Average Strength
 
9.84
41.19%
14.87
53.18%
26.01
71.15%
29.87
81.42%
32.06
86.15%
45 : 55
 
1
9.42
14.93
25.96
28.98
33.42
2
8.18
14.93
25.42
30.22
35.38
3
9.07
14.04
26.49
29.87
33.60
Average Strength
 
8.89
37.22%
14.64
52.33%
25.96
70.99%
29.69
80.94%
34.13
91.72%
* Estimated value due to experimental error
Table: 3 Shows Growth of Compressive Strength (N/mm2) with Age
Cement: processed Fly Ash Proportion (%)
 
3 Days
7 Days
28 Days
56 Days
90 Days
%
%
%
%
%
100 : 00
 
1
24.89
26.49
36.44
36.80
36.09
2
23.82
28.09
37.16
36.98
38.40
3
22.93
29.33
36.09
36.27
37.16
Average Strength
 
23.88
65%
27.97
76%
36.56
100%
36.68
100.32%
37.21
101.78%
60 : 40
 
1
14.04
19.02
30.04
34.13
2
13.87
19.20
28.80
35.91
3
13.33
18.67
29.16
35.38
Average Strength
 
13.75
46.87%
18.96
64.65%
29.33
100%
35.14
119.80%
38.10*
129.89%*
55 : 45
 
1
10.13
16.36
28.27
32.00
34.67
2
10.84
17.24
28.98
30.93
35.38
3
10.84
16.00
27.73
30.93
37.33
Average Strength
 
10.61
37.45%
16.53
58.37%
28.33
100%
31.29
110.46%
35.79
126.36%
50 : 50
 
1
10.31
15.64
25.78
30.40
31.47
2
9.96
14.40
26.84
29.87
32.71
3
9.24
14.58
25.42
29.33
32.00
Average Strength
 
9.84
37.81%
14.87
57.18%
26.01
100%
29.87
114.81%
32.06
123.23%
45 : 55
 
1
9.42
14.93
25.96
28.98
33.42
2
8.18
14.93
25.42
30.22
35.38
3
9.07
14.04
26.49
29.87
33.60
Average Strength
 
8.89
34.25%
14.64
56.39%
25.96
100%
29.69
114.38%
34.13
131.51%
* Estimated value due to experimental error

 

Probable areas of application
 
Depending upon structural requirements related to age of incidence of loads replacement of OPC by pfa can be designed. For example Footing, Raft Foundation, Pile Foundation and like where age of full load and formwork retention is not an issue, 50% or more replacement of OPC by pfa can be done. For other elements, considering the parameters of age-load relationship, actual loads on the members, formwork retention requirement etc. replacement can be 40% to 45%.
The study was carried out on mortar specimens. This suggests that in mortar for masonry and plasters the replacement of the order of 45 to 50% can be worked out.
For non-structural grades of concrete, replacement of 50% appears feasible.
Green Construction can be achieved by these replacements.
Acknowledgement
The authors are thankful to Dr. R.N.Vakil, Director, CEPT University for encouraging the study at the Lab of Faculty of Technology.
Thanks are also due to Mr. S.B.Dangayach, CEO, Sintex, Kalol, for providing OPC & pfa samples.
Thanks are also due to Mr. Rakesh Shah & Bhavesh Shah of Fairmate Chemical Pvt Ltd, Vadodara for providing the water reducing admixture.
The authors are greatful to Ms.Reshma Shah, Head of the Department, U.G.Programme to allow the use of Laboratory.
Mr. Vijay Patel assisted in the programme.

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