Advantages of using Fly Ash in Concrete

PPC or site mixing of OPC and fly ash site By

Anil Banchhor, S. Krishnan

The Associated Cement Companies Limited, Cement House, 121, M. K.

Road, Mumbai, 100 020

The paper presents the results of an experimental investigation that was carried out

to evaluate the effectiveness of key performance characteristics of concrete

incorporating fly ash by means of, inter-ground PPC and site mixing OPC and fly

ash. A host of properties of concrete were studied. Both drum mixer and pan mixer

were used in the stud to evaluate the mixing efficiency. The results of investigations

indicate that the performance of concrete using Fly Ash is better than the OPC

concrete, especially with respect to durability indicator due to pozzolanic action of

fly leading to pore refinement and denser concrete matrix The beneficial effects of

fly ash are seen more pronounced in case of factory-ground Portland Pozzolona

Cement (PPC) than site mixing of fly ash with OPC under normal Indian

construction site conditions. The inter-grinding of fly ash with clinker and gypsum

maximizes the pozzolanic potential of fly ash with more consistent product with

good control on variability, leading to a better performance of PPC concrete.

1.0 INTRODUCTION

Concrete has become an indispensable construction material and it is now used in

greater quantities than any other material. In the present context durability and

sustainable development are key issues for development. Ordinary Portland cement

has a high calcium base affecting the microclimate of concrete and mortar. The

interface bond between the cement paste and aggregates can be improved with

better pore structure and minimised micro cracks using mineral admixtures like fly

ash granulated blast furnace slag rice husk, silica fume etc. Out of the above, the

use of fly ash has gained prominence due to growing awareness about the benefits

and easy availability of the good quality fly ash, RMC plants and bigger construction

sites are using fly ash with OPC in batching plants

An experimental study was conducted to measure the effectiveness of key

performance characteristics of concrete made with OPC, inter-ground PPC and site

mixed OPC & Fly Ash. The comparative performance has been studied for the fresh

and hardened properties of concrete like slump, slump retention, water demand,

compressive strength, , water permeability, drying shrinkage etc. for concrete of

grade M 20 and M-25. The microstructure of the concrete was also studied.

In this study, OPC and the fly ash used were from the same source, as that of

underground PPC. The effect of efficiency of mixing on the properties of concrete

was understood by carrying out mixing in a drum mixer and in a pan mixer The

study will continue for one-year duration.

It has been found that the properties of fresh and hardened concrete of inter-

ground PPC is better than the site blend of OPC and fly ash. One of the major

factors contributing to this appears to be the homogeneity of the blend of fines

particles namely cement and fly ash. The normal drum mixer will not be able to

produce a highly homogenous mix of fine powders whereas, inter-grinding of fly

ash with clinker and gypsum produces a highly homogenous mix with a high level

of inter-particle contact between cement and fly ash grains. This process also

maximises the pozzolanic potential of the fly ash with more consistent product with

good control on variability, which results in to better performance characteristics of

concrete. The intergrinding makes fly ash particles finer, which imparts additional

reactive surfaces for hydration and improves the overall particle size distribution of

the resultant cement In this respect, The ordinary drum mixer is found to be less

efficient as compared to the Pan mixer with respect to homogeneity.

2.0 EXPERIMENTAL PROGRAMME In this study, normal grades of concrete viz. M20 & M25, which are being used, at

most of the construction sites, have been taken, and the findings can be used

effectively at construction sites.

Another important parameter is the use of Pan mixer and drum Mixer, so that the

construction practice of using batching plant at big projects/ RMC and rotating drum

mixer of smaller sites can be co-related with the performance of the concrete based

on mixing efficiency of mixers.

2.1 Materials for Study

All the cementitious materials from the same source were used to compare the

performance characteristics of concrete made by separate blending and inter-

ground PPC. OPC and PPC from ACC’s Wadi Cement Plant were used for the study

purpose. Fly ash from Raichur Thermal Power Plant confirming to IS 3812-2003 was

used for as a separate mineral additive; because fly ash from the same source and

the same quality is being used at Wadi to manufacture inter-ground PPC. The fly

ash having surface area of 289 sq. m. was sieved through 90micron sieve and

coarse particles were removed to bring the fly ash particle to required specific

surface area of 329 sq. m. as per IS 3812-2003. In making concrete for the study,

crushed granite from New Mumbai area was used as coarse aggregates along with

fine aggregates from river Vaitarna.

2.11 Cement & Fly ash

Source ACC - Wadi Cement Works. Test results given in Table 1. Fly Ash from

Raichur Thermal Plant was tested

And results are given in Table 2.

2.13 Aggregates:

Coarse aggregate of Crushed Granite from New Mumbai

Maximum size Metal 1 10 mm ; Metal 2 20 mm

Fine aggregate

Type: River sand

Zone: II

Retention on 600 micron: 48.0%

Source: Vaitrana Riverbed

Table 1 : Properties of cement Table 2: Properties of fly ash as

per IS 3812-2003

Property PPC OPC-53

Std. Consistency (%)

29.5 27.3

Specific Surface (m2/kg)

351 334

Setting Time (minutes) Initial Final

165 310

210 280

Soundness test Le Chatelier (mm) Autoclave (%)

0.5 Nil

0.5 Nil

Comp. Strength (M Pa) 3 Days

7 Days 28 Days

30.0 43.0 52.0

41.0 52.5 62.0

2.2 Testing of Fresh concrete

Fresh concrete was tested for Slump, slump retention at 30 minutes, bleeding, and

yield. Slump was maintained around and not less than 100mm to represent the

typical conditions prevalent at most small/medium construction sites in the country.

2.3 Testing of Hardened concrete

Hardened concrete testing includes compressive strength at ages 1D, 3D, 7D, 14D,

28D, 56D, 90D , 180D and 360D.. Rapid Chloride penetration Test (RCPT) as per

ASTM C-1202-97, Initial Surface Absorption Test as per (ISAT) as per BS 1881

1970 , Water Permeability as per DIN 1048 1991 , UPV and hammer test, pH of

concrete, and Modulus of elasticity. were carried out for 28D,

2.4 Testing of mortar

Physical Analysis of Fly Ash Specific Gravity 2.14 Residue on 45 micron percent

23

Specific Surface in m2/ kg 289 S. Surface after sieving with 90 micron sieve & before use

329

Lime Reactivity (M Pa) 4.53 Cement Reactivity ( %) 85%

Chemical Analysis of Fly ash Constituent Percentage SiO2 62.1 Al2O3 26.0 Fe2O3 4.5 CaO 2.7 MgO 1.4 IR 86.3 LOI 0.6 Na2O 0.16 K2O 1.54 Chloride 0.003

The mortar separated from concrete by sieving the same through a 4.75 mm sieve

as taken for evaluation. These includes compressive strength and water absorption

of mortar cubes at 1,3, 7 and 28D, 56D, 90D, 180D and 360D,

2.5 Mix Proportions

Two series of mixes, one for M-20 and the other for M-25 grade of concrete were

used. In each grade, concrete with only OPC, only factory ground PPC and site

mixed OPC-fly ash blend were made. Total Cementitious material; content per cubic

metre was kept constant for all mixes in each grade. Corrections were made only

for specific gravity differences. Each of the mixes was made in a typical tilting

drum one bag drum mixer and a lab mixer of 80-litre capacity. The study is on the

performance of factory ground PPC (containing 25% fly ash) and site blended OPC

and fly ash in concrete. OPC was included in the study as a reference. Table -3

Table 3: Mix Proportions

Grade M - 20 Grade M 25

Type of concrete OPC PPC OPC

+ Fly

ash

OPC PPC OPC

+ Fly

ash

Material kg/m3

kg/m3 kg/m3 kg/m3 kg/m3 kg/m3

Cement 325 325 244 357 357 268

Fly ash -- (25%

)

81 -- (25%

)

89

Sand 855 845 845 827 815 815

Coarse aggregate

10 mm

518 518 518 487 487 487

Coarse aggregate

20mm

557 557 557 580 580 580

Water 179 179 179 187 187 187

Water to binder

ratio

0.55 0.55 0.55 0.52 0.52 0.52

3.0 OBSERVATIONS:

3.1 Fresh Concrete Properties:

3.11 Slump: The slump of concrete was measured immediately after discharge

from the mixer and also at 30 minutes. All the mixes were cohesive and the initial

slump of PPC concrete was found to be higher as compared to OPC+FA and OPC

concrete. The slump loss after 30 minutes also found to be lower in case of PPC

concrete. Figures 1 to 4

Figure 1. Slump and Retention M 20 - Drum Mixer

100

120

90

4050

30

0

20

40

60

80

100

120

140

OPC+FA PPC OPC

Type of Concrete

Slum

p m

m

INITIAL

30 MIN

Figure 2: Slump and Retension M 25 -Drum Mixer

90

120

80

4055

30

0

20

40

60

80

100

120

140

OPC+FA PPC OPC

Type of Concrete

Slum

p m

m

INITIAL

30 MIN

Fi gur e 3 . S l ump a nd Re e nt i on M - 2 0 P a n M i x e r

10 5

13 0

10 0

3 05 0

4 0

0

20

40

60

80

100

120

140

OP C+FA P P C OP C

Ty pe of Conc r e t

I N I T

3 0 M I N

Figure 4: Slump and Retention M25- Pan Mixer

90

4055

30

120100

0

20

40

60

80

100

120

140

OPC+FA PPC OPC

Type of Concrete

Slum

p m

m

INITIAL

30 MIN

3.12 Bleeding: In concrete, after compaction in place bleed water comes on the

top, indicating formation of channels, which would affect durability. The bleeding of

PPC concrete was found to be lower than the OPC and the mix of OPC and FA, in

drum mixed concrete. Bleeding of OPC-Fly ash concrete was lower for pan mixer

as compared to the drum mixer. However, with both the mixer types, PPC had the

lowest bleeding. ( see table 4) Allowing the fresh concrete to stand in a measuring

cylinder and removing the bleed water by a pipette after 1hr bleeding was

determined

Table 4: Bleeding in ml after 1 hour . M 20 Drum Mixer M 20 Pan Mixer M 25 Drum Mixer M 25 Pan Mixer OPC+F

A PPC

OPC

OPC+FA

PPC

OPC OPC+FA

PPC

OPC

OPC+FA

PPC

OPC

15 7 17 6 5 11 12 6 15 5 5 10

3.2 Hardened Concrete Properties:

3.21 Compressive strength: Compressive strength of concrete of both the grades is given in Figures 5-8. Strength development of OPC + Fly ash concrete lags at all ages, compared to factory made inter-ground PPC. However, these differences are much less in pan mixed concrete.

Figure 5: Comcrete Compressive Strength M 20 Drum Mixer

5.77

8.69.7

1417

14.216.4

21.3

2831

25

0

5

10

15

20

25

30

35

OPC +FA PPC OPC Type of Concrete

Com

pres

sive

str

engt

h m

Pa

1

3

7

28

Figure 6: Comcrete Compressive Strength M 25 Drum Mixer

5.98

9.210.5

16

20

16

19

25.4

2932

34

0

5

10

15

20

25

30

35

OPC +FA PPC OPC

Type of ConcreteC

ompr

essi

ve S

tren

gth

mPa

13

7 28

Figure 7: Concrete Compressive Strength M 20 Pan Mixer

6.2 79

1315

1817 18

23

3032

27

0

5

10

15

20

25

30

35

OPC +FA PPC OPC

Type of Concrete

Com

pres

sive

Str

engt

h m

Pa

1 3

7

28

F ig ure 8 : C o mcret e C o mp ressive St reng t h M 2 5 Pan M ixer

6.58.6 9.5

1416

201921

27

31 3234

0

5

10

15

20

25

30

35

40

OP C +FA P P C OP C

Ty pe of Conc r e t e

1 3

7

28

3.22 Modulus of elasticity: Modulus of elasticity values of the drum mixed concrete are given in Table. 5 The values were determined on a 150mm dia x 300 m length cylinder in wet condition. Higher E values of PPC was observed because of pore filling effect of fly ash.

3.23 Water permeability: Water permeability tests were carried out as per DIN

1048 specification on 150 mm cube specimens. The specimens were air dried in

laboratory conditions, before being subjected to the tests, as this will be closer to

field conditions of normal concrete. The values of permeability are given in Table 6.

Table 5: Modulus of elasticity and Water Permeability- Drum Mixer 28 day

Type of concrete

Modulus of elasticity in Gpa at 28 days

Water Permeability in mm at 28 days

M-20 OPC + FA 32.9 10 M-20 PPC 34.5 9 M-20 OPC 30.17 12 M25 OPC + FA 32.9 10 M 25 PPC 34.5 8 M25 OPC 32.9 10

3.24 Initial Surface Absorption (ISAT): This test was carried out as per BS

1881 on 150 mm cube specimens. The specimens were kept for open-air sun

drying for 48 hrs, before being subjected to the tests, as this will be closer to field

conditions of normal concrete. The test was carried out on a side surface as the top

surface in a laboratory specimen tends to be rich in paste up to a few mm of depth,

due to trowelling action, while finishing the specimen in the mould. The values of

permeability are given in Table 7

Table 7: Initial surface absorption - Drum Mixer 28 day

Initial surface absorption in ml/ m2/ sec

Type of Concrete

Time of measurement from start in minutes

10 30 60 120 M-20 OPC + FA 0.045 0.03 0.02 0.015M-20 PPC 0.035 0.0250.0180.012M-20 OPC 0.05 0.04 0.0250.018M25 OPC + FA 0.04 0.0250.0180.013M 25 PPC 0.03 0.0220.0150.011M25 OPC 0.043 0.028 0.02 0.015

3.25 Optical Microscopy: Optical microscopic examination of polished sections of concrete brings out the following.

In all the samples hard and partly hydrated, un hydrated cement grains are seen In the case of OPC + Fly ash site mixed concrete, fly ash grains are not well distributed, whereas in the case of PPC concrete, Fly ash grains are well distributed in the mass of concrete.

PPC based concrete is showing better distribution of fly ash particles and

compactness of binder phase

Concrete samples which are made in a pan mixer appears to be comparatively

more homogeneously distributed than that of drum mixer, in reference to

aggregate and binder phases

3.26 pH of concrete: The concrete samples was ground and mixed together before drawing a representative sample. Water was boiled to remove dissolved air and cooled before mixing with the sample. PH meter was used to measure the pH of the solution Table for M-25 grade concrete.

Table for M-25 grade concrete (( or we can also mention that similar results have been obtained for M-25 grade concrete) 3.3 Mortar Properties: Compressive Strength: The compressive strength values of mortar separated from

concrete is given in Table 8.

The trends are similar to those of concrete.

Water absorption: Mortar cubes made from concrete were dried at 110 deg C for

24 hrs. to constant weight. Then these were immersed in water for 30 minutes

and at the end to this period, they were removed, surface dried and weighed. The

Grade of concrete Cementitious mix

Age of hydration

pH of 5% solution

OPC+ Fly ash 7 days 12.128 days 12.1

PPC 7 days 12.128 days 12.2

OPC 7 days 12.328 days 12.5M 20

M 20

M 20

percentage increase in weight is recorded as percentage water absorption. These

were given on Table 9.

The results are in line with the trend observed on concrete, with factory made ppc

having a lower level of absorption.

Table 8: Mortar Compressive strength MPa Drum Mixer Pan Mixer

M 20 M 20 Type 1

day 3 day

7 day 28 day

1 day 3 day 7 day 28 day

OPC + FA 6.3 12.2 16.2 27.6 7 14 17.5 29 PPC 7.2 14.2 19.1 33.6 7.5 15.8 21.2 35 OPC 7.6 17 24 34 7.8 18 25 36

M 25 M 25 OPC + FA 6.5 13 17 31 7.5 14 18 32 PPC 7.5 15 21 35 7.8 16 23 35 OPC 7.6 17 25 36 8 20 27 36

Table 9. Percentage water absorption of mortar – Drum Mixer

Type of Concrete

Percentage water absorption

M-20 OPC + FA 6.8 M-20 PPC 5.6 M-20 OPC 6.5

M-20 OPC + FA 6.1 M-20 PPC 5.6 M-20 OPC 6.1

4.0 DISCUSSIONS:

4.1 Fresh Concrete Properties

1. Workability: the concrete made with Fly ash show improved workability and this aspect can be used for making concrete of higher strength by reducing the water content. The workability of interground PCC is found to be better than OPC+FA mix because the fly ash particles of PPC become much finer due to intergrinding. The slump retention of PPC is also found to be better than OPC+FA mix and OPC. This is due to retention of water within the finer mass of concrete and improved water holding capacity of the PPC because of improved particle size distribution.

2. Bleeding: The addition of fly ash reduces the rate of bleeding. Since the

fly ash particles are finer than cement particles, they modify the minute space in wet concrete and block the flow of water in the channels and

thus reduce the bleeding. It was also observed that the control of internal bleeding plays an important part in determining the strength of the transition zone between aggregates and the cement paste and therefore the mechanical properties of the concrete.

4.2 Hardened Concrete Properties

1. Compressive Strength: Compressive strength of factory ground

blended cement values were higher than the site mixed concrete and this appears to be due to the finer and desirable particle size distribution of the fly ash particles due to grinding as well as the optimum level of gypsum presence.

2. Elastic Modulus: Values of fly ash blended concrete were higher

probably due to the pore filling effect of the fly ash particles, compared to OPC and this is more pronounced in the case of factory ground PPC due to the enhanced effect of this parameter

3. Water permeability: The values of water permeability are lower for fly

ash blended concrete due to pore refinement and this is more pronounced in factory produced PPC concrete, due to greater secondary reaction products, because of more number of reactive fly ash particles and their desirable particle size distribution.

4. ISAT: The values of initial surface absorption test are lower for fly ash

blended cement concrete due to pore refinement and this is more pronounced in factory produced PPC concrete, due to greater secondary reaction products, because of more number of reactive fly ash particles and their desirable particle size distribution.

4.3 Effect of Intergrinding of Fly Ash with clinker for manufacturing PPC:

1. Improved Sphericity: The intergrinding creates new reactive surfaces for the fly ash and improves the sphericity of the coarse angular fly ash particle. In short it can be attributed to the rounding of corners of fly ash and also breaking of larger particles to smaller sizes having near spherical shapes. This also creates larger surface for reaction as compared to un-ground fly ash.

Spherical fly ash particles Grinding to smaller rounded particles

2. Improved Particle size distribution: The intergrinding of clinker with Fly ash delivers much better particle size distribution as compared to separately mixed fly ash with OPC. Crow & Dungstan concluded that: fineness appeared moré critical for the reactivity of low calcium ash than to those with high calcium fly ash. However apart from than fineness, many other fly ash related variables influence strength development and there in an optimum level of fineness (450 to 480 Blaine) up to which the strength increases. Beyond this the water demand increases and thereby a drop in the strength.

3. Gypsum Content: Gypsum is known to activate the alumnus phase of fly

ash thereby improving pozzolanicity and strength/ durability. During intergrinding, gypsum is added as percentage of total mass of clinker and fly ash. But in the case of site mixed fly ash, extra gypsum is not added towards fly ash and thereby the aluminous phase in not utilised to fullest extent. The site mixing of gypsum is possible at RMC plants, where good quality control is possible which is not available at most of the construction sites, where ordinary drum mixers are used..

4.4 Mixing efficiency of the concrete mixers and the prevalent construction

practice

1 Mixing efficiency: The mixing efficiency of the ordinary drum mixers at

site is poor as compared to Pan mixer / twin shaft mixers of batching

plant, particularly at low water content. The mixing in the ordinary drum

mixer is due to free falling action of the ingredient and the shearing action

of baffle plates are not very effective. Pan mixers and high RPM twin shaft

mixers of batching plant are much better in terms of high efficiency.

2 Quantity of water and mixing time: The quantity of water is not

controlled in most of the small construction sites where drum mixers are

being used and the operator tend to add more water in the mix than

recommended to reduce the mixing time and quickly unload the concrete

for the waiting workers gang to transport it. Masons also prefer concrete

mix of large slump so that their efforts for compaction and finishing are

reduced. Hence the mixer operator is constantly under pressure from the

concreting gang for early unloading of the mix from mixer and the

mason’s demand for higher slump. Hence presence of a supervisor is a

must for control in the quantity of water. In many sites, it has been

observed that as soon as the supervisor leaves mixer location for some

other work, the control on the quantity of water is lost.

3 Addition of required Quantity of fly ash: Strict control on the quantity

of the fly ash also needs to be exercised in case of ordinary drum mixer.

This points to the fact that at smaller construction sites fly ash should not

be added in making concrete with ordinary drum mixer, if a strict quality

control is not possible.

4 Variation in quality of fly ash: Moreover the quality of fly ash will not

be uniform if it is the collected from in different fields and variations may

also be due to variations in quality of coal, grinding & burning conditions

etc.. Therefore testing facility of fly ash as well as expertise in testing is

required before using fly ash at construction sites for proper quality

control.

5.0 CONCLUSIONS:

• It has been found that addition of fly ash to OPC in concrete improves the

properties of fresh concrete and enhance parameters of which indicate

durability. Here also, the properties of fresh and hardened concrete of inter-

ground PCC is better than the site blend of OPC and fly ash.

• The inter-grinding of fly ash with clinker and gypsum maximise the

pozzolanic potential of the fly ash with more consistent product with good

control on variability, which results in to better performance characteristics

of concrete. The variability in type & quality of fly ash, control on doses and

mixing plays a vital role in imparting strength and durability to the concrete.

• The intergrinding makes fly ash particles finer, which imparts additional

reactive surfaces for hydration and improves the particle size distribution of

the resultant cement.

• The site mix fly ash concrete also do not have adequate gypsum, as

compared to inter-ground PCC in which gypsum is added based on the

overall quantity of clinker and fly ash while grinding. The gypsum (SO3) is

know to activate the aluminous phase of the fly ash and it is absent in the

site mixed fly, whether it is a batching plant or a smaller construction site.

• The fly ash should be mixed at site in the batching plant only since mixing

efficiency of the ordinary drum mixers is poor at low water content.

Improper distribution of fly ash particles may result in to some catastrophe.

• Proper precautions on quantity of water and mixing time needs to be taken

care along with strict control on the quantity and quality of the fly ash also

needs to be exercised.

• The testing facility of fly ash as well as expertise in testing is required before

using fly ash at construction sites apart from quality control at site in

batching.

Factory interground PPC takes care of all the problems mentioned above, wherein

fly quality is checked regularly. The performance of the cement is also checked at

regular interval, which takes care of all the ingredients and the product

performance is guaranteed as per the BIS code 1489 part –1. Fly ash should be

added only in the batching plant, where good control on quantity and quality can be

ensured. Ordinary drum mixer should not be used for site mixed fly ash concrete.

References

1. Mehta P. K. & Moteiro Paulo J. M.- Concrete Microstructure, Properties &

Materials

2. IS 456 –2000, Plain & Reinforced Concrete Code of Practice

3. IS 3812 –2003 Pulverised Fuel Ash- Specification

4. Mullick, A. K. Use fo Fly Ash in Structural Concrete: Part II- How much?, The Indian Concrete Journal, June 2005 Vol.. 79, No. 6, PP. 10-14

5. Malhotra V.M. and Mehta P. K. - High-Performance, High Volume Fly Ash

Concrete: Materials, Mixture Proportioning, Properties, Construction Practice and Case Histories

6. Banchhor Anil & S. Krishnan – Performance Evaluation of interground PCC

and site Mixed Fly Ash Concrete, ICI-Asian conference, Mumbai ACECON-2005 Sept.22-25, 2005,

7. Banchhor Anil, Krishnan S. Khadilkar S. A. and Karandikar M. V. Study on

concrete made with PPC, OPC & site mixed fly ash with OPC – 9th NCB international seminar at Delhi 11-13