Usage of Fly Ash in Cement Industry

7/28/2019 Usage of Fly Ash in Cement Industry

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USAGE OF FLY ASH IN CEMENT INDUSTRY

The requirement of power has increased manifold mainly due to increasedpopulation and rapid industrial growth. Nearly 73% of India's total installed capacityis thermal, of which coal-based generation is about 90%. At present about 90 milliontones of fly ash is being generated in India annually. It is envisaged that by the endof 11th five year plan, the fly ash generation in the country is estimated to reach 110- 120 million TPA. India's dependence on coal as a source of energy remainsunchanged. Thus, it is but natural that fly ash management in the country would beimportant and of national concern. Such huge quantities of generation would posechallenging problems, in the form of land usage, health hazards and environmentaldisturbances.

The utilization of fly ash in the country has registered a significant increase from 3 -5% in the late eighties and early nineties to the present level of about 15%.

However, this growth trend is too minimal considering the total quantity generated.Various options of furthering the increased usage of fly ash are being continuouslyreviewed in the country by various government and semi government bodies, whichis resulting in a gradual increase in awareness and acceptance levels of the fly ashbased products and hence gradual increase in fly ash utilization levels.

The use of fly ash as supplementary cementitious material was originally motivatedby its consistent pozzolanic activity and for sustainable development in the cementindustry. However, there is now a growing acceptance that fly ash based cementsprovide several distinct advantages because of which they can be recommended ontheir on merit. The improved understanding of the beneficial properties imparted by

fly ash for both the plastic and hardened stage of blended cement concrete has ledto several builders specifying the use of fly ash based cements for their projects. It isnow beyond doubt that the blended cement concrete either with blending componentas a part in the cement or alternatively as site blended at the ready - Mix locations isthe option for a durable civil structure.

However, the properties of fly ash based cement and its distinct advantages overOPC cements are basically governed by the physico-chemical properties of fly ash.This paper briefly describes the effect of fineness and combustibles of fly ash on theproperties of cement and concrete.

Fly ash used for the manufacture of Portland Pozzolana Cement and as anadmixture in cement mortar and concrete should satisfy the Indian StandardSpecification IS: 3812 - 1981. The physical and chemical requirements of fly ash asper this specification are given in the following tables.


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REQUIREMENTS OF FLY ASH AS PER IS: 3812 - 1981

(SPECIFICATION FOR FLY ASH FOR USE AS POZZOLANA AND ADMIXTURE)

CHEMICAL REQUIREMENTS

Sr.No. Characteristic Requirement

1 SiO2 + Al2O3 + Fe2O3, percent by mass, Min. 70.0

2 Silicon Dioxide (SiO2), percent by mass, Min. 35.0

3 Magnesium Oxide (MgO), percent by mass, Max. 5.0

4 Total sulphur as sulphur trioxide (SO3), percent by mass,Max.

2.75

5 Available alkalis as sodium oxide (Na2O), percent by mass,Max (Note 1)

1.5

6 Loss on ignition (LOI), percent by mass, Max. 12.0

Note 1 - Applicable only when reactive aggregates are used in concrete andare specially requested by the purchaser.

PHYSICAL REQUIREMENTS

Sr.No.

Characteristic RequirementGrade of Fly Ash

I II

1 Fineness - Specific surface in m /kg by Blaine'spermeability method, Min.

320 250

2 Lime Reactivity - Average compressive strength inN/mm2 (M Pa), Min.

4.0 3.0

3 Compressive strength at 28 days (cementreplacement test) in N/mm2 (M Pa), Min.

Not less than 80%of the strength ofcorresponding plaincement mortarcubes.

4 Drying Shrinkage, percent, Max. 0.15 0.10

5 Soundness by autoclave test expansion ofspecimens, percent, Max.

0.8 0.8

Note: Fly ash of grade II may be used for manufacture of Portland PozzolanaCement subject to the requirements of IS: 1489-1991


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IS 1489 (Part 1) : 1991

4.1.2 Fineness and average compressive strength in lime reactivity of flyash that is to be blended with finished Portland cement to produce

Portland Pozzolana Cement, when tested in accordance with the procedurespecified in IS 1727: 1967, shall not be less than 320 m2/kg and 4.0 M Parespectively. Average compressive strength in lime reactivity test of suchfly ash shall be carried out at the fineness at which pozzolana has beenreceived for blending.

4.1.3 Average compressive strength in lime reactivity of fly ash that is to beinterground with Portland cement clinker for manufacture of PortlandPozzolana Cement shall not be less than 4.0 M Pa when tested at thefineness of Portland Pozzolana Cement manufactured out of it or at thefineness in "as received" condition, whichever is greater, tested in

accordance with the procedure specified in IS 1727: 1967.

Before going to the different properties of fly ash, it will be interesting to see howIndian fly ashes vary in their properties.


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INDIAN FLY ASHES

ParameterNo. ofvalues Min. Max. Mean SD c.v.

SiO2 47 50.0 68.9 59.3 3.79 6.4

Al2O3 47 15.1 33.7 26.6 4.06 15.3

Fe2O3 47 3.5 10.2 5.5 1.52 27.5

CaO 47 0.2 6.4 1.5 1.17 79.7

MgO 47 0.1 3.3 1.0 0.78 81.9

LOI 47 0.3 12.2 3.4 3.23 94.8

SO3 45 0.1 1.0 0.2 0.17 80.3

IR 37 78.0 96.1 88.4 4.59 5.2

Na2O 42 0.01 0.40 0.10 0.07 76.8

K2O 43 0.06 1.99 1.07 0.30 27.6

TiO2 21 0.6 1.8 1.4 0.26 19.1

Cl- 11 0.01 0.01 0.01 0.001 13.5

Sp Sur. -m2/kg

37 286 642 388 89.92 23.2

LR- M Pa 40 2.0 8.9 5.4 1.58 2.92

CR - % 37 70 93 84 5.56 6.6


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COMBUSTIBLES IN FLY ASH

Colour of fly ash

The colour of the fly ash depends on the Fe2O3 content and carbon content. Themost significant factor is the unburned coal content corresponding to loss of ignition.The carbon content is responsible for the black or gray appearance of the concrete.The presence of large amount of Fe2O3 (brown) in fly ash contributes to the darkcolour of concrete.

Normal concrete without fly ash admixture is light gray in colour and this isinfluenced by colour of the fine aggregate in concrete. However, in case of highcarbon content fly ashes, it was observed that even with 10% addition the colour ofthe concrete changes to blackish. In case of other fly ashes with around 6-7%carbon content colour of the concrete changes at the level of 25% addition and with

low carbon content fly ashes (LOI 0-2%), the colour of the concrete changes ataround 35-40% addition level.

Water requirement

The amount of water necessary to obtain normal consistency in fly ash concrete/mortar varies considerably in accordance with the carbon content.

In order to assess the water requirement with respect to carbon content, flowabilitystudies were conducted at ACC's research station, RCD Thane, on cement mortarusing flow table test. Cement mortar was prepared with 25% replacement of fly ashat fixed water cement ratio of 0.5%.

In another experiment, the carbon content in a low carbon content fly ash wasincreased by addition of carbon black and the carbon content in the sample wasincreased to 2, 4, 6 & 8% as confirmed by loss on ignition of corresponding fly ashsamples.

It was observed from the both above experiments that up to 4% carbon content flowcharacteristics of fly ash samples are comparable or even better than OPC controlbut at the level of 6% and above levels of carbon contents flow properties of fly ashreduces substantially. Therefore threshold value for carbon content in fly ash may beconsidered as 6%. The details of the experiments and observations are given under:


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CEMENT MORTAR FLOWABILITY TEST TO STUDY THE EFFECTOF CARBON CONTENT

Experiment - 1

Experimental ConditionsA) Control - OPC 53 GradeB) Fly ash replacement - 25%C) Water Cement Ratio - 0.5D) Mixing Procedure - ASTM C-109E) Number of drops on flow table - 25 in 18 seconds

Sr.No.

Sample Details Loss on Ignition offly ash

AverageReading

% Flow

1 OPC Grade (control) - 408 49.7

2 OPC + Fly ash No.1 0.6 402 51.1

3 OPC + Fly ash No.2 0.4 298 76.64 OPC + Fly ash No.3 0.7 429 44.4

5 OPC + Fly ash No.4 2.9 400 51.6

6 OPC + Fly ash No.5 4.0 421 46.7

7 OPC + Fly ash No.6 6.2 486 30.5

8 OPC + Fly ash No.7 6.9 503 26.2

9 OPC + Fly ash No.8 8.8 549 14.9

10 OPC + Fly ash No.9 15.7 562 2.0

11 OPC + Fly ash No.10 13.6 562 2.0

Observation:

1. Upto 4% of carbon content flow characteristics are comparable with OPC2. Drop in flow was observed for 6% land above levels of carbon content

Experiment - 2a) Blend - 1 = Fly ash sample + 1% carbon blackb) Blend - 2 = Fly ash sample + 3% carbon blackc) Blend - 3 = Fly ash sample + 5% carbon blackd) Blend - 4 = Fly ash sample + 7% carbon blackTest Results

Sr. No. Sample Details Loss on

Ignition

Average

reading

% Flow

1 OPC + 25% of Blend 1 2.1 487 30.2

2 OPC + 25% of Blend 2 4.0 501 26.7

3 OPC + 25% of Blend 3 6.0 536 18.2

4 OPC + 25% of Blend 4 8.0 550 14.6


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Observation:Drop in flow was observed beyond 4% increase in loss on ignition.

FINENESS OF FLY ASH

The strength characteristics of fly ash are influenced by its both chemical andphysical properties. In order to have an understanding between fineness of fly ashes(residue on 45- sieve) and water demand, a term called fineness factor has beenintroduced and which is calculated by the following formula.

Fineness Factor (FF) = 10,000/(a X b)

Where, a = water requirement for 110 +/- 5% flow of mortar in percent and b =residue of fly ash on 45 sieve in percent.

Normally the fly ash samples with higher fineness factor are considered to be having

increased reactivity.

Effect of +45 micron & -45 micron fractions of fly ash onquality of cement.

In order to understand behaviour of coarseness of fly ashes, studies were conductedby separating out +45 & -45 fractions from three sources of fly ashes. The -45micron fraction separated from the three fly ashes were blended with OPC cement inthe proportion of 70:30. The +45 coarse fraction separated by sieving fromrespective fly ash samples was ground in the lab ball mill and pass through 45-sieve. These ground fractions were blended with the same OPC cement and in thesame ratio 70:30. The PP cement thus prepared by blending +45 (ground) & -45 as such separated fractions were evaluated for their physical properties.

The reactive silica content in both the fractions (+45 7 -45 ) was evaluated andtest results are tabulated below.

Sample Details Compressive Strength ( M Pa)

1 Day 3 days 7 days 28 days

OPC Control 23.0 34.5 52.0 63.5

PPC with 30% Fly ash - Sample -1 9.0 20.0 27.0 48.5

OPC + 30% (+45 - Fly ash Sample - 1) 9.5 20.0 25.0 37.5OPC + 30% (-45 - Fly ash Sample - 1) 15.5 250 33.5 50.0


OPC + 30% (+45 - Fly ash Sample - 2) 10.5 19.0 24.5 41.5

OPC + 30% (-45 - Fly ash Sample - 2) 12.0 23.5 31.0 48.0


OPC + 30% (+45 - Fly ash Sample - 3) 10.0 20.0 27.5 41.5

OPC + 30% (-45 - Fly ash Sample - 3) 15.5 25.5 33.5 51.0


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REACTIVE SILICA CONTENT - EN METHOD

Sample Details Reactive Silica

Sample No.1, +45 fraction 8.1

Sample No.1, -45 fraction 11.2Sample No.2 +45 fraction 12.4

Sample No.2, -45 fraction 15.0

Sample No.3 +45 fraction 11.2

Sample No.3, -45 fraction 14.0

It can be seen from the above tables that the +45 fraction of fly ashes contributesless to the strength development of PP Cement.

CONCLUSION

The fly ash usage in cement and in other building materials is governed by itsphysical and chemical properties. The quality of fly ashes widely varies from sourceto source due to different type of processes in operation at different power stations.The fineness and the combustibles are the key parameters, which have highinfluence on fly ash quality. Higher fineness and low combustibles leads to higherusage of fly ash. The recently amended IS specification (IS: 1489) for PPC hasallowed usage of fly ash in cement up to 35%. Presently the average usage of flyash in cement is about 18 - 20%. The improvement measures being taken by thepower plants will help in bringing up this level to the maximum possible which will bebeneficial for the efficiencies Power Plant, cement factories and the nation as awhole.

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Usage of Fly Ash in Cement Industry