Introduction.
Fly ash or pulverized fuel ash (PFA) is the residue from the combustion of pulverized coal collected by mechanical or electrostatic separators from the flue gases or power plants.
It constitutes about 75 per cent of the total ash produced. The properties and composition of fly ash vary widely, not only between different plants but from hour to hour in the same plant. Its composition depends on type of fuel burnt and on the variation of load on the boiler.
Fly ash obtained from cyclone separators is comparatively coarse and contains a large proportion of unburnt fuel, whereas that obtained from electrostatic precipitators is relatively fine having a specific surface of about 3500 cm2/g and may be as high as 5000 cm2/g. Normally it is rather finer than Portland cement.
Fly ash consists generally of spherical particles, some of which may be like glass and hollow and of irregularly shaped particles of unburnt fuel or carbon. It may vary in colour from light grey to dark grey or even brown. Carbon content in fly ash is important consideration for use with cement; it should be as low as possible. The fineness of fly ash should be as high as possible. The silica contained in fly ash should be present in finely divided state since it combines slowly over a very long period with the lime liberated during the hydration of the cement. Curing at a temperature of 38°C has been found to greatly accelerate its contribution to the strength of concrete.
Curing at high pressure and temperature in autoclave promotes the reaction between the lime liberated during hydration of cement and the silica in the fly ash. However, this reaction should tend to prevent the release of free lime to reduce efflorescence. Fly ash is supplied in two grades; grade I and grade II. There general use is incorporating it in cement mortar and concrete and in lime pozzolana mixture.
However, only grade I is recommended for manufacture of Portland pozzolana cement.
Effect of fly ash on cement concrete.
1.On amount of mixing water.
The use of fly ash in limited amounts as a replacement for cement or as an addition to cement requires a little more water for the same slump because of fineness of the fly ash. It is generally agreed that the use of fly ash, particularly as an admixture rather than as a replacement of cement, reduces, segregation and bleeding. If the sand is coarse the addition of fly ash produces beneficial results; for fine sands, its addition may increase the water requirement for a given workability.
2. On strength in compression.
Since the puzzolanic action is very slow, an addition of fly ash up to 30 per cent may result in lower strength at 7 and 28 days, but may be about equal at 3 months and may further increase at ages greater than 3 months provided curing is continued.
3. On modulus of elasticity.
It is lower at early ages and higher at later ages.
4. On curing conditions.
It is similar to Portland cement concrete. 5. On shrinkage of concrete.
Coarser fly ashes and those having a high carbon content are more liable to increase drying shrinkage than the finer fly ashes and those having a low carbon content.
6. On permeability.
The permeability of concrete reduces on addition of fly ash to cement. 28 days pulverised fly-ash-concrete may be three times as permeable as ordinary concrete but after 6 months it may be less than one quarter permeable.
7. On resistance to chemical attacks.
Fly ash slightly improves the resistance of sulphate attacks.
8. On heat of hydration.
Fly ash reduces the heat of hydration in concrete. A substitution of 30 per cent fly ash may result in a reduction of 50-60% heat of hydration.
9. On air entrainment.
The presence of fly ash reduces the amount of air entraining agent.
10. Setting time.
A 30 per cent substitution of fly ash may result in an increase of initial setting time up to 2 hours.
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