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Fly ash consists of spherical glassy particles ranging from 1 to 150 m, most of which passes
through a 45 m sieve. More than 40 per cent of the particles, which are under 10 microns
contribute to early age strength (7 and 28 day). Particles of sizes 10 to 45 microns reacts slowly
and are responsible for gain in strength from 28 days to one year. Physical and chemical
requirements of fly ash are given in Tables 9.1 and 9.2.
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2w 2X 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.
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X 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.
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2i X It is lower at early ages and higher at later ages.
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X It is similar to Portland cement concrete.
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X 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.