Flyash mostly used in fly ash bricks and concrete. the Flyash or pulverized fuel ash is the residue from the combustion of pulverized coal collected by flue gases or power plants. The properties and composition of fly ash vary on different plants. Its composition depends on the type of fuel burnt and on the variation of load on the boiler.
Flyash 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. It is generally finer than Portland cement. Fly ash consists generally of spherical particles, like glass and hollow and of irregularly shaped particles of unburnt fuel or carbon. It may vary in color from light grey to dark grey or even brown.
Fly ash consists of spherical glassy particles ranging from 1 to 150 mm, most of which pass through a 45 mm sieve. More than 40 percent of the particles, which are under 10 microns contribute to early-age strength (7 and 28 days). Particles of sizes 10 to 45 microns react slowly and are responsible for gain in strength from 28 days to one year. Physical and chemical requirements of fly ash are given in IS 3806.
Effects of Fly Ash on Cement Concrete
On Amount of Mixing Water: The use of a limited amount of fly ash as a replacement for cement or as an addition to cement requires slightly more water for the same slump because of the fineness of fly ash. It is generally agreed that the use of fly ash, especially as an admixture rather than replacement of cement, reduces, segregation and bleeding. If sand other than fly ash is found to have beneficial results, For fine sand, it may additionally increase the water requirement for a given workability. 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.
On Strength in Compression: Since the Pozzolanas action is very slow, an addition of fly ash up to 30 percent may result in lower strength at 7 and 28 days, but maybe about equal at 3 months and may further increase at ages greater than 3 months provided curing is continued. On Modulus of Elasticity: It is lower at an early age and higher at later ages.
On Curing Conditions: It is similar to Portland cement concrete. 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.
On Permeability: The permeability of concrete reduces the addition of fly ash to cement. 28 days pulverized fly-ash-concrete maybe three times as permeable as ordinary concrete but after 6 months it may be less than one-quarter permeable.
On-Resistance to Chemical Attack: Fly ash slightly improves the resistance of concrete to sulfate attack.
On Heat of Hydration: Fly ash reduces the heat of hydration in concrete. The substitution of 30 percent of fly ash may result in a reduction of SO-60% heat of hydration.
On Air Entrainment: The presence of fly ash reduces the amount of air-entraining agents. Setting Time: A 30 percent substitution of fly ash may result in an increase of initial setting time up to 2 hours.
Read more : Testing of flyash
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