Fly ash or pulverized fuel ash (PFA) is the residue from pulverized coal combustion collected by mechanical or electrostatic flue gas separators or power plants. It constitutes about 75 percent of the total ash produced.
Characteristics and composition
The properties and composition of the Fly ash they vary widely, not only between different plants but also from hour to hour on the same plant. Its composition depends on the type of fuel burned and the load variation in the boiler.
The fly ash obtained from cyclone separators is relatively coarse and contains a large proportion of unburned fuel, while that obtained from electrostatic precipitators is relatively fine and has a specific surface area of about 3,500 cm2/g and can reach 5,000 cm2/g.
It is normally finer than Portland cement. Fly ash is generally composed of spherical particles, some of which may be glass-like, and irregularly shaped particles of unburned fuel or carbon.
It can range in color from light gray to dark gray or even brown.
Properties of a good fly ash
The carbon content in the fly ash is an important consideration for use with cement; it should be as low as possible. The fineness of the fly ash should be as high as possible.
The silica contained in the fly ash must be present in a finely divided state as it slowly combines over a very long period with the lime released during the hydration of the cement.
Setting at a temperature of 38 °C has been found to greatly accelerate its contribution to the strength of concrete.
Setting at high pressure and temperature in an autoclave promotes the reaction between the lime released during the hydration of the 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.
Specifications and requirements
Fly ash consists of spherical glassy particles ranging from 1 to 150 μm, most of which pass through a 45 μm sieve. More than 40 percent of the particles, which are less than 10 microns, contribute to early age strength (7 and 28 days). Particles with sizes from 10 to 45 microns react slowly and are responsible for the increase in strength from 28 days to one year.
Effects of fly ash on cement concrete
| In the amount of water mix | Using fly ash in limited amounts as a replacement for cement or as an addition to cement requires slightly more water for the same slump due to the fineness of the fly ash. It is generally accepted that the use of fly ash, particularly as a mix rather than as a replacement for cement, reduces segregation and bleed. 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. |
| In stress, hardness and compression | Since pozzolanic action is very slow, a fly ash addition of up to 30 percent may result in lower strength at 7 and 28 days, but may be about the same at 3 months and may increase further at older ages. after 3 months, provided that the curing is continued. |
| In the modulus of elasticity | It is less at early ages and greater at later ages. |
| In the setting conditions | It is similar to Portland cement concrete. |
| in concrete shrinkage | Coarser, high-carbon fly ashes are more likely to increase drying shrinkage than finer, low-carbon fly ashes. |
| in the patency | The permeability of concrete is reduced by adding fly ash to the cement. 28 days of pulverized fly ash concrete can be three times more pervious than ordinary concrete, but after 6 months it can be less than one fourth pervious. |
| In resistance to chemical attack | Fly ash slightly improves the resistance of concrete to sulfate attack. |
| in hydration | Fly ash reduces the heat of hydration in concrete. A 30 percent fly ash replacement can result in a 50-60% reduction in heat of hydration. |
| in air drag | The presence of fly ash reduces the amount of air entrainment agent. |
| setting time | A 30 percent replacement of fly ash can result in an increase in initial set time of up to 2 hours. |
Applications
Its general use is to incorporate it into cement and concrete mortar and lime pozzolan mix.
However, only grade 1 is recommended for the manufacture of pozzolan Portland cement.