Fly ash or pulverized fuel ash (PFA) is flue gases or residue from the combustion of pulverized coal collected by mechanical or electrostatic separators from power plants. It accounts for about 75 percent of the total ash produced. The properties and composition of fly ash vary from time to time not only among different plants but also within the same plant.
Its composition depends on the type of fuel burned and the variation of load on the boiler. Fly ash from cyclone separators is comparatively coarse and contains a large proportion of unburned fuel, whereas ash from electrostatic precipitators is relatively fine, with a specific surface area of about 3500 cm.2/g and 5000 cm. as high as it can be2/Yes. It is generally finer than Portland cement.
Fly ash usually consists of spherical particles, some of which may be glassy and hollow, and may be irregularly shaped particles of unburned fuel or carbon. It can vary in color from light brown to dark brown or even brown.
Fly ash is supplied in two grades; Grade I And Grade II, The common use there is to incorporate it into cement mortar and concrete, and to lime pozzolana mixtures. However, only Grade I Portland Pozzolana is recommended for the manufacture of cement.
Effect of fly ash on concrete
1. on the amount of water added
Use of fly ash as a cement replacement or as an addition to cement in limited quantities requires slightly more water for the same slump due to the fineness of fly ash. It is generally believed that the use of fly ash, particularly as an admixture rather than as a replacement for cement, reduces separation and bleeding. If the sand is coarse, adding fly ash gives beneficial results; For fine sand, the addition may increase the water requirement for a given working capacity.
2. on strength in compression
Since pozolanic action is very slow, adding up to 30 percent fly ash may reduce strength at days 7 and 28, but may be approximately equal at 3 months and further increase beyond 3 months of age provided treatment is continued.
3. on modulus of elasticity
It is less at a young age and more at a later age.
4. on treatment status
It is similar to Portland cement concrete.
5. on shrinkage of concrete
Coarse fly ash and finer fly ash with higher carbon content are more liable to increase drying shrinkage than those with lower carbon content.
6. on permeability
Addition of fly ash to cement reduces the permeability of concrete. 28 days Pulverized fly-ash-concrete can be three times as permeable as ordinary concrete but after 6 months it can be less than a quarter permeable.
7. on resistance to chemical attack
Fly ash slightly improves the resistance of concrete to sulfate attack.
8. on the heat of hydration
Fly ash reduces the heat of hydration in concrete. The replacement of 30 percent fly ash can result in a 50-60% reduction in the heat of hydration.
9. on air entrance
The presence of fly ash reduces the amount of air permeating agent.
10. on time
A 30 percent replacement of fly ash can result in an increase in initial setting time of up to 2 hours.