Use of Fly ash - in cement making etc.:
Fly ash is the mineral residue resulting from the combustion of powdered coal in power generating plants. Fly ash consists of mostly of silicon dioxide, aluminum oxide and iron oxide. It is pozzolanic in nature, meaning it reacts with calcium hydroxide and alkali to form cementitious compounds. In the coal fired power plant the fly ash is collected by using electrostatic precipitators or filter bags. Initially, this collected ash was disposed of in ash ponds or landfills. Once its pozzolanic properties were discovered however, it became useful as a replacement of Portland cement in concrete. Fly ash can replace up to 50% by mass of Portland cement. Generally less expensive than Portland cement, it also presents advantages in a host of applications.
- Fly ash can be used to improve workability and pumpability of concrete. Due to its generally slower rate of hydration, fly ash also lowers the heat of hydration and is important in mass concrete structures, such as large foundations, bridges, and piers.
- High fly ash concrete shows less bleeding and shrinkage than straight cement mixes.
Fly ash is also used as a component in the production of flowable fill, which is used as self-leveling, self-compacting backfill material in lieu of compacted earth or granular fill.
More and more fly ash is being used beneficially as a recycled material, although it is still far from fully utilized as more than 75% of fly ash produced from coal power plants is disposed in landfills. This amounts to hundreds of megatons in India and China, creating environmental problems. Though it is costly to retrofit older coal-fired power plants to filter fly ash from the atmosphere, over time the economic incentive of selling the captured fly ash pays for the initial expense of installation.