Fly Ash is one of the coal combustion products, which is composed of the fine particles driven out of the boiler with the flue gases. The ash that falls in the bottom of the boiler is called bottom ash. In modern coal-fired power plants, fly ash is generally captured by electrostatic precipitators or other particle filtration equipment before the flue gases reach the chimneys. Together with bottom ash it is known as coal ash. The Fly Ash practicaly enhances the functionality of concrete workability and reduces requirement of water.
 
And in concrete the benefits are mainly these:  Permeability of concrete;  Carbonation of concrete;  Stability of concrete dependent on constant cycles of freezing and thawing;  Abrasion and erosion of fly ash concrete; Sulfate resistance of concrete;  Alkali aggregate reactions in concrete;  The erosion of steel reinforcement in concrete; Concrete unprotected in seawater

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