Fly ash is a byproduct of coal-fired electric generator plants. The coal is pulverized and blown into a burning chamber where it ignites to heat boiler tubes. Heavier ash particles (bottom ash or slag) fall to the bottom of the burning chamber and the lighter ash particles (fly ash) remain suspended in the exhaust gases. It is the finely divided residue that results from the combustion of pulverized coal and is transported from the combustion chamber by exhaust gases. Before leaving the stack, these fly ash particles are removed by an electrostatic precipitator, or bag house. 

Fly ash is a pozzolan, meaning it is a siliceous or siliceous and aluminous material that, in the presence of water, will combine with an activator (lime, Portland cement or kiln dust) to produce a cementitous material. Currently, approximately 10.5 million tons of coal fly ash are used annually for engineering applications. The greatest volumes of ash are used in cement and concrete products, structural fills, and road bases. Research has demonstrated the many benefits of incorporating fly ash into Portland cement concrete mixes. It must be emphasized that not all benefits will be realized in every case. Fly ashes and Portland cements vary, as do field conditions, but some of the benefits are;

• Significant strength gain
• Improved workability
• Reduced bleeding
• Reduced heat of hydration
• Reduced permeability
• Increased resistance to sulfate attack
• Increases resistance to alkali-silica reactivity
• Lower cost



Some of the cautions that should be observed when using fly ash in concrete are:



• Decreased air entraining ability

• Decreased early strength
• Seasonal limitations
These limitations should not discourage the use of fly ash in concrete

However, they should be considered during design and practice. Evaluate each mix to determine which benefits and cautions apply.