Fly ash is a by-product of coal burning at the utility plants. As coal is burned, non-combustible mineral impurities in coal evaporate andcondense into tiny particles of glass, almost totally spherical inshape. The individual particles are very small, like talcum powderand are carried up and out of the boiler in the flow of exhaust gasesleaving the boiler after the coal is consumed.

The fly ash particlesare removed from the exhaust stream in bag houses or electrostatic precipitators and then stored for later shipment

The difference between fly ash and Portland cement becomes apparentunder a microscope. Fly ash particles are almost totally spherical inshape allowing them to flow and blend freely in mixture. Thatcapability is one of the properties making fly ash adesirable admixture of concrete.