Fly ash is a by-product of coal burning at the utility plants. As coal is burned, non-combustible mineral impurities in coal evaporate and condense into tiny particles of glass, almost totally spherical in shape. The individual particles are very small, like talcum powder and are carried up and out of the boiler in the flow of exhaust gases leaving the boiler after the coal is consumed. The fly ash particles are 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 apparent under a microscope. Fly ash particles are almost totally spherical in shape allowing them to flow and blend freely in mixture. That capability is one of the properties making fly ash a desirable admixture of concrete.