Chemical & Physical Properties of Fly Ash

Fly ash is a fine, powdery material and by far the largest-volume coal combustion product. Its particles are spherical in shape, glassy, with a size similar to that of silt. The chemical make-up of fly ash depends primarily on the chemical properties of the coal burned, but also on the grinding equipment, furnace, combustion process, and nitrogen oxide control equipment used. The properties of fly ash are the key factor in determining its potential use in manufacturing and construction applications.
Ash produced from coal-fired power plants is much like volcanic ash. It consists of lime-stone, iron, aluminum, silica sand, and clay. Coal ash contains trace quantities of the oxidized forms of other naturally occurring elements. The same elements exist in soil, rock and coal. These trace elements include arsenic, boron, cadmium, chromium, copper, lead, selenium, and zinc which can have adverse effects on human health if inhaled or ingested in large quantities.  
Types of Fly Ash
The American Society for Testing and Materials classifies fly ash into two categories: Class F and Class C. Combustion of bituminous and anthracite coals produces Class F fly ash. Class F Fly ash is, pozzolanic containing silicon, aluminum, and high levels of iron.
The combustion of subbituminous and lignite coals produces Class C, a pozzolanic and cementitious ash high in calcium or lime. Pozzolanic materials, a term originally applied to volcanic ash, form cement when mixed with water.