Fly ash is a powdery, toxic residue consisting of ultra-fine particles of burned fuel that are emitted along with various gases from the combustion of coal or other substances, such as solid waste, in electric power generating plants. It accounts for roughly half of the volume of coal ash, which also includes bottom ash, which falls to the bottom of the combustion chamber, and it is one of the biggest industrial waste products in terms of volume.
Fly ash consists largely of silicon dioxide, aluminum oxide and calcium oxide along with traces of numerous toxic elements and compounds, including arsenic, beryllium, cobalt, dioxins, gallium, hexavalent chromium, lead, mercury, and polycyclic aromatic hydrocarbons. The exact composition and ratios of substances vary according to the type of coal or other substances burned.
In the developed countries fly ash previously was generally released into the atmosphere, but air pollution standards now mandate that it be captured by using particle filtration equipment. It is commonly stored at coal-fired electric power plants or buried in landfills, which can result in leakage of the toxic substances into the environment.
There are numerous applications for fly ash, particularly for use in construction materials, and a substantial amount is now recycled, especially by using it in concrete. Good quality fly ash can result in stronger and more durable concrete, whereas poor quality fly ash can have the opposite effect.