Fly ash. Coloured scanning electron micrograph (SEM) of a coal fly ash particle4. Fly ash is part of the non-combustible waste residue produced in the furnace of coal-fired power stations and is light enough to go up the flue or chimney. All fly ash particles contain silicon dioxide (SiO2) and calcium oxide (CaO). Coal ash particles may contain large quantities of toxic metals, including mercury, arsenic, beryllium, cadmium, chromium, nickel, and selenium. It can also be recycled and used as an additive for concrete, cinder blocks, and asphalt. Magnification: x1890, when printed 10 centimetres wide.