Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal. Fly ash is generally captured byelectrostatic precipitators or other particle filtration equipments before the flue gases reach the chimneys of coal-fired power plants. Since the particles solidify while suspended in the exhaust gases, fly ash particles are generally spherical in shape and range in size from 0.5 µmto 100 µm. They consist mostly of silicon dioxide (SiO₂), which is present in two forms: amorphous, which is rounded and smooth, and crystalline, which is sharp, pointed and hazardous; aluminium oxide (Al₂O₃) and iron oxide (Fe₂O₃). Fly ashes are generally highlyheterogeneous, consisting of a mixture of glassy particles with various identifiable crystalline phases such as quartz, mullite, and various iron oxides.

The ways of fly ash utilization include (approximately in order of decreasing importance):

• Concrete 
• Embankments 
• Grout and Flowable fill production
• Waste stabilization and solidification
• Cement clinkers production - (as a substitute material for clay)
• Mine reclamation
• Stabilization of soft soils
• Road subbase construction
• As Aggregate substitute material (e.g. for brick production)
• Mineral filler in asphaltic concrete