Fly ash is an unwanted unburnt residue of coal combustion in a coal thermal power plant. It is emitted along with flue gases during the burning of coal in a furnace and collected using the electrostatic precipitators.
Fly ash consists primarily of oxides of silicon, aluminum iron and calcium. Magnesium, potassium, sodium, titanium, and sulfur are also present to a lesser degree.
Fly ash is found to be more enriched with several toxic elements as compared to its parent coal because many of trace elements present in parent coal is vaporized during the combustion.
Fly ash is a pozzolan, a substance containing aluminous and siliceous material that form cement in the presence of water. When mixed with lime and water, fly ash forms a compound similar to Portland cement.
A statutory notification by the Ministry of Environment and Forest under the provisions of Environment Protection Act requires 100% utilisation and disposal of fly ash.