Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature.

Calcium oxide is used to manufacture of pulp and paper, aluminium, magnesium, calcium carbide and other calcium salts. It has also uses in water treatment, neutralisation of acid waste effluents, insecticides, fungicides, dehairing of hides, sugar refining, poultry feeds, food additive, glass manufacture, refractory, sewerage treatment (phosphate removal, pH control), building and construction materials (bricks, plaster, mortar, stucco and cement), drilling fluids, lubricants, carbon dioxide absorbent and laboratory reagent.