Calcium oxide (formula: CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound.

The broadly used term lime connotes calcium-containing inorganic compounds, in which carbonates, oxides, and hydroxides of calcium, silicon, magnesium, aluminium, and iron predominate.

Calcium oxide that survives processing without reacting in building products, such as cement, is called free lime.

This is accomplished by heating the material to above 825 °C (1,517 °F),[6][7] a process called calcination or lime-burning, to liberate a molecule of carbon dioxide (CO2), leaving quicklime behind.

In 80 BC, the Roman general Sertorius deployed choking clouds of caustic lime powder to defeat the Characitani of Hispania, who had taken refuge in inaccessible caves.

[23] A similar dust was used in China to quell an armed peasant revolt in 178 AD, when lime chariots equipped with bellows blew limestone powder into the crowds.

Although quicklime is not considered a fire hazard, its reaction with water can release enough heat to ignite combustible materials.