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. The broadly used term "
lime" connotes calcium-containing inorganic materials, in which carbonates, oxides and hydroxides of calcium, silicon, magnesium, aluminium, and iron predominate. By contrast, "quicklime" specifically applies to the single chemical compound calcium oxide. Calcium oxide which survives processing without reacting in building products such as cement is called
free lime.