Calcium oxide (CaO), commonly known as lime, quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic and alkaline crystalline solid. As a commercial product lime often also contains magnesium oxide, silicon oxide and smaller amounts of aluminium oxide and iron oxide.
Calcium oxide is usually made by the thermal decomposition of materials, such as limestone, that contain calcium carbonate (CaCO3). This is accomplished by heating the material to around 500°C and so removing the carbon dioxide in a reversible reaction. It is one of the first chemical reactions discovered by man and was known in prehistory: see limekiln.
As hydrated or slaked lime, Ca(OH)2, it was used in mortar and plaster to increase the rate of hardening. Hydrated lime is very simple to make as lime is a basic anhydride and reacts vigorously with water. Lime was also used in glass production and its ability to work with silicates is also used in modern metal production (steel, magnesium, aluminium and other non-ferrous metals) industries to remove impurities as slag.
It is also used in water and sewage treatment to reduce acidity, to soften, as a flocculant and to remove phosphates and other impurities; in paper making to dissolve lignin, as a coagulant and in bleaching; in agriculture to improve acid soils; and in pollution control - in gas scrubbers to desulfurize waste gases and to treat many liquid effluents. It is a refactory and a dehydrating agent and is used to purify citric acid, glucose, dyes and as a CO2 absorber. It is also used in pottery, concrete, paints and the food industry.