Calcium oxide is usually made by the thermal decomposition of materials such as limestone, that contain calcium carbonate (CaCO₃; mineral calcite) in a lime kiln. This is accomplished by heating the material to above 825 °C, a process called calcination or lime-burning, to liberate a molecule of carbon dioxide (CO₂); leaving CaO. This process is reversible, since once the quicklime product has cooled, it immediately begins to absorb carbon dioxide from the air, until, after enough time, it is completely converted back to calcium carbonate

Usage

As hydrated or slaked lime, Ca(OH)₂ (mineral portlandite), it is used in mortar and plaster. Hydrated lime is very simple to make as lime is a basic anhydride and reacts vigorously with water. Lime is also used in glass production and its ability to react with silicates is also used in modern metal production industries (steel in particular) to remove impurities as slag.

It is also used in water and sewage treatment to reduce acidity, to harden, 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 acidic soils; and in pollution control, in gas scrubbers to desulfurize waste gases and to treat many liquid effluents. It has traditionally been used in the burial of bodies in open graves, to hide the smell of decomposition, as well as in forensic science, to reveal fingerprints. It is a refractory and a dehydrating agent and is used to purify citric acid, glucose, dyes and as a CO₂ absorber. It is also used in pottery, paints and the food industry. Furthermore, quicklime is used in epidemics, plagues, and disasters to disintegrate bodies in order to help fight the spread of disease. CaO is a key ingredient in the nixtamalization process used to create corn hominy and masa or tortilla dough. In ancient India, before the discovery of soap, it was mixed with sand and used to clean one’s body, while it was also used to build houses.

A relatively inexpensive substance, CaO produces heat energy by the formation of the hydrate, calcium hydroxide, as in the following equation:

CaO (s) + H₂O (l) Ca(OH)₂ (aq) (ΔH_r = −63.7 kJ/mol of CaO)

The hydrate can be reconverted to calcium oxide by removing the water in the reversible equation. If the hydrated lime is heated to redness, the CaO will be regenerated to reverse the reaction. As it hydrates, an exothermic reaction results. One liter of water combines with approximately 3.1 kg of calcium oxide to give calcium hydroxide plus 3.54 MJ of energy. This process can be used to provide a convenient portable source of heat, as for on-the-spot food warming in a self-heating can.

When quicklime is heated to 2,400 °C (4,300 °F), it emits an intense glow. This form of illumination is known as a limelight and was used broadly in theatrical productions prior to the invention of electric lighting.

Annual worldwide production of calcium oxide is around 283 million metric tons. China is by far the world’s largest producer, with a total of around 170 million metric tons per year. The United States is the next largest with around 20 million metric tons per year.

Use as a weapon

Historian and philosopher David Hume, in his history of England, recounts how during early in the reign of Henry III the English Navy destroyed an invading French fleet, by blinding the enemy fleet with “quicklime,” the old name for calcium oxide:

D’Albiney employed a stratagem against them, which is said to have contributed to the victory: Having gained the wind of the French, he came down upon them with violence; and throwing in their faces a great quantity of quicklime, which he purposely carried on board, he so blinded them, that they were disabled from defending themselves.

Also, quicklime is thought to be a component in Greek fire to make it ignite on contact with water