Calcium carbonate is found in nature as the mineral limestone. When heated, it undergoes thermal decomposition to produce calcium oxide, also known as lime, and carbon dioxide:
`CaCO_3 -> CaO + CO_2`
Water is then added to calcium oxide to produce calcium hydroxide:
`CaO + H_2O -> Ca(OH)_2`
Some uses of lime are in concrete, mortar and plaster and to remove silicate impurities during the production of steel. Plaster of Paris is a familiar form of lime that people have typically used. It heats up when added to water, showing the exothermic nature of the reaction that forms calcium hydroxide. Calcium hydroxide is also known as slaked lime and is used as an agricultural product to decrease soil acidity.
The decomposition of calcium carbonate is reversible in the presence of oxygen. Hydrated lime products such as brick mortar and cement harden over time as they react with carbon dioxide and form a limestone like material.