Lime is an inorganic material composed primarily of calcium oxides and hydroxides. It is also the name for calcium oxide which is used as an industrial mineral and is made by heating calcium carbonate in a kiln. Calcium oxide can occur as a product of coal-seam fires and in altered limestone xenoliths in volcanic ejecta. The International Mineralogical Association recognizes lime as a mineral with the chemical formula of CaO. The word lime originates with its earliest use as building mortar and has the sense of sticking or adhering. These materials are still used in large quantities in the manufacture of steel and as building and engineering materials (including limestone products, cement, concrete, and mortar), as chemical feedstocks, for sugar refining, and other uses. Lime industries and the use of many of the resulting products date from prehistoric times in both the Old World and the New World. Lime is used extensively for wastewater treatment with ferrous sulfate. The rocks and minerals from which these materials are derived, typically limestone or chalk, are composed primarily of calcium carbonate. They may be cut, crushed, or pulverized and chemically altered. Burning (calcination) of calcium carbonate in a lime kiln above 900 °C (1,650 °F) converts it into the highly caustic and reactive material burnt lime, unslaked lime or quicklime (calcium oxide) and, through subsequent addition of water, into the less caustic (but still strongly alkaline) slaked lime or hydrated lime (calcium hydroxide, Ca(OH)2), the process of which is called slaking of lime. When the term lime is encountered in an agricultural context, it usually refers to agricultural lime, which today is usually crushed limestone, not a product of a lime kiln. Otherwise it most commonly means slaked lime, as the more reactive form is usually described more specifically as quicklime or burnt lime.
Production
In the lime industry, limestone is a general term for rocks that contain 80% or more of calcium or magnesium carbonate, including marble, chalk, oolite, and marl. Further classification is done by composition as high calcium, argillaceous (clayey), silicious, conglomerate, magnesian, dolomite, and other limestones. Uncommon sources of lime include coral, sea shells, calcite and ankerite. Limestone is extracted from quarries or mines. Part of the extracted stone, selected according to its chemical composition and optical granulometry, is calcinated at about 900 °C (1,650 °F) in lime kilns to produce quicklime according to the reaction:
CaCO 3 calcium carbonate → heat CaO calcium oxide + CO 2 carbon dioxide {\displaystyle {\ce {{\overset {calcium~carbonate}{CaCO3}}->[][{\text{heat}}]{\overset {calcium~oxide}{CaO}}+{\overset {carbon~dioxide}{CO2}}}}}
Before use, quicklime is hydrated (combined with water, slaking). Hydrated lime is also known as slaked lime, and is produced according to the reaction:
CaO + H 2 O water ⟶ Ca ( OH ) 2 calcium hydroxide {\displaystyle {\ce {{CaO}+{\overset {water}{H2O}}->{\overset {calcium~hydroxide}{Ca(OH)2}}}}}
Dry slaking is slaking quicklime with just enough water to hydrate the quicklime, but to keep it as a powder; it is referred to as hydrated lime. In wet slaking, a slight excess of water is added to hydrate the quicklime to a form referred to as lime putty. Lime is commonly used as a binding mortar in masonry due to its adhesive properties with bricks and stones. It is also used in whitewashing as a wall-coat to allow the whitewash to adhere the wall.
Cycle
The process by which limestone (calcium carbonate) is converted to quicklime by heating, then to slaked lime by hydration, and naturally reverts to calcium carbonate by carbonation is called the lime cycle. The conditions and compounds present during each step of the lime cycle have a strong influence of the end product, thus the complex and varied physical nature of lime products. An example is when slaked lime (calcium hydroxide) is mixed into a thick slurry with sand and water to form mortar for building purposes. When the masonry has been laid, the slaked lime in the mortar slowly begins to react with carbon dioxide to form calcium carbonate (limestone) according to the reaction:
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