Lime

Lime is made by burning limestone or chalk in kilns at over 900 °C. In the heat, the calcium carbonate in the stone gives off carbon dioxide and turns into calcium oxide, or quicklime. Adding water to quicklime releases a great deal of heat and produces slaked lime (calcium hydroxide). Slaked lime has been used for thousands of years in mortar, plaster and whitewash; on the wall it slowly absorbs carbon dioxide from the air and turns back into calcium carbonate, in a sense becoming stone again. Both forms of lime are strong alkalis: they can burn skin and cause lasting eye damage.

Building: Lime mortar and lime plaster have been used in historic buildings for thousands of years, and lime is still found in mortars, plasters and some types of cement. Whitewash: Slaked lime thinned with water is brushed onto walls, giving a cheap, breathable white coating. Industry: It is used in steelmaking to remove impurities, and in the paper and chemical industries. Water treatment: It is used in drinking water and wastewater treatment to adjust acidity and settle out pollutants. Agriculture: It is used to correct acid soils.

Quicklime comes as white or greyish lumps or powder. In contact with water it heats up, gives off steam and falls apart into powder, so it must never be tested with bare hands or without eye protection. Slaked lime is a fine white powder, or a paste mixed with water (lime putty). Limestone is hard and fizzes when a drop of vinegar is added. Fresh lime does not fizz, because it is not a carbonate, although after long exposure to air its surface turns back into carbonate. The white crust that builds up in kettles and taps is popularly called “lime” (limescale), but it is calcium carbonate deposited from hard water. Chemically it is closer to limestone than to lime.

On the “street to diamond” value ladder it sits on step 2 of nine: Raw material by the tonne. Lime is an industrial raw material sold by the tonne. Its value depends on its calcium oxide content (purity), its reactivity (how quickly it slakes with water), its particle size and the purity of the limestone burned. Burning lime takes a great deal of energy, so fuel costs matter too, and the carbon dioxide released from the stone adds to lime’s climate footprint.

Quicklime and slaked lime are strong alkalis: they burn the skin, can cause lasting eye damage, and their dust irritates the airways. Quicklime becomes very hot when it meets water, which can cause burns. When working with lime, wear gloves, safety goggles and a dust mask, and keep lime away from children. If lime gets into the eyes, rinse them at once with plenty of clean water for a long time and seek medical help without delay.