Some Important Compounds of Calcium

Calcium's industrially important compounds are calcium oxide, calcium hydroxide, calcium carbonate, calcium sulphate (Plaster of Paris) and cement — each manufactured on a large scale by processes described below.

Calcium Oxide (Quick Lime), $CaO$

Quick lime is manufactured commercially by heating limestone ($CaCO_3$) in a rotary kiln at 1070–1270 K:

$CO_2$ is continuously removed as it forms so that the reaction can go to completion. Calcium oxide is a white amorphous solid melting at 2870 K; left exposed to the atmosphere, it absorbs both moisture and carbon dioxide:

Adding a limited amount of water to quick lime breaks up the lumps — a process called slaking of lime — and slaking quick lime with soda gives solid soda lime. As a basic oxide, $CaO$ also combines with acidic oxides at high temperature:

$CaO$ is the cheapest available alkali and the primary raw material for cement manufacture; it is used to make sodium carbonate from caustic soda, and in the purification of sugar and the manufacture of dyestuffs.

Calcium Hydroxide (Slaked Lime), $Ca(OH)_2$

Made simply by adding water to quick lime, calcium hydroxide is a white amorphous powder, sparingly soluble in water — its saturated solution is called lime water, while a suspension of the solid in water is called milk of lime. Passing $CO_2$ through lime water turns it milky, as insoluble calcium carbonate precipitates:

Passing further, excess $CO_2$ redissolves this precipitate by converting it to soluble calcium hydrogencarbonate:

Milk of lime also reacts with chlorine gas to give calcium hypochlorite, the active ingredient of bleaching powder:

It is used to make mortar (a building material), as a disinfectant whitewash, and in glass-making, tanning, bleaching-powder manufacture and sugar purification.

Calcium Carbonate, $CaCO_3$

Found naturally as limestone, chalk and marble, calcium carbonate can also be prepared in the laboratory by passing $CO_2$ through slaked lime or by adding sodium carbonate to calcium chloride solution:

Excess $CO_2$ must be avoided in the first route, since it would redissolve the product as soluble calcium hydrogencarbonate. Pure calcium carbonate is a white, fluffy, almost water-insoluble powder that decomposes to release $CO_2$ when heated to 1200 K:

and reacts with dilute acids to liberate $CO_2$:

As marble it is a building material; it is also the source of quick lime, and combined with magnesium carbonate it is used as a flux in extracting metals such as iron. Specially precipitated $CaCO_3$ finds heavy use in high-quality papermaking, and it also serves as an antacid, a mild abrasive in toothpaste, an ingredient of chewing gum, and a filler in cosmetics.

Calcium Sulphate (Plaster of Paris), $CaSO_4\cdot\tfrac{1}{2}H_2O$

This hemihydrate is obtained by heating gypsum, $CaSO_4\cdot 2H_2O$, to 393 K:

Heated further, past 393 K, all the water of crystallisation is driven off to give fully anhydrous calcium sulphate, known as "dead burnt plaster." Plaster of Paris has the distinctive property of setting when mixed with water — an adequately wetted batch forms a plastic mass that hardens into a solid within 5 to 15 minutes. Its largest use by far is in the building and plastering industries; it is also used to immobilise fractured or sprained limbs, and in dentistry, ornamental work and casting statues and busts.

Cement …