Important compounds of calcium are calcium oxide, calcium hydroxide, calcium sulphate, calcium carbonate and cement. These are industrially important compounds.
The large scale preparation of these compounds and their uses are described below.
Calcium Oxide or Quick Lime, CaO
It is prepared on a commercial scale by heating limestone (CaCO3) in a rotary kiln at 1070-1270 K.
CaCO3 → CaO + CO2
The carbon dioxide is removed as soon as it is produced to enable the reaction to proceed to completion. Calcium oxide is a white amorphous solid.
It has a melting point of 2870 K. On exposure to atmosphere, it absorbs moisture and carbon dioxide.
CaO + H2O → Ca(OH)2
CaO + CO2 → CaCO3
The addition of limited amount of water breaks the lump of lime. This process is called slaking of lime. Quick lime slaked with soda gives solid sodalime. Being a basic oxide, it combines with acidic oxides at high temperature.
CaO + SiO2 → CaSiO3
6CaO + P4O10 → 2Ca3(PO4)2
It is an important primary material for manufacturing cement and is the cheapest form of alkali.
It is used in the manufacture of sodium carbonate from caustic soda.
It is employed in the purification of sugar and in the manufacture of dye stuffs.
Calcium Hydroxide (Slaked lime), Ca(OH)2
Calcium hydroxide is prepared by adding water to quick lime, CaO. It is a white amorphous powder. It is sparingly soluble in water.
The aqueous solution is known as lime water and a suspension of slaked lime in water is known as milk of lime. When carbon dioxide is passed through lime water it turns milky due to the formation of calcium carbonate.
Ca(OH)2 + CO2 → CaCO3 + H2O
On passing excess of carbon dioxide, the precipitate dissolves to form calcium hydrogen carbonate.
CaCO3 + CO2 + H2O → Ca(HCO3)2
Milk of lime reacts with chlorine to form hypochlorite, a constituent of bleaching powder.
2 Ca(OH)2 + 2Cl2 → CaCl3 + Ca(OCl)2 + 2H2O
It is used in the preparation of mortar, a building material.
It is used in white wash due to its disinfectant nature.
It is used in glass making, in tanning industry, for the preparation of bleaching powder and for purification of sugar.
Calcium Carbonate, CaCO3
Calcium carbonate occurs in nature in several forms like limestone, chalk, marble etc. It can be prepared by passing carbon dioxide through slaked lime or by the addition of sodium carbonate to calcium chloride.
Ca(OH)2 + CO2 → CaCO3 + H2O
CaCl2 + Na2CO3 → CaCO3 + 2NaCl
Excess of carbon dioxide should be avoided since this leads to the formation of water soluble calcium hydrogencarbonate. Calcium carbonate is a white fluffy powder.
It is almost insoluble in water. When heated to 1200 K, it decomposes to evolve carbon dioxide.
CaCO3 → CaO + CO2
It reacts with dilute acid to liberate carbon dioxide.
CaCO3 + 2HCl → CaCl2 + H2O + CO2
CaCO3 + H2SO4 → CaSO4 + H2O + CO2
Uses: It is used as a building material in the form of marble and in the manufacture of quick lime. Calcium carbonate along with magnesium carbonate is used as a flux in the extraction of metals such as iron.
Specially precipitated CaCO3 is extensively used in the manufacture of high quality paper. It is also used as an antacid, mild abrasive in tooth paste, a constituent of chewing gum, and a filler in cosmetics.
Calcium Sulphate (Plaster of Paris), CaSO4·½ H2O
It is a hemihydrate of calcium sulphate. It is obtained when gypsum, CaSO4·2H2O, is heated to 393 K.
2(CaSO4.2H2O) → 2(CaSO4).H2O + 3H2O
Above 393 K, no water of crystallisation is left and anhydrous calcium sulphate, CaSO4 is formed. This is known as ‘dead burnt plaster’.
It has a remarkable property of setting with water. On mixing with an adequate quantity of water it forms a plastic mass that gets into a hard solid in 5 to 15 minutes.
The largest use of Plaster of Paris is in the building industry as well as plasters. It is used for immoblising the affected part of organ where there is a bone fracture or sprain. It is also employed in dentistry, in ornamental work and for making casts of statues and busts.
Cement: Cement is an important building material. It was first introduced in England in 1824 by Joseph Aspdin. It is also called Portland cement because it resembles with the natural limestone quarried in the Isle of Portland, England.
Cement is a product obtained by combining a material rich in lime, CaO with other material such as clay which contains silica, SiO2 along with the oxides of aluminium, iron and magnesium.
The average composition of Portland cement is : CaO, 50- 60%; SiO2, 20-25%; Al2O3, 5-10%; MgO, 2- 3%; Fe2O3, 1-2% and SO3, 1-2%. For a good quality cement, the ratio of silica (SiO2) to alumina (Al2O3) should be between 2.5 and 4 and the ratio of lime (CaO) to the total of the oxides of silicon (SiO2) aluminium (Al2O3) and iron (Fe2O3) should be as close as possible to 2.
The raw materials for the manufacture of cement are limestone and clay. When clay and lime are strongly heated together they fuse and react to form ‘cement clinker’.
This clinker is mixed with 2-3% by weight of gypsum (CaSO4·2H2O) to form cement.
Thus important ingredients present in Portland cement are dicalcium silicate (Ca2SiO4) 26%, tricalcium silicate (Ca3SiO5) 51% and tricalcium aluminate (Ca3Al2O6) 11%.
Setting of Cement: When mixed with water, the setting of cement takes place to give a hard mass. This is due to the hydration of the molecules of the constituents and their rearrangement.
The purpose of adding gypsum is only to slow down the process of setting of the cement so that it gets sufficiently hardened.
Uses: Cement has become a commodity of national necessity for any country next to iron and steel. It is used in concrete and reinforced concrete, in plastering and in the construction of bridges, dams and buildings.