Calcium Sulphate

What is Calcium Sulphate? 

Calcium sulphate is an inorganic compound which is known for its hydrated forms gypsum and plaster of Paris. Chemical formula of calcium sulphate is CaSO4. It is a highly useful compound in its anhydrous form and hydrated form as well. It is also called drierite. It is poorly soluble in water and causes permanent hardness in water. 

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Structure of Calcium Sulphate 

Calcium sulphate contains one atom of calcium and Sulphur and four atoms of oxygen. It is an ionic compound which constitutes one calcium cation and one sulphate anion (polyatomic ion). Calcium ion has +2 valency while sulphate polyatomic ion shows -2 valency. So, on combining they form a neutral compound calcium sulphate (CaSO4). Structure is given below-

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Hydrous and Anhydrous Forms of Calcium Sulphate

Hydrous Forms of Calcium Sulphate 

Anhydrous Form of Calcium Sulphate 

CaSO4.12 H2O – Calcium Hemihydrate or Plaster of Paris 


CaSO4.2H2O – Calcium dihydrate or Gypsum 

Plaster of Paris and Gypsum 

Plaster of Paris and gypsum are hydrated forms of calcium sulphate. These two are well known hydrated forms of calcium sulphate as these are largely used in various fields especially medicinal and construction fields. 

Plaster of Paris is a white powder which is hydrated calcium sulphate salt and on mixing with water changes into gypsum. Its chemical name is calcium sulphate hemihydrate. It is also known as POP (abbreviated form of Plaster of Paris). It is used by doctors as plaster to support the fractured bones in the right position. It is prepared by heating gypsum at 373K. reaction is given below –

CaSO4.2H2O heat (373K) → CaSO4.12 H2O + 1.5H2

Gypsum                                            Plaster of Paris 

Chemical name of gypsum is calcium sulphate dihydrate. Its chemical formula is CaSO4.2H2O. It is used for protective and decorative coating of walls, ceilings and decorative elements. It cannot be molded into different shapes. It occurs naturally while Plaster of Paris is a man-made material. Although gypsum can also be prepared by plaster of Paris. When water is mixed with Plaster of Paris it becomes hard and forms gypsum. Reaction is given below –

CaSO4.12 H2O + 112H2O 🡪 CaSO4.2H2O

Plaster of Paris                     Gypsum 

Uses of Calcium Sulphate 

Calcium sulphate is used in various ways in numerous fields. Some of its uses are given below-

  • It is used in construction work.

  • It is used in decorative materials.

  • It is used as mortar.

  • It is used in surgical splints, casting, molds and modeling.

  • It is used in fertilizers and soil conditioners.

  • It is used as alabaster in sculptures. 

  • It is a major source of dietary calcium. 

  • It is used for adding hardness to water used for brewing.

  • It is used in baking.

  • It is a component of Portland cement.

  • It is a common ingredient in making mead.

  • It is used in foot creams, shampoos and other products.

  • It is largely used in dentistry.

  • It is used in mushroom cultivation.

  • Gypsum can be used to remove cancerous pollutants such as lead or arsenic from contaminated water. 

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