Q.Why is gypsum added to cement?

Gypsum is added to cement clinker so its calcium sulphate retards the fast-setting tricalcium aluminate, giving a workable setting time.

Cement clinker contains compounds such as tricalcium aluminate (3CaO.Al2O3), which reacts very rapidly with water ("flash setting"), hardening the cement almost instantly and leaving no time for it to be mixed, transported or placed properly.

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Limestone (CaCO3) is a bulk industrial raw material; Plaster of Paris (CaSO4·½H2O) is valued for its ability to set into a strong, expanding solid when mixed with water.

(i) Limestone (CaCO3):

• Principal raw material for manufacturing Portland cement (heated with clay in a kiln).
• Used in the manufacture of glass (as a source of CaO in the glass batch).
• Used as a flux in the blast furnace extraction of iron, where it reacts with silica (SiO2) impurities in the ore to form slag (CaSiO3), which separates from the molten iron.
• On heating (calcination), limestone gives quicklime (CaO), used in mortar, whitewashing and many other industrial processes.

(ii) Plaster of Paris (CaSO4·½H2O):

• Obtained by heating gypsum (CaSO4·2H2O) at about 393 K.
• Its most important property is that when mixed with water it sets into a hard mass, with a slight expansion, which makes it excellent for taking sharp impressions. …

A small amount of gypsum is mixed with cement to slow down its rate of setting, giving enough working time before it hardens.

Cement is made by heating limestone and clay, and it sets and hardens when mixed with water because its components (mainly calcium silicates and calcium aluminate) react with water (hydration).

If pure cement is used, the tricalcium aluminate present in it reacts with water very rapidly, so the cement would set almost immediately (flash set). This would leave no time to mix the cement, transport it and place it in the required shape.

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