Necessity of Adding Gypsum (CaSO4) in Cement Manufacture
Portland cement is obtained from grinding clinkers and usually sets and hardens immediately after the addition of water. In order to slow down the setting time of cement, gypsum @ 3% to 4% is added during the process of clinker grinding.
If the quantity of gypsum(CaSO4) is more, the cement becomes very slow setting. It also hardens slowly which results in delay the removal of formwork. Gypsum combines with tricalcium aluminate and prevents flash setting.
But, if more gypsum is added with cement, the excess amount after combining with tricalcium aluminate remains free in cement. It expands and makes the cement unsound.
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Chemical Composition of Cement