Gypsum is added into the Portland Cement to:

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(B) Portland cement consists of $C_3A$ (tricalcium aluminate),which reacts very rapidly with water,causing the cement to set almost immediately. To pr

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Slow down the setting process

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(B) Portland cement consists of $C_3A$ (tricalcium aluminate),which reacts very rapidly with water,causing the cement to set almost immediately. To prevent this flash setting and to allow sufficient time for mixing and placing the concrete,a small amount of gypsum $(CaSO_4 \cdot 2H_2O)$ is added. Gypsum acts as a retarder,which slows down the hydration of $C_3A$,thereby delaying the setting process of the cement.

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