JEE Chemistry 2023 Question with Solution

Given: The question asks the purpose of adding gypsum to cement.

Find: The correct role of gypsum in cement.

Gypsum, $\mathrm{CaSO_4\cdot 2H_2O}$, is added to cement to regulate the setting time. Without gypsum, cement would set immediately upon mixing with water, leaving no time for concrete placement.

Gypsum reacts with tricalcium aluminate, $\mathrm{C_3A}$, in cement to form insoluble calcium sulfoaluminates, which slows down the hydration reaction.

This delay ensures sufficient time for the mixing, transportation, and placement of concrete before it hardens. Therefore, gypsum is added to slow down the process of setting. The correct option is D.

Given: Cement contains tricalcium aluminate, and gypsum is added during manufacture.

Find: Why gypsum is added.

If gypsum were absent, cement would undergo very rapid setting when mixed with water. That would make the cement unusable for practical handling.

By reacting with $\mathrm{C_3A}$, gypsum controls this rapid reaction and delays hardening long enough for proper use. Hence, its purpose is not to speed up setting or merely to aid hydration, but to retard the setting process.

So, the correct answer is to slow down the process of setting.