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Gypsum (CaSO4·2H2O) is added to Portland cement in order to:

1. Stop setting process
2. Slow down the setting process
3. Fasten the setting process
4. Adjust the viscosity of the slurry

Correct answer: Slow down the setting process

Solution

Gypsum reacts with tricalcium aluminate (Ca3Al2O6) in cement to form calcium sulphoaluminate (ettringite), which forms a protective coating on C3A particles. This slows down the initial rapid hydration (flash setting), giving workers adequate time to use the cement.

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