Question

Addition of gypsum to cement:
A. reduces setting time of cement
B. produces very light colour cement
C. increases setting time of cement
D. shining surface is formed

Answer 1

Gypsum is a very good water absorbing molecule. Thus, when it is added to cement, it tends to absorb the water poured on the cement mixture. As we know, the amount of water added decides how fast the cement solidifies.

Complete step by step answer:
Tricalcium aluminate, otherwise known as $C3A$, is one of the major components of cement, especially Portland’s cement. This compound can react readily with water and cause the cement to solidify at a very fast rate. This process is known as flash setting. Let us see why this happens:
$C3A$ dissolves in water to form a type of calcium aluminate hydrate, which will destroy the purpose of cement, as this is a clotting agent. Thus, we need to add something which will prevent this.
As we know, gypsum is a good absorber of water. Thus, it is added to cement to prevent the reaction between $C3A$ and water, and is known as a retarder. Let us see how gypsum works:
When the cement is mixed with water, the gypsum present quickly reacts with the $C3A$ to produce another compound known as calcium sulfoaluminate hydrate. These molecules get deposited and form protective layers over the cement particles, thereby cutting off the reaction between $C3A$ and water. This in turn, leads to a larger setting time.

So, the correct answer is Option C .

Note: The excess addition of gypsum will produce an accelerating effect on the setting time, since gypsum too can produce a clotting agent when present in sufficient amounts. A larger setting time is generally required so that the cement can be moulded to the mason’s wishes. Also note that excessive addition of gypsum can hinder the structural quality of the cement.