Portland cement contains 'X' to enhance the setting time. What is 'X'?

We are asked to identify substance 'X' in Portland cement that enhances (controls) the setting time.

Portland cement is manufactured by heating a mixture of limestone and clay in a rotary kiln. The calcium silicates and aluminates formed during this process would normally set (harden) very quickly when mixed with water. To regulate and slow down this setting process, a small amount of gypsum is added during the grinding stage.

Gypsum has the chemical formula $$CaSO_4 \cdot 2H_2O$$ (calcium sulphate dihydrate). It reacts with tricalcium aluminate ($$C_3A$$) to form ettringite, which coats the aluminate particles and prevents them from reacting too fast with water. This gives the mason adequate working time before the cement hardens.

Now let us examine the options. Option A, $$CaSO_4 \cdot \frac{1}{2}H_2O$$, is Plaster of Paris — it is not added to Portland cement. Option C, anhydrous $$CaSO_4$$, and Option D, $$CaCO_3$$ (limestone), are also not the standard additives used to control setting time. The correct additive is gypsum, $$CaSO_4 \cdot 2H_2O$$.

Hence, the correct answer is Option B.