The correct answer is: B. Decrease the rate of setting of cement.
Gypsum is added to cement clinker to control the setting time of cement. It does this by reacting with the calcium aluminate trihydrate (C3A) in the cement to form ettringite, which is a gel that takes up space and slows down the hydration process.
Option A is incorrect because gypsum does not increase the tensile strength of cement. In fact, it can slightly decrease the tensile strength of cement.
Option C is incorrect because gypsum does not facilitate the formation of colloidal gel. The formation of colloidal gel is a result of the hydration of the cement clinker, and gypsum does not play a role in this process.
Option D is incorrect because gypsum does not bind the particles of calcium silicate. The particles of calcium silicate are bound together by the hydration products of the cement clinker, and gypsum does not play a role in this process.