Which of the following is the main ingredient of cement?

Correct answer: (a) Limestone

Explanation

Limestone, chiefly calcium carbonate (CaCO₃), is the main raw ingredient in Portland cement manufacture.

It supplies calcium, which combines with silica, alumina and iron during firing in the kiln.

Gypsum is added only in a small amount at the end to control setting time, not as the main ingredient.