The correct answer is: C. Preventing rapid setting of cement.
Gypsum is added to portland cement to control the setting time of the 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 setting process.
A. Increasing the strength of cement is not the main purpose of adding gypsum to portland cement. However, it does have a slight effect on strength, as it helps to form a more dense and uniform cement paste.
B. Rapid setting of cement is not the desired effect of adding gypsum to portland cement. Gypsum is added to prevent rapid setting, as this can make it difficult to work with the cement and can lead to weak concrete.
D. Reduction in the cost of cement is not the main purpose of adding gypsum to portland cement. However, it does have a slight effect on cost, as gypsum is a relatively inexpensive material.