The correct answer is A. Ordinary Portland cement.
Dicalcium silicate (C2S) is the main constituent of ordinary Portland cement, accounting for about 60-70% of its weight. It is a hard, brittle mineral that reacts with water to form calcium silicate hydrate (C-S-H), the main binding agent in concrete.
Low heat cement is a type of Portland cement that has been modified to produce less heat during hydration. This is done by adding materials such as limestone or gypsum to the cement clinker. Low heat cement is used in applications where heat generation is a concern, such as in mass concrete structures.
Rapid hardening cement is a type of Portland cement that has been modified to accelerate the rate of hydration. This is done by adding materials such as calcium chloride or aluminum sulfate to the cement clinker. Rapid hardening cement is used in applications where early strength is required, such as in precast concrete products.
Sulfate resisting cement is a type of Portland cement that has been modified to resist the attack of sulfates in the environment. This is done by adding materials such as gypsum or anhydrite to the cement clinker. Sulphate resisting cement is used in applications where sulfate exposure is a concern, such as in foundations and marine structures.