Question

Gypsum is added to cement because:
A. It makes the cement very hard.
B. It increases the pace of cement setting.
C. It slows down the initial setting of cement when water is added
D. None of the above.

Answer 1

Hint: Gypsum slows down the hydration process of cement once it is mixed with water.

Complete answer:
In the cement manufacturing process clinkers are formed, these cement clinkers are cooled down and added with a small amount of gypsum, after that mixture is sent to the final grinding process.
During this manufacturing process gypsum is added to slow down the hydration process of cement once it is mixed with water, actually when water is added to cement it starts reacting with \[{\text{C3A}}\] (tri calcium aluminates) and get hardens and this process occurs in very less time which does not allow for transporting, mixing and placing, but when gypsum is added then reaction with ${\text{C3A}}$ particles takes place to form ettringite(hydrous calcium aluminum sulfate) resulting in formation of very fine grained crystals which forms a coating on the surface of ${\text{C3A}}$, these crystals are too small to bridge the gaps between the particles of cement hence slows down the initial setting of cement when water is added and if not added the cement will set immediately leaving no time for concrete placing, therefore the cement remains plastic and workable.

We can say that gypsum slows down the initial setting of cement when water is added hence the correct option is C.

Note: Gypsum is a naturally occurring mineral which is mined from the ancient sea beds as raw material, it is composed of calcium sulfate and water which plays a very important role in controlling the hardening rate of cement and retards the setting time that's why also known as retarding agent of cement.