Question

Q3: Why is gypsum added to the cement clinker? Typically, how much is the amount of added gypsum? Q4: The presence of high free-lime in Portland cement can lead to unsoundness. What is meant by the term, "unsoundness"? Which other compound can cause unsoundness in Portland cement products?

Answer 1

Gypsum is a mineral known as hydrated calcium sulphate in chemical form. Gypsum controls the rate of hardening of the cement. During the manufacturing process of cement, during the cooling of clinker, a small amount of gypsum is added during the final grinding process.

Gypsum is introduced to control the “setting of cement”, slows down the hydration process of cement when it is mixed with water. If gypsum is not added then cement will set immediately after mixing of water leaving no time for concrete placing.

For ordinary Portland cement (OPC), it remains between 3 to 4% and in case of Quick setting cement, it is reduced up to 2.5%.

The process involved in hydration of cement is that, when the water is added into cement, it starts reacting with the C3A (tricalcium aluminate) and hardens. The cement gets hardened immediately which causes difficulty in mixing and transportation.

Gypsum is added to clinker through grinding process cause delaying the reaction of C₃A (tricalcium aluminate) with water by its reaction with C₃A to form insoluble calcium sulfoaluminate
(3CaO-A1,03-3CaSO4-2H,0)

- ettringite ,around C₃A particles, which permits enough time for the hydration of C₃Sthat its reaction is slower than C₃A and permits the occurring of natural setting

Reaction of gypsum in cement is given

3CaO.A1,0, +3CaSO4 + 2H, 0 3CaO-A1,03-3CaSO4-2H,0 (ettringite) Which is insoluble calcium sulfoaluminate