Ncert Exercises & Solutions

By adding gypsum to cement

1. Setting time of cement becomes less

2. Setting time of cement increases

3. Colour of cement becomes light

4. Shining surface is obtained

Hint: Gypsum added to cement to prevent early hardening

Raw materials for cement-limestone, clay, gypsum. Cement is a dirty greyish heavy powder containing calcium aluminates and silicates.

Gypsum

({CaSO}_{4} \cdot 5 H_{2} O)

is added to the components to increase the setting time of cement so that it gets sufficiently hardened. Setting of cement is an exothermic process and involves hydration of calcium aluminates and silicates.

Gypsum prevents Flash Setting of cement during manufacturing.
It retards the setting time of cement.
Allows a longer working time for mixing, transporting and placing.
When water is mixed to cement Aluminates and sulfates get react and evolve some heat but gypsum acts as coolant and brings down the heat of hydration.
Gypsum cements possess considerably greater strength and hardness when compared to non gypsum cement.
Water required in gypsum based cement for hydration process is less

The amount of gypsum should account for 3% ~ 5% of the cement's mass.
If the content of gypsum exceeds the limit, it will lower the strength of cement and it can even lead to poor dimensional stability, which will cause the expanded destruction of cement paste.

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