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The reason for adding gypsum in cement is:


1. To avoid flash set
2. To increase the heat of hydration
3. To increase the rate of hydration
4. To decrease the soundness of cement

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Correct Answer - Option 1 : To avoid flash set

Explanation:

(i) Gypsum is basically a Hydrated calcium sulphate (CaSO4.2H2O). It acts as a retarder in the cement and increases the setting time of cement. In places where setting time of cement required to be more, Gypsum is added in to that cement.

(ii) It is used to avoid flash set in cement.

Important information about Gypsum: 

(i) Gypsum is a mineral and is hydrated calcium sulphate in chemical form.

(ii) Gypsum plays a very important role in controlling the rate of hardening of the cement.

(iii) During the cement manufacturing process, upon the cooling of clinker, a small amount of gypsum is introduced during the final grinding process. Gypsum is added to control the “setting of cement”. If not added, the cement will set immediately after mixing of water leaving no time for concrete placing.

Flash set:

(i) It is defined as the rapid development of permanent rigidity of the portland cement paste, mortar, or concrete along with high heat evolution. This rigidity cannot be dispelled nor can be plasticity be regained, by further mixing without the addition of water.

(ii) Causes of the flash set are:

  • Presence of high Tricalcium aluminate.
  • Inadequate amount of gypsum added to cement
  • Presence of alkalis.

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