Gypsum is hydrated calcium sulphate in the chemical form and it plays a very important role in controlling the setting and hardening time of cement.
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Ordinary Portland cement is hydraulic cement which hardens very fast due to a chemical reaction with water. In order to prevent the early hardening time or to control the flash setting of cement gypsum is added to the cement. If gypsum is not added the cement will set very rapidly and the cement shall not get any working time for concrete setting.
In case of underwater construction or construction of bridges the setting time has to be very quick so a less quantity of gypsum is used in the cement.
Hence the quantity of gypsum to be added to cement is indirectly proportional to the setting time of concrete.
Suggested Read: Hydraulic Cement vs. Non Hydraulic Cement