Why we add gypsum to cement?
The gypsum added to cement in a small quantity in the range of 3.5-4.0 % in terms of SO3. Gypsum playing an essential role in improving cement properties. In the absence of gypsum. Ground clinker or cement will experience flash setting. Flash setting occurs due to rapid hydration of calcium aluminate to form calcium aluminate hydrate. The rapid hydration creates excessive exothermic heat. The heat generated by the rapid hydration will make the mix stiff, and it will reduce the remixing chances. Thus, under this condition, it will be infeasible to use cement because the mix will become stiff in a short time. The early stiffing will cause difficulties in placing and dealing with cement or concrete mix.
To avoid the flash setting and early stiffing of the mix. The reaction of cement and water has to be changed. The adding of sulfate salt will prevent the reaction of aluminate with water. The aluminate will tend to react with sulfate salt, which prevents the flash setting of the mix. Gypsum identified as the most suitable form of sulfate salt to control the hydration of calcium aluminate (C3A), which resulted in better workability for a longer duration. The chemical reaction in the presence of gypsum is given below.
3CaO. Al2O3 + 3CaSO4. 2H2O + nH2O ----------> 3CaO. Al2O3. 3CaSO4. 32H2O
(Ettringite: calcium trisulpho aluminate hydrate) + moderate exothermic heat
The gypsum will alter the cement properties, and it will prevent the flash set resulted from the hydration of calcium aluminate. Also, it will produce a mix with good workability for a longer time. Adding gypsum should be limited to a certain level. The European code stipulates the dosage of SO3 to 3.5-4% of the cement mass. Adding more gypsum doesn’t mean retarding the hydration process and maintaining higher workability for a longer time. The formation of ettringite as a result of adding gypsum of a certain quantity may accelerate the hardening process and reduce the mix workability.