Which one among the following is the chemical formula of gypsum which is an ingredient of cement?

Gypsum is a hydrated sulphate of calcium that occurs naturally as a mineral in sedimentary rocks [4]. Chemically, it is known as calcium sulphate dihydrate, represented by the formula CaSO4·2H2O [4]. In the cement industry, gypsum is a critical ingredient added to clinker (typically around 3-5%) to act as a setting regulator [5]. It functions by reacting with tricalcium aluminate (C3A) to form ettringite, which creates a protective film that slows down the hydration rate and prevents 'flash setting' [5]. While Plaster of Paris is the hemihydrate form (CaSO4·1/2H2O), gypsum contains two molecules of water of crystallisation [1]. Other options like CaO (calcium oxide) and Ca2SiO4 (dicalcium silicate) are components of the cement clinker itself rather than the additive gypsum.

Sources

1. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 13: Types of Rocks & Rock Cycle > 13.5. Some Rock-Forming Minerals > p. 175
2. [4] https://cronfa.swan.ac.uk/Record/cronfa59456/Download/59456__22459__c96fd6f8c3f84a06a44d645465625442.pdf
3. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Plaster of Paris > p. 33
4. [5] https://www.sciencedirect.com/science/article/abs/pii/S0950061817326235