What is the purpose of adding 2% gypsum to cement?

Cement is a fine inorganic material that, when mixed with water, forms a hard and strong solid mass due to chemical reactions known as hydration reactions. The major components of cement are tricalcium silicate (C₃S), dicalcium silicate (C₂S), tricalcium aluminate (C₃A), and tetra-calcium aluminoferrite (C₄AF). Among these, C₃A reacts very rapidly with water, leading to flash setting — an immediate hardening of the cement paste that prevents proper mixing and placing.

To control this rapid reaction, about 2% gypsum (CaSO₄·2H₂O) is added during the final grinding of cement clinker. The role of gypsum is to regulate the setting time of cement by slowing down the hydration of tricalcium aluminate. It reacts with C₃A to form calcium sulfoaluminate (ettringite), which forms a thin protective layer around C₃A particles, reducing their immediate reactivity with water. This controlled reaction allows sufficient time for mixing, transportation, and application before the cement starts to harden.

Without gypsum, the cement would set almost instantly after adding water, making it unusable for construction purposes. The addition of a small, controlled amount (typically 2–5%) ensures a balance between proper setting and hardening. Excess gypsum, however, can cause expansion and cracking upon drying due to the formation of unwanted crystalline products.

Gypsum thus plays a crucial role in the manufacture and performance of Portland cement. It ensures that the cement remains workable for a reasonable period and hardens uniformly, leading to durable and strong construction materials. This adjustment of setting time by gypsum is one of the key chemical controls in the cement industry and forms a fundamental concept in inorganic industrial chemistry