Explain the Role of Gypsum in the Setting of Cement

Role of Gypsum in the Setting of Cement

Gypsum (CaSO₄·2H₂O) is added to cement mainly to control and regulate its setting time. It prevents the cement from setting too rapidly after water is added.

When water is added to cement, the tricalcium aluminate (C₃A) reacts very rapidly with water. If gypsum is not present, this reaction can cause flash setting, which leaves insufficient time for mixing, transportation, and proper placement of the cement paste.

Gypsum reacts with tricalcium aluminate in the presence of water and produces ettringite (calcium sulfoaluminate hydrate). The formation of ettringite slows down the rapid hydration of C₃A and thereby provides a suitable setting time for cement.

C₃A + 3CaSO₄·2H₂O + 26H₂O → C₆ĀS₃H₃₂

Thus, gypsum acts primarily as a setting-time regulator. It prevents flash setting by controlling the rapid hydration of tricalcium aluminate and allows sufficient time for the cement to be mixed, transported, placed, and compacted properly.

In Short

• Gypsum is added to cement to regulate its setting time.
• It controls the rapid hydration of C₃A.
• It prevents flash setting of cement.
• It promotes the formation of ettringite.
• It provides sufficient time for the proper handling and placement of cement.