What is the most common admixture used as a retarder?

• A. Lignosulfonate
• B. Calcium chloride
• C. Gypsum
• D. Air-entraining agent

Answer: (C)

Retardation is achieved by substances that slow down the hydration of cement, extending the workable period. Gypsum, a form of calcium sulfate, is added to cement to regulate the hydration of aluminate compounds. It reacts with C3A to form ettringite, which delays the rapid initial setting and provides a longer open time, preventing flash setting. That makes gypsum the most common retarder used in typical cement blends.

Lignosulfonates are mainly used as water-reducing admixtures to improve workability, with only a minor retarding effect in some cases. Calcium chloride speeds up setting and early strength (an accelerator), not retardation. Air-entraining agents introduce microscopic air voids to improve freeze-thaw durability and do not serve to slow the setting process.