Cement Additives: Correct Statement Identification

Cement additives are crucial for controlling the properties of cement, particularly its setting time. Understanding their specific roles is key to selecting the correct statement.

Statement Analysis

• Option 1: Calcium chloride acts as a retarder
  Calcium chloride is generally known to accelerate the setting and hardening of cement, especially in colder temperatures. Therefore, this statement is incorrect.
• Option 2: Gypsum (calcium sulphate) acts as an accelerator
  Gypsum's primary function is to regulate the setting time of cement. It slows down the rapid hydration reaction of Tricalcium aluminate (C₃A), preventing flash setting. This means it acts as a retarder, not an accelerator. Thus, this statement is incorrect.
• Option 3: Gypsum (calcium sulphate) acts as a retarder
  Gypsum (calcium sulphate, often CaSO₄·2H₂O) is added to cement clinker precisely to control the setting time. It reacts with C₃A to form a protective layer, slowing down the hydration process and preventing premature stiffening (flash set). This statement accurately describes the function of gypsum.
• Option 4: None of these
  Since Option 3 is a correct statement, this option is incorrect.

Conclusion

Based on the analysis, the statement correctly identifying the role of Gypsum is Option 3.