Step 1: Understanding the Question:
The question asks for the functional purpose of adding Gypsum ($CaSO_4 \cdot 2H_2O$) during the final grinding stage of cement clinker.
Step 2: Detailed Explanation:
The Flash Set Problem: Tricalcium Aluminate ($C_3A$) is one of the phases in cement clinker. It reacts violently and almost instantly with water. If pure cement clinker is mixed with water, it will stiffen in seconds, making it impossible to work with. This is called a "flash set."
Role of Gypsum: About $2-5%$ gypsum is added during the grinding of clinker. When water is added to this cement, the gypsum reacts with $C_3A$ to form a protective coating of insoluble "ettringite" around the cement particles.
Retarding Effect: This coating slows down the initial hydration reaction. It prevents the flash set and gives the workers enough time (usually $30$ to $60$ minutes) to mix, transport, and place the concrete before it begins to harden. Thus, Gypsum acts as a retarder.
Analyzing Other Options:
- Accelerate (A): Accelerators (like $CaCl_2$) do the opposite; they are used in cold weather.
- Increase strength (C): Gypsum does not contribute to the final compressive strength.
Step 3: Final Answer:
Gypsum is added to cement specifically to retard the initial setting time and prevent flash setting. Therefore, option (B) is the correct answer.