Question

Gypsum is added to clinker during cement manufacture to ________

• A. make the cement impervious
• B. bind the particles of calcium silicate
• C. decrease the rate of setting of cement
• D. to facilitate the formation of colloidal gel

Hint:

Gypsum acts as a chemical retarder in cement.
Without gypsum, cement undergoes flash set (instantaneous hardening), making it impossible to work with on a construction site.
Always remember that gypsum targets the highly reactive tricalcium aluminate (\(\text{C}_3\text{A}\)) phase.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the primary function of adding gypsum to cement clinker during the final stage of cement manufacturing.
Portland cement is manufactured by burning a mixture of calcareous and argillaceous materials at high temperatures to form clinker, which is then finely ground.
Understanding the role of additives like gypsum is essential to controlling the physical properties of concrete during construction.

Step 2: Key Formula or Approach:
During the hydration of cement, several mineral phases react with water at different rates:
- Tricalcium Aluminate (\(\text{C}_3\text{A}\) or \(3\text{CaO}\cdot\text{Al}_2\text{O}_3\)) is the most reactive phase.
- In the absence of an inhibitor, \(\text{C}_3\text{A}\) reacts instantly with water, causing "flash set" (an immediate hardening of the cement paste with high heat evolution).
- To control this rapid reaction, gypsum (\(\text{CaSO}_4\cdot2\text{H}_2\text{O}\)) is interground with the clinker in small quantities (usually \(2\%\) to \(3\%\) by weight).

Step 3: Detailed Explanation:
Let us examine the chemical mechanism of gypsum addition during cement hydration:
1. When water is added to cement, gypsum quickly dissolves and releases calcium (\(\text{Ca}^{2+}\)) and sulfate (\(\text{SO}_4^{2-}\)) ions into the solution.
2. These sulfate ions react with the highly active tricalcium aluminate (\(\text{C}_3\text{A}\)) on the surface of the cement particles to form an insoluble, crystalline compound called ettringite (calcium sulfoaluminate):
\[ 3\text{CaO}\cdot\text{Al}_2\text{O}_3 + 3\text{CaSO}_4\cdot2\text{H}_2\text{O} + 26\text{H}_2\text{O} \to 3\text{CaO}\cdot\text{Al}_2\text{O}_3\cdot3\text{CaSO}_4\cdot32\text{H}_2\text{O} \] 3. The ettringite forms a protective barrier layer around the unhydrated \(\text{C}_3\text{A}\) grains, preventing water from reaching them directly.
4. This barrier slows down the initial hydration rate of the aluminate phase, thereby extending the setting time of the cement paste.
5. Delaying the setting allows sufficient time for transport, mixing, placing, and compacting the concrete in structural molds.
6. Therefore, gypsum serves as a retarder to decrease the rate of setting of cement.

Step 4: Final Answer
Thus, the correct option is (C).