To retard the initial setting time of cement, the compound responsible is
- ATricalcium silicate
- BGypsumCorrect
- CDicalcium silicate
- DTricalcium aluminate
Solution
Concept
Cement setting time retarding agent
Formula
NONE
Step-by-step solution
1. During cement manufacturing, clinker undergoes rapid hydration due to tricalcium aluminate (C3A), leading to flash set. 2. Gypsum (calcium sulfate dihydrate) is added during fine grinding of clinker. 3. Gypsum reacts with C3A to form ettringite on particle surfaces, retarding the initial setting time of cement. Answer: B
Explanation
Gypsum is added during cement grinding to act as a retarding agent for initial setting. It reacts with tricalcium aluminate to form calcium sulfoaluminate, which coats the cement grains and delays their rapid reaction with water. This process slows down the initial setting time so the concrete can be mixed, transported, and placed.
Why the other options are wrong
A) Tricalcium silicate contributes to early strength development rather than retarding setting time. C) Dicalcium silicate is responsible for long-term strength gain in concrete. D) Tricalcium aluminate causes rapid flash setting if not controlled by gypsum.
Shortcut
Gypsum = Retarder that delays initial setting time of cement.
Common mistake
Confusing Gypsum, which acts as a retarder, with Tricalcium Aluminate, which causes rapid initial setting.
Correct answer: B) Gypsum