Q.16 Which of the following compounds is responsible for slow hardening, less heat of hydration, and greater resistance to the chemical attack in OPC Cement?
- AC4AF
- BC3S
- CC3A
- DC2SCorrect
Solution
Concept
C2S (dicalcium silicate) hydrates slowly. It gives low heat, later-age strength and better resistance to chemical attack.
Step-by-step solution
1. C3A reacts fastest and gives the highest heat. It has poor sulphate resistance. 2. C3S hardens fast and gives early strength with high heat. 3. C4AF has a small role and gives the colour of cement. 4. C2S hardens slowly, gives less heat of hydration and has good resistance to chemical attack. 5. So the answer is C2S.
Explanation
C2S reacts slowly with water, so it gives slow hardening and low heat. It produces less Ca(OH)2, so it resists chemical attack better. Low-heat cement has a high C2S content.
Why the other options are wrong
A: C4AF has little effect on strength; it mainly affects colour. B: C3S gives fast hardening and high heat. C: C3A gives the fastest reaction, the highest heat and poor sulphate resistance.
Shortcut
Slow and low heat = C2S; fast and early strength = C3S; flash set and max heat = C3A.
Common mistake
Confusing C2S with C3S.
Correct answer: D) C2S