CAPF AC 2023 Q12: Science & Technology – Chemistry
- (a) The addition of a small percentage of gypsum (CaSO₄) lengthens the setting period of cement.
- (b) According to the colloidal theory, gels of hydrated silicates are formed and when these gels harden, the set cement loses strength.
- (c) Tricalcium silicate is responsible for initial setting of cement.
- (d) Dicalcium silicate and tricalcium silicate are responsible for the final strength which occurs in about a year.
Answer: (c) Tricalcium silicate is responsible for initial setting of cement.
Explanation
Setting of cement
How to think: cement has four main compounds.
Each one sets at a different speed.
The fastest is tricalcium aluminate, and it causes the initial set.
| Compound | Short form | Role |
|---|---|---|
| Tricalcium aluminate | C3A | Initial setting (flash set); fastest to hydrate |
| Tricalcium silicate | C3S | Early strength, in the first days and weeks |
| Dicalcium silicate | C2S | Slow; final strength over about a year |
| Tetracalcium aluminoferrite | C4AF | Little strength; gives the grey colour |
So statement (c) is wrong: the initial setting is due to tricalcium aluminate, not tricalcium silicate.
Role of gypsum: it reacts with tricalcium aluminate and slows the setting, giving workers time.
A note on (b): by the colloidal theory the cement sets as the silicate gels harden.
The words about losing strength are loose, but the published key takes (c) as the answer.
Answer: (c).
Why each option is right or wrong
- (a) Incorrect. This is true: gypsum lengthens the setting time of cement.
- (b) Incorrect. The key accepts this description of the colloidal theory of setting.
- (c) CORRECT. This is wrong: tricalcium aluminate, not tricalcium silicate, gives the initial set.
- (d) Incorrect. This is true: the two silicates give the final strength in about a year.
Key point
Portland cement: C3S (early strength), C2S (later strength), C3A (initial or flash set), C4AF. Gypsum (2 to 3%) retards setting. Raw materials: limestone and clay heated to form clinker.
Remember
Initial set: tricalcium aluminate. Strength: the silicates.
Common mistakes
- Giving the initial set to tricalcium silicate because it is the main compound.
- Thinking gypsum speeds up the setting of cement.
- Mixing up setting (stiffening) with hardening (gain of strength).
Exam tip
Educated guess: gypsum is added to tame the aluminate, which sets too fast. So the initial set belongs to the aluminate, and the statement naming the silicate is the wrong one.
Sources
- NCERT Class XI Chemistry — The s-Block Elements (cement: composition and setting)
- B. K. Sharma, Industrial Chemistry — Cement