2023 · Q12
Which one of the following statements is not correct regarding the setting of cement?
general-sciencechemistry
- AThe addition of a small percentage of gypsum (CaSO₄) lengthens the setting period of cement.
- BAccording to the colloidal theory, gels of hydrated silicates are formed and when these gels harden, the set cement loses strength.
- CTricalcium silicate is responsible for initial setting of cement.
- DDicalcium silicate and tricalcium silicate are responsible for the final strength which occurs in about a year.
Explanation
The colloidal theory of cement setting states that hydration forms colloidal gels of calcium silicate hydrate (C-S-H), and it is the hardening of these gels that gives set cement its strength and durability. The cement therefore gains strength as the gels harden; it does not lose strength, so statement B is the one that is not correct. Gypsum retards the flash hydration of C3A (A is correct), tricalcium silicate C3S hydrates rapidly and drives initial setting and early strength (C is correct), and C2S with C3S builds final strength over about a year (D is correct).