Which one of the following statements is not correct regarding the setting of cement?

Correct — C, the claim that tricalcium silicate is responsible for the initial setting of cement. The compound that governs the initial set is tricalcium aluminate. It hydrates almost the moment water is added and would flash-set the paste on its own, which is precisely why a few per cent of gypsum is ground in with the clinker to hold it back. Tricalcium silicate also hydrates quickly, but what it delivers is early strength over roughly the first week, not the initial set. So the statement misattributes the initial setting to the wrong compound, and it is the one that is not correct.

• (a)The addition of a small percentage of gypsum (CaSO₄) lengthens the setting period of cement. — This is correct and is standard practice. Gypsum retards the hydration of tricalcium aluminate, which is what makes cement workable for long enough to be placed.
• (b)According to the colloidal theory, gels of hydrated silicates are formed and when these gels harden, the set cement loses strength. — The first half is the colloidal account of setting, and it is correctly described — gels of hydrated silicates form and stiffen. The tail clause runs the wrong way, since hardening gels are what give the set mass its strength, so this option is not a clean statement either. The examiner's target is the compound error in the option about the initial set, and the official key marks that one.
• (d)Dicalcium silicate and tricalcium silicate are responsible for the final strength which occurs in about a year. — Matches the standard account. The silicates carry the long-term strength, dicalcium silicate hydrating slowly over months and continuing to add strength for about a year.

Portland cement is made by burning limestone with clay to a clinker and grinding it with a little gypsum. Four compounds do the work: tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium alumino-ferrite. Setting and hardening are hydration reactions, not drying — cement gains strength under water — and each compound has its own timetable. Tricalcium aluminate reacts first and fastest, tricalcium silicate carries the first week's strength, and dicalcium silicate goes on hydrating for months.

One honest caution before you use this item as a model answer. On a strict reading two of the four statements are open to challenge: the option about the initial set names the wrong compound, and the option describing the colloidal theory ends by saying the set cement loses strength, which reverses what hardening gels do. UPSC did not drop this question — it dropped three others on this paper — and its key marks the tricalcium silicate statement. Author to that, and remember the underlying chemistry rather than the sentence: the aluminate sets, the silicates strengthen, and the gypsum buys time.

• The four clinker compounds are tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium alumino-ferrite.
• Tricalcium aluminate hydrates first and causes flash set; two to three per cent of gypsum is added to retard it.
• Tricalcium silicate carries early strength, roughly the first seven days.
• Dicalcium silicate hydrates slowly and carries strength gained over months, up to about a year.
• Setting and hardening are hydration, so cement cures under water and does not need to dry.

• Attributing the initial set to a silicate rather than to the aluminate.
• Thinking cement hardens by drying, when it hardens by hydration.
• Assuming more gypsum always helps; too much causes expansion and cracking.

Asked as a which-one-is-not-correct item over four textbook sentences, so the examiner is testing the compound-by-compound timetable rather than the process as a whole.

• practice — not a real PYQ

  Why is a small quantity of gypsum ground with cement clinker?

  • (a)To accelerate the setting of the cement
  • (b)To retard the setting so that the cement stays workable
  • (c)To give the cement its grey colour
  • (d)To make the cement waterproof

  Answer(b) To retard the setting so that the cement stays workable — gypsum holds back the rapid hydration of tricalcium aluminate.

• practice — not a real PYQ

  Which compound of cement clinker contributes most to strength gained in the first week after mixing?

  • (a)Dicalcium silicate
  • (b)Tricalcium silicate
  • (c)Tetracalcium alumino-ferrite
  • (d)Calcium sulphate

  Answer(b) Tricalcium silicate — it hydrates quickly and carries the early strength, while dicalcium silicate works over months.