Which one of the following is NOT present in cement?
- (a)2CaO·SiO2
- (b)3CaO·SiO2
- (c)4CaO·SiO2
- (d)3CaO·Al2O3
Correct — C, 4CaO·SiO2. The main compounds in Portland cement are dicalcium silicate, 2CaO·SiO2 (C2S, 'belite'); tricalcium silicate, 3CaO·SiO2 (C3S, 'alite'); tricalcium aluminate, 3CaO·Al2O3 (C3A); and tetracalcium aluminoferrite, 4CaO·Al2O3·Fe2O3 (C4AF). There is no 'tetracalcium silicate' (4CaO·SiO2) among cement's compounds — that phase does not exist — so it is the one NOT present.
- (a)2CaO·SiO2 — Dicalcium silicate (C2S, belite) is a genuine cement compound; it contributes to the slow, long-term gain of strength.
- (b)3CaO·SiO2 — Tricalcium silicate (C3S, alite) is the major cement compound and is chiefly responsible for early strength and setting.
- (d)3CaO·Al2O3 — Tricalcium aluminate (C3A) is a real cement compound; it reacts fast with water, which is why gypsum is added to control the setting time.
Portland cement is essentially a mix of calcium silicates and aluminates formed when limestone (for lime), clay or shale (for silica and alumina) and iron oxide are heated into clinker. Its four principal Bogue compounds are C3S (alite), C2S (belite), C3A and C4AF. 'Tetracalcium silicate', 4CaO·SiO2, is not one of them — the calcium silicates in cement stop at the tri- form.
Three of the four options are legitimate cement phases; the odd one out just extends the pattern one step too far, from 3CaO·SiO2 to a non-existent 4CaO·SiO2. Recognising the real Bogue compounds isolates the impostor.
- Portland cement's essential raw ingredients supply lime, silica, alumina and iron oxide.
- Its principal compounds are C3S (3CaO·SiO2), C2S (2CaO·SiO2), C3A (3CaO·Al2O3) and C4AF (4CaO·Al2O3·Fe2O3).
- C3S (alite) gives early strength; C2S (belite) gives long-term strength.
- Gypsum is added to clinker to retard the fast-setting C3A and control the setting time.
C2S, C3S and C3A are real Bogue compounds; tetracalcium silicate (4CaO·SiO2) is not — so it is the one not present.
- Assuming a 'tetracalcium silicate' exists just because di- and tri-calcium silicates do.
- Mixing up tetracalcium aluminoferrite (real — 4CaO·Al2O3·Fe2O3) with a supposed tetracalcium silicate.
Asked as which formula is, or is not, a genuine constituent/compound of Portland cement.
No directly related past PYQ was found.
- practice — not a real PYQ
Which compound of Portland cement is mainly responsible for its early strength?
- (a)dicalcium silicate (C2S)
- (b)tricalcium silicate (C3S)
- (c)tricalcium aluminate (C3A)
- (d)tetracalcium aluminoferrite (C4AF)
Answer(b) tricalcium silicate, C3S (alite).
- practice — not a real PYQ
Gypsum is added to cement clinker mainly to
- (a)increase strength
- (b)slow down (retard) the setting time
- (c)change the colour
- (d)make it waterproof
Answer(b) slow down the setting time by controlling the fast-reacting C3A.