Ordinary Portland cement is manufactured principally from
Answer: A
The two supply the calcium, silica, alumina and iron that the clinker minerals require. Gypsum is added later, after burning, to control the setting time. Burning the two together at high temperature is what forms the clinker minerals.
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The four principal compounds of Portland cement clinker are commonly known as
Answer: B
Bogue devised the calculation that estimates their proportions from an oxide analysis. Blaine, Vicat and Le Chatelier all gave their names to test apparatus instead. The Bogue calculation remains the standard way of estimating the phases.
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Tricalcium silicate, the principal strength-giving compound of Portland cement, is abbreviated as
Answer: C
In cement chemists' notation C stands for calcium oxide and S for silica, so C3S is three parts lime to one of silica. It usually makes up around half of the clinker. The abbreviations are a shorthand used throughout the cement industry.
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Which clinker compound contributes most to early strength?
Answer: D
It hydrates quickly and supplies most of the strength gained in the first week. Dicalcium silicate acts far more slowly and contributes chiefly to later strength. The two silicates together dominate the strength of the finished cement.
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Which clinker compound is chiefly responsible for strength gained after about 28 days?
Answer: A
Its slow hydration continues for months, which is why concrete keeps gaining strength long after it is placed. The two silicates together account for most of the strength of the cement. Their proportions can be adjusted to suit the intended use.
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State whether the following is true or false.
Dicalcium silicate contributes more to early strength than tricalcium silicate does.
Answer: B
It is the other way round, with C3S dominating the first week and C2S the later gain. Confusing the two is the commonest error in this part of the subject. Remembering that three is faster than two is a serviceable mnemonic.
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Assertion (A): Concrete continues to gain strength for months after it has been placed.
Reason (R): Dicalcium silicate hydrates slowly and keeps contributing long after setting.
Answer: C
Both are true and the reason gives the mechanism: the slower of the two silicates is still reacting well beyond the twenty-eight days at which strength is conventionally specified. The twenty-eight day figure is a convention rather than a final value.
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Tricalcium aluminate is significant chiefly because it
Answer: D
Left unchecked it would cause a flash set, which is why gypsum is interground with the clinker. It is also the compound most vulnerable to sulphate attack. Low-aluminate cements are specified where sulphates are present in the ground.
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Gypsum is added to cement clinker in order to
Answer: A
Without it the aluminate would react almost instantly and the cement would flash set. The quantity is controlled carefully, since too much causes expansion in the hardened paste. The standards therefore set an upper limit on total sulphate content.
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State whether the following is true or false.
Cement without added gypsum would set too slowly to be useful.
Answer: B
It would set far too quickly, flash setting within minutes as the tricalcium aluminate reacts. Gypsum is a retarder, so its absence accelerates rather than delays the set. A cement flash setting in the mixer would be useless on site.
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