State whether the following is true or false.
A pozzolana has cementing properties on its own without any addition.
Answer: A
It has little or none by itself but reacts with lime in the presence of water to form cementing compounds. The definition of a pozzolana rests on precisely that dependence. Volcanic ash from Pozzuoli gave the class its name.
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Portland slag cement uses which material as its blending component?
Answer: B
Correct answer: ground granulated blast furnace slag
The slag is a by-product of iron making, quenched rapidly so that it solidifies as a glass rather than crystallising, then ground. That glassy state is what makes it latently hydraulic: activated by the alkali and lime from the Portland fraction, it hydrates to give a denser paste with lower heat evolution and much better resistance to sulphate and chloride.
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If a blended cement contains 65 per cent clinker and 35 per cent fly ash, how much clinker is in a 50 kg bag?
Answer: C
Sixty-five per cent of 50 kilograms is 32.5. Replacing clinker in this way is the most direct means of reducing the carbon dioxide associated with a tonne of cement. Every tonne of clinker not made saves roughly its own weight in emissions.
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Hydrophobic cement is produced by
Answer: D
The film protects the cement from moisture during storage and is broken during mixing. It is useful where cement must be stored for long periods in humid conditions. The protection lasts through storage but not through mixing.
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High alumina cement differs fundamentally from Portland cement in that it is based on
Answer: A
Correct answer: calcium aluminates rather than calcium silicates
High alumina cement is made by fusing limestone with bauxite, and its principal phase is monocalcium aluminate rather than the calcium silicates of Portland cement. The different chemistry gives very rapid strength gain - the strength of ordinary cement at 28 days reached within a day - and excellent refractoriness and acid resistance, along with the conversion problem the next item describes.
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State whether the following is true or false.
High alumina cement can lose strength over time under warm and damp conditions.
Answer: B
Correct answer: True
The metastable hydrates formed initially convert over time to a stable but denser form, and because the solid occupies less volume the paste becomes more porous and loses strength. Warmth and moisture accelerate the conversion greatly. A series of structural failures in the 1970s ended its use in structural concrete in Britain, and it is now used for refractory and rapid-repair work where the effect is allowed for.
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Consider the following statements about the Bogue compounds.
1. C3S contributes most to early strength.
2. C3A reacts rapidly and evolves a great deal of heat.
3. C4AF is the principal source of long-term strength.
Answer: C
The first two are correct. The third is wrong, since long-term strength comes chiefly from C2S, while the ferrite phase contributes mostly colour and a lower melting point. Its contribution to strength is comparatively minor.
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The lime saturation factor of a cement raw mix expresses
Answer: D
Too high a value leaves free lime in the clinker, which causes unsoundness, while too low a value wastes potential strength. It is one of the principal controls on the raw mix. It is calculated from the oxide analysis before burning begins.
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State whether the following is true or false.
Free lime in cement is desirable because it adds to the available calcium.
Answer: A
Free lime hydrates slowly and expands after the concrete has hardened, causing unsoundness and cracking. Its content is limited by specification for exactly that reason. Unsoundness of this kind appears only after the concrete has hardened. That delay is exactly what makes the fault so difficult to detect early.
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Assertion (A): Free lime in clinker causes cracking in hardened concrete.
Reason (R): It hydrates slowly and expands after the paste has already set.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. Free lime that escaped combination in the kiln hydrates slowly, long after the paste has set, and the hydration is accompanied by a volume increase that the hardened material cannot accommodate. The result is unsoundness - cracking and disruption from within - which is why free lime is limited and why the Le Chatelier soundness test exists.
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