Ultra-high performance concrete achieves its very high strength largely through
Answer: C
Superplasticisers permit water to cement ratios far below the usual range, and fine powders fill the spaces between the cement grains. Steel or polymer fibres are generally added to give it some tensile capacity as well.
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Assertion (A): Ultra-high performance concrete usually contains fibres.
Reason (R): Very high compressive strength does nothing to improve the brittleness of the material.
Answer: D
Both are true and the reason gives the need: a stronger matrix is if anything more brittle, so fibres are added to provide the ductility that would otherwise be entirely absent from such a dense mix. Fibre type and dosage are part of the mix design rather than an extra.
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Consider the following statements about special cements.
1. Calcium aluminate cement is used in refractory castables.
2. Magnesium phosphate cement is used where rapid reopening matters.
3. Sorel cement is preferred for exterior structural work.
Answer: A
The first two are correct. The third is wrong, since magnesium oxychloride cement has poor water resistance and is confined to protected interior applications such as flooring. Water resistance is its defining weakness.
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Waterproof or water-repellent cement is produced by
Answer: B
Correct answer: intergrinding a water-repellent admixture with the ground clinker
A stearate or similar water-repellent is ground in with the clinker, so the cement itself carries the admixture and no site addition is needed. The effect is to reduce capillary absorption at the surface rather than to change the fundamental porosity of the paste.
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State whether the following is true or false.
Waterproof cement makes concrete completely impermeable to water.
Answer: C
Correct answer: False
The name promises more than the material delivers: water-repellent cement reduces capillary absorption, but concrete remains permeable and water under pressure will pass through it. Genuine watertightness comes from a low water-cement ratio, proper compaction and curing, and where necessary a membrane.
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Belite-rich cements are proposed as a lower-carbon option because belite
Answer: D
Correct answer: forms at a lower burning temperature than alite
Belite forms by solid-state reaction from about 1,200 degrees, while alite needs a liquid phase at around 1,450. A belite-rich cement therefore burns cooler, saving fuel, and needs less lime, saving process emissions. The cost is slow early strength, which is why such cements suit mass concrete rather than general use.
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Assertion (A): Belite-rich cement has not displaced ordinary Portland cement despite its lower emissions.
Reason (R): Belite hydrates slowly, so early strength is much lower than the construction industry expects.
Answer: A
Both are true and the reason gives the obstacle: construction schedules are built around early strength, and a binder that takes months to reach a usable figure changes the economics of a project far more than the cement price does.
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State whether the following is true or false.
A lower-carbon cement will be adopted whenever its emissions are lower.
Answer: B
Correct answer: False
Adoption depends on cost, on standards permitting the material, on raw material supply, on contractors and testing houses knowing how to handle it, and on clients accepting a binder without decades of field record. Lower emissions is one consideration among several, and rarely the decisive one.
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Portland clinker production emits roughly 800 kg of carbon dioxide per tonne. If a special cement halves that and replaces 10 per cent of world production, what proportion of total cement emissions is saved?
Answer: C
Correct answer: about 5 per cent
Halving the emission on a tenth of production saves \[ 0.10 \times 0.50 = 0.05, \] that is five per cent of the total. The arithmetic is worth doing because it shows the scale of the difficulty: a material that halves emissions must reach a large share of world production before the effect on the total is substantial.
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Oil well cement retarders are needed because
Answer: D
Conditions several kilometres down would set an unretarded slurry long before it reached its destination. The retarder is matched to the measured downhole temperature rather than chosen by habit. Every well is designed against its own measured conditions.
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