Supersulphated cement is made principally from
Answer: C
The slag is activated by sulphate rather than by lime, so the clinker content is only a few per cent. It gives good resistance to sulphate and to weak acids but needs careful curing and is sensitive to high temperature.
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Limestone calcined clay cement, often abbreviated LC3, reduces clinker content by combining clinker with
Answer: D
The calcined clay and the limestone react synergistically, so the pair performs better than either alone at the same replacement level. Suitable clays are far more widely available than slag or fly ash, which is the main argument for it.
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Assertion (A): Calcined clay is a more promising clinker substitute than fly ash for long-term use.
Reason (R): Suitable clays are widely distributed, whereas fly ash depends on coal-fired power generation.
Answer: A
Both are true and the reason gives the difference in prospects: one supply is geological and the other industrial, and the industry supplying fly ash is contracting in most countries that are trying hardest to reduce emissions.
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If LC3 allows clinker content to fall from 95 per cent to 50 per cent, by what proportion is the clinker per tonne reduced?
Answer: B
Correct answer: by about 47 per cent
\[ \frac{95 - 50}{95} = 0.474. \] The reduction is taken as a proportion of what was there before, not as the difference in percentage points - 45 points is not 45 per cent. Since nearly all the emission is in the clinker, the saving in carbon dioxide is close to the same figure.
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State whether the following is true or false.
A reduction from 95 to 50 per cent clinker is a 45 per cent reduction in clinker use.
Answer: C
It is a fall of 45 percentage points, but as a proportion of the original figure the reduction is about 47 per cent. The two are commonly confused and give noticeably different answers when the starting figure is not a hundred.
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Refractory castables based on calcium aluminate cement are used because the binder
Answer: D
Correct answer: retains strength at temperatures that would destroy Portland cement
Portland cement dehydrates and loses strength above a few hundred degrees. Calcium aluminate cement converts to ceramic bonds instead and continues to serve well beyond a thousand, which makes it the standard binder for refractory castables. It is considerably more expensive and sets faster, not slower.
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State whether the following is true or false.
Ordinary Portland cement concrete is suitable for furnace linings.
Answer: A
Its hydrates decompose on heating and the concrete loses strength and spalls. Refractory work uses calcium aluminate binders or purely ceramic materials designed for the temperature. Hydraulic and ceramic bonding are quite different mechanisms.
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Coloured cements are usually produced by
Answer: B
Correct answer: adding pigments to white cement
Colour is obtained by grinding in mineral pigments - iron oxides, chromium oxide, cobalt compounds - at a few per cent by mass. The pigment must be stable in the alkaline environment and fast to light, which restricts the palette considerably compared with paints.
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Assertion (A): Coloured cement is normally based on white cement rather than grey.
Reason (R): The grey background would dull and shift any pigment added to it.
Answer: C
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. Grey clinker owes its colour to iron compounds, and that grey shows through any pigment added, muting it and shifting the hue. White cement is made from low-iron raw materials at greater cost, and the pigment then gives the colour it was chosen for.
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Photocatalytic cement containing titanium dioxide is intended to
Answer: D
Correct answer: break down pollutants and keep surfaces clean
Titanium dioxide is a photocatalyst: under ultraviolet light it generates radicals that oxidise nitrogen oxides and organic matter on the surface. The result is a facade that stays clean and removes some pollution from the air passing it, which is why the material has been used on prominent buildings.
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