Cement and Refractories Questions and Answers

Answer: A

Alumino-silicate clay is abundant and cheap, and it serves wherever the temperature is moderate and the slag is not strongly basic. The severe duties are given to magnesite, chromite or zirconia at many times the price.

Answer: B

Correct answer: attack by molten metals and glass

Zircon and zirconia resist wetting and chemical attack by molten glass and by many molten metals, and their low thermal expansion gives good resistance to spalling under thermal cycling. That combination is why they are used in glass tank blocks and in continuous casting nozzles despite their cost.

Answer: C

Correct answer: the calcium hydroxide liberated during hydration

Hydration of the silicates produces calcium hydroxide, which contributes nothing to strength and is the phase most readily leached or attacked. A pozzolana - fly ash, silica fume, calcined clay - reacts with it to form more calcium silicate hydrate. The result is more of the useful phase, less of the vulnerable one, and hence a denser and more durable concrete.

Answer: D

Correct answer: very rapid strength gain and resistance to chemical attack

High alumina cement reaches in a day the strength Portland cement takes a month to develop, and it resists sulphate and acid attack well, so it is used for refractory concretes and urgent repairs. It has a large heat of hydration, is expensive, and is subject to conversion of its hydrates in warm damp conditions with loss of strength - which is why its structural use is restricted.

Answer: A

Correct answer: uses an industrial by-product and lowers the clinker factor

Ground granulated blast furnace slag replaces a large proportion of the clinker, so less limestone is calcined and less fuel burned - a substantial saving in both cost and carbon dioxide. The resulting cement gains strength more slowly at first but ends up denser and more resistant to sulphate and chloride attack.

Answer: B

Furnace and kiln linings must hold their shape and strength at temperatures that would destroy any ordinary material, and resist the slag or gas in contact. Chemical inertness toward that contact is as important as the temperature rating.

Answer: C

Correct answer: basic slag

An acidic refractory is silica-rich and a basic slag is lime-rich, so the two react and the lining is fluxed away. The rule is to match the lining to the process chemistry: acidic refractories with acidic slags, basic with basic, and neutral ones such as chromite and carbon where the environment is mixed or uncertain.

Answer: D

Correct answer: acidic refractories

Silica and fireclay are silica-rich and therefore acidic; they stand up to acidic slags and are attacked by basic ones. Magnesite, dolomite and lime are the basic refractories, and chromite, carbon and alumina are usually classed as neutral. The classification is chemical, not thermal, and it is what decides where a brick may be used.

Answer: A

Correct answer: basic refractories

Magnesite and dolomite are rich in magnesia and lime, so they are basic and are used where the slag is basic - in steelmaking converters and furnaces, and in the burning zone of a cement kiln. Their weakness is hydration: dolomite in particular will slake in damp storage, so the bricks must be kept dry until they are built in.

Answer: B

Being neither strongly acidic nor basic they resist attack from either kind of slag, which suits them to the boundary between an acid and a basic lining. Carbon and silicon carbide also conduct heat well, which is sometimes wanted and sometimes not.