Cement and Refractories Questions and Answers

Answer: C

Correct answer: the surface dried before hydration was complete

A dusty friable surface is the classic symptom of inadequate curing: the water evaporated from the top few millimetres before the cement there had hydrated, so that layer never developed any strength. It is a curing failure rather than a materials fault, and it is prevented by covering, wetting or applying a curing membrane promptly after finishing.

Answer: D

The polymorphic changes of silica involve abrupt volume changes, and cycling through them breaks the brick apart. A material of low thermal expansion, such as a suitable fireclay or silicon carbide, would survive the cycling.

Answer: A

Correct answer: both the assertion and the reason are true and the reason explains the assertion

Both are true and the reason is precisely the mechanism. Tricalcium aluminate reacts with water almost instantly, and without a retarder the paste would stiffen before it could be placed. Gypsum reacts with it to form a coating of ettringite around the grains, which slows the reaction to a manageable rate - which is why gypsum is ground in with the clinker rather than added later.

Answer: B

Refractoriness alone is not enough: the brick carries load while hot, and its refractoriness under load may be hundreds of degrees lower. Slag resistance and spalling behaviour must also be checked before it is specified.

Answer: C

Correct answer: slower early strength but better durability

The pozzolana reacts only with the calcium hydroxide released as the cement hydrates, so it contributes nothing at first and strength develops more slowly in the early weeks. In return it converts that vulnerable calcium hydroxide into more calcium silicate hydrate, giving a denser, less permeable paste with better resistance to sulphate and chloride attack.

Answer: D

Correct answer: expansion caused by free lime or magnesia

The soundness test measures delayed expansion from free lime or magnesia, which hydrate long after the concrete has hardened and disrupt it. Setting time, fineness and compressive strength are each measured by their own separate tests, and none of them would reveal a fault that only appears weeks or months after placing.