Special Cements Questions and Answers

Answer: A

Correct answer: False

The reaction is photocatalytic, so it requires ultraviolet light to proceed and stops entirely in the dark. That limits the effect to surfaces which are exposed and reasonably well lit, and it is one of the reasons measured pollution removal in the field falls short of laboratory figures.

Answer: B

Correct answer: expand slightly during early hydration

A controlled expansion during hardening puts the concrete into compression against its restraint, which offsets the drying shrinkage that follows. The object is a member that ends up at or near its original length rather than cracked, and reinforcement is essential to provide the restraint the expansion works against.

Answer: C

Both are true and the reason gives the mechanism: unrestrained expansion simply makes the member larger, whereas restrained expansion stores a compressive stress that later shrinkage has to overcome before cracking can begin.

Answer: D

Its lower strength and greater capacity to accommodate movement make it the material of choice for repairing historic masonry. A Portland cement mortar in such a wall is generally too strong and drives damage into the stone.

Answer: A

Both are true and the reason gives the principle: the joint should be the sacrificial element, and a cement mortar reverses that arrangement in a wall built for something gentler. Compatibility matters more than strength in conservation work.

Answer: B

Correct answer: False

A mortar stronger than the units it beds becomes the strong element, so movement is accommodated by cracking the stone or brick instead of the joint. Repointing should use a mortar weaker than the units and no stronger than the original, which is why lime mortars are specified for historic masonry rather than cement ones.

Answer: C

The clay already present in the stone supplied the silica and alumina, so no separate raw mix was needed. It set quickly and was much used for stucco before Portland cement displaced it. Its rapid set suited the decorative work of the period.

Answer: D

Deeper wells mean higher temperature and pressure, which shortens the thickening time and demands a different formulation. The classification exists so that a cement can be selected against the conditions it will meet.

Answer: A

Correct answer: a retarder is needed or another class must be chosen

Two hours forty is less than the three hours required, so the slurry would begin to stiffen while still being pumped - which risks losing the well. The remedy is a retarder or a slower class of cement, and an accelerator would make matters very much worse.

Answer: B

Correct answer: True

A well cement is placed at depths where the temperature may exceed 150 degrees and the pressure many hundreds of bar, both of which shorten the thickening time greatly. Testing at the surface would give a figure with no bearing on the job, so the consistometer reproduces the downhole schedule.