Cement and Refractories Questions and Answers

Answer: C

Correct answer: loss of plasticity soon after mixing

Setting is the stiffening of the paste in the first hours, the point at which it ceases to be workable. Hardening is the development of strength that follows and continues for weeks and months. Both are consequences of the same hydration reactions, but they are separate stages and are measured by quite different tests.

Answer: D

Correct answer: hydration of the clinker compounds

Cement sets by chemical reaction with water, not by drying. The silicates and aluminates hydrate to form calcium silicate hydrate gel and calcium hydroxide, and it is the interlocking gel that gives strength. This is why cement hardens perfectly well under water, and why allowing concrete to dry out is a defect rather than a step in the process.

Answer: A

Correct answer: stop the hydration reaction prematurely

Hydration needs water, and if the concrete dries out the reaction stops wherever the water has gone, leaving unhydrated cement and a weak, permeable structure. The surface is the first to suffer, which is why an uncured slab dusts and crazes. Curing keeps the water available until enough of the cement has reacted.

Answer: B

Correct answer: thermal cracking as the interior cools

In a large pour the heat of hydration cannot escape, so the interior rises many tens of degrees above the surface and expands. As it later cools it contracts against a surface that has already set, and the resulting tensile stress cracks the mass. A low-heat cement, higher in dicalcium silicate and often blended with pozzolana, releases the same heat far more slowly.

Answer: C

Correct answer: 30 minutes

The initial setting time is specified as a minimum, not a maximum, and it must be at least about 30 minutes so that there is time to mix, transport, place and compact the concrete before it stiffens. The final setting time is specified as a maximum, usually 600 minutes, so that the work can proceed.

Answer: D

Correct answer: excess free lime or magnesia causing later expansion

Free lime and magnesia hydrate very slowly, long after the concrete has hardened, and the hydration products occupy more volume than the oxides, so the hardened paste expands and disrupts. The Le Chatelier test accelerates this by boiling the specimen and measures the expansion. It detects a delayed fault, which is why it cannot be replaced by a setting or strength test.

Answer: A

Correct answer: rate of hydration and early strength

Hydration proceeds from the surface of each grain inward, so finer grinding exposes more surface and the reaction goes faster. Early strength therefore rises with fineness, as does the rate of heat release. The ultimate strength changes little, since given time even coarse grains hydrate, and finer grinding costs more energy and makes the cement more prone to deterioration in store.

Answer: B

Correct answer: lime

The clinker liquid in the burning zone is strongly basic, being rich in lime, and an acidic silica lining would be attacked by it directly. A basic refractory - magnesite or magnesia-chrome - is chemically compatible with the melt and survives. The general rule is that the lining must be of the same chemical character as whatever it is in contact with.

Answer: C

Correct answer: water content giving a standard paste

The standard consistency test finds the percentage of water at which a Vicat plunger penetrates to a specified depth, typically 26 to 33 per cent by weight of cement. It is not an end in itself: the setting time and soundness tests are both performed on a paste of standard consistency, so the figure must be determined first for the results to be comparable.

Answer: D

The poorly crystalline gel formed by both silicates fills the space between the grains and binds them together. The calcium hydroxide liberated alongside it contributes little strength and is the component a pozzolana consumes.