Types of Cement and Composition Questions and Answers

Answer: C

Both are true and the reason gives both limits: the specification brackets the sulphate content from each side, which is why it is stated as a maximum in the standards and controlled closely in manufacture. Both faults are serious, so the range is narrow in practice.

Answer: D

The iron compound gives cement its characteristic colour, and reducing it is how white cement is made. Its contribution to strength is comparatively small. It also lowers the temperature at which the kiln charge begins to melt.

Answer: A

Iron gives ordinary cement its grey colour, so china clay and limestone of low iron content are used instead. The cement is correspondingly more expensive to make. It is specified for architectural finishes and for terrazzo work.

Answer: B

More of the fast-reacting silicate and a greater surface area together speed the early strength gain. The final strength is not necessarily higher than that of ordinary cement. The gain is in the rate rather than in the final figure.

Answer: C

It hardens faster, gaining strength more quickly, but its setting time is much the same. Setting and hardening are distinct processes and the name refers only to the second. A rapid hardening cement still allows normal time for placing.

Answer: D

Both are true and the reason gives the distinction: one cement reaches usable strength sooner and the other stiffens sooner, and confusing them can lead to a material quite unsuited to the work in hand. The two properties are specified separately for good reason.

Answer: A

Correct answer: releases less heat during hydration

In a large pour the heat cannot escape and the core rises well above the surface temperature, so the mass cracks as it cools against its own restraint. Low heat cement is formulated with less tricalcium silicate and less tricalcium aluminate, the two most exothermic phases, so the peak temperature is lower and reached more slowly. It gains strength more slowly too, which mass concrete can afford.

Answer: B

The aluminate reacts with sulphates to form expansive ettringite, so limiting it limits the damage. Cements for marine and sulphate-bearing ground are specified on this basis. Foundations in gypsum-bearing soils are a common case.

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 cause. Sulphate entering the paste reacts with the calcium aluminate hydrates to form ettringite, which occupies a much larger volume and disrupts the concrete from within. Limiting the tricalcium aluminate content - typically to five per cent or less - removes most of what the sulphate can attack, which is precisely how the cement is specified.

Answer: D

The pozzolana reacts with the calcium hydroxide released during hydration to form further binding compounds. Strength develops more slowly but durability is often better. The reaction also consumes lime that would otherwise be leached out.