Hydration of Cement Questions and Answers

Answer: C

Keeping the water constant gives a more workable mix, while keeping the workability constant allows less water and a higher strength. Which benefit is taken is a decision for the mix designer. The admixture provides the option rather than dictating the outcome.

Answer: D

Correct answer: resistance to freezing and thawing

Entrained air introduces a system of fine, closely spaced, deliberately unconnected bubbles that give freezing water somewhere to expand into, relieving the hydraulic pressure that would otherwise disrupt the paste. The spacing factor matters more than the total air content, since every point in the paste must be within reach of a bubble.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason states both effects of the same bubbles. The voids relieve freezing pressure, which is the point, and they also reduce the solid volume of the paste, costing roughly five per cent of compressive strength for each one per cent of air. In an exposed structure that is a trade well worth making, since a concrete destroyed by frost has no strength at all.

Answer: B

The first two are correct. The third reverses the effect, since entrained air slightly reduces strength and is used for freeze-thaw durability rather than for load-bearing capacity. Air content is specified as a range rather than as a minimum.

Answer: C

Entrained air consists of deliberately introduced fine bubbles of controlled size, whereas entrapped air is larger voids left by incomplete compaction. One is designed for and the other is a defect. Compaction removes the second but cannot create the first.

Answer: D

Correct answer: forming expansive compounds with the aluminate phase

Sulphate reaching the paste reacts with the calcium aluminate hydrates to form ettringite, which occupies a much greater volume than the phases it replaces, so the paste expands and cracks from within. The defences follow from the chemistry: a sulphate-resisting cement low in tricalcium aluminate, or a low permeability that keeps the sulphate out in the first place.

Answer: A

Reactive silica in certain aggregates reacts with the alkalis in the pore solution to form a gel that takes up water and expands. The damage appears years after construction, which makes prevention far more practical than cure.

Answer: B

The first two identify the two mechanisms correctly. The third is precisely backwards, since a higher ratio gives a more permeable concrete and makes every form of chemical attack easier. Permeability rather than strength is the governing property here.

Answer: C

A denser paste with fewer connected capillaries admits aggressive solutions far more slowly. Permeability rather than strength is what governs most durability problems, though the two go together. Specifications often set the ratio as well as the strength class.

Answer: D

Both are true and the reason gives the logic: strength and permeability are related but not identical, and it is the transport of chloride, sulphate and carbon dioxide into the concrete that actually causes the deterioration.