Hydration of Cement Questions and Answers

Answer: A

Strength depends steeply on porosity, so halving the capillary volume raises strength considerably. This is the underlying reason the water to cement ratio matters so much to the finished concrete. Porosity is the link between the mix design and the result.

Answer: B

Calcium hydroxide brought out by moving water reacts with carbon dioxide to leave a white deposit. It is usually a blemish rather than a structural problem, though it indicates that water has been moving through the concrete.

Answer: C

It is normally a surface blemish caused by water movement and evaporation. It does show that water is passing through the concrete, which may be worth investigating for other reasons. Persistent efflorescence often points to a leak somewhere.

Answer: D

Excessive bleeding leaves a weak layer at the surface and can trap water beneath aggregate particles. It is reduced by better grading, by finer cement or by a lower water content. Some bleeding is normal and only excess is a problem.

Answer: A

Correct answer: speed early strength gain in cold weather

An accelerator brings forward setting and early strength, which is what cold weather working needs: formwork struck sooner, and the concrete carried past the point where frost could damage it. Extending the placing time is a retarder, reducing the water is a plasticiser, and entraining bubbles is an air-entraining agent - four different admixtures for four different problems.

Answer: B

Chloride ions destroy the passive film on reinforcement and cause corrosion, so chloride-free accelerators are used instead. The prohibition is one of the firmest rules in concrete practice. Unreinforced work is the only place the old practice survives.

Answer: C

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

Both are true and the reason is the mechanism. The pore solution’s high alkalinity holds a passive oxide film on the steel; chloride ions penetrate that film locally and initiate pitting, and the rust formed occupies several times the volume of the steel consumed, so it splits the cover off. Calcium chloride is the cheapest and most effective accelerator known, and is barred from reinforced work for exactly this reason.

Answer: D

Delaying the set keeps the concrete workable long enough for a large pour to be placed without a cold joint. High temperatures accelerate hydration, so the need is greatest in exactly the conditions that shorten the time available.

Answer: A

Cement grains flocculate and trap water between them, and the admixture disperses them so that the trapped water becomes available for workability. The same slump can then be reached at a lower water content and a higher strength.

Answer: B

Twelve per cent of 200 is 24 litres, leaving 176. With the cement content unchanged the ratio falls and the strength rises correspondingly, which is why these admixtures are so widely used. Nearly all structural concrete now contains one.