Hydration of Cement Questions and Answers

Answer: C

Water not consumed by hydration leaves capillary pores behind as it evaporates, and strength falls sharply as porosity rises. This single ratio does more to determine the quality of concrete than any other decision in the mix.

Answer: D

Correct answer: False

Extra water beyond what hydration needs does not react; it occupies space and, on evaporating, leaves capillary pores. Strength falls roughly in proportion as the water-cement ratio rises, and permeability rises with it, so durability suffers as well. Where more workability is wanted the correct remedy is a water-reducing admixture, which gives it without the water.

Answer: A

Dividing 175 by 350 gives 0.50. Ratios below about 0.45 are usual where durability matters, while much above 0.6 gives a porous paste of limited strength. The ratio is the single most influential number in a mix design.

Answer: B

Correct answer: a quarter of the mass of the cement

About 0.23 to 0.25 of the cement mass is chemically combined in complete hydration. In practice a somewhat higher ratio is needed - around 0.38 - because the gel pores must also be filled for hydration to proceed, and higher still for workability without admixtures. Everything above that requirement becomes capillary porosity, which is why the water-cement ratio governs strength so directly.

Answer: C

Both are true and the reason gives the compromise: workability demands water that the chemistry does not need, and the surplus leaves the capillary pores that limit strength and durability. Every litre beyond the chemical requirement leaves pores behind.

Answer: D

Such a mix is barely damp and cannot be compacted by ordinary means. Superplasticisers are what make very low ratios workable, and they are the basis of high-strength concrete. Chemistry rather than more water is what buys the workability.

Answer: A

Hydration stops if the paste dries, and strength gain stops with it, permanently. Poor curing is among the commonest causes of concrete failing to reach the strength its mix design promised. Curing is cheap to do well and expensive to get wrong.

Answer: B

Correct answer: False

Hydration stops when the paste dries, and the products already formed do not undo themselves - but the capillary structure set up by early drying is permanent. Re-wetting restarts hydration in whatever pore space remains, so some strength is recovered, but the concrete never reaches what continuous curing would have given. That irreversibility is the whole argument for protecting a pour from the first hour.

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. Something like half the eventual strength develops in the first three days and around three quarters within a week, and it is in that period that the capillary network is being closed off by hydration products. Curing later cannot repair a structure that was allowed to form coarse and open, which is why the first few days are worth more than all the weeks after them.

Answer: D

Correct answer: larger and represent space once occupied by water

Capillary pores are the remains of the water-filled space between cement grains that hydration products have not filled, typically tens of nanometres upward, and they are what govern permeability and strength. Gel pores are far smaller and are an intrinsic part of the calcium silicate hydrate itself, so their volume is fixed by how much gel there is. Only the capillary porosity can be reduced, by lowering the water-cement ratio and curing properly.