Hydration of Cement Questions and Answers

Answer: A

Often written as C-S-H, it is a poorly crystalline gel that fills the space between the grains and gives the paste its strength. Calcium hydroxide is formed alongside it but contributes far less to the strength of the hardened material.

Answer: B

The high pH forms a passive film on embedded steel that resists corrosion. Carbonation gradually consumes the hydroxide and lowers the pH, which is what eventually puts the reinforcement at risk. Cover depth is specified to delay that front reaching the steel.

Answer: C

Both are true and the reason gives the mechanism: the passive film on the steel depends on a high pH, so once the carbonation front reaches the bar the protection is lost and corrosion can begin in the presence of moisture and oxygen.

Answer: D

Correct answer: False

Strength comes from calcium silicate hydrate, the poorly crystalline gel that makes up roughly half the volume of hardened paste and binds everything together. Calcium hydroxide is a by-product, forming perhaps a quarter of the solids as large crystals that contribute little strength and are the phase most readily leached or attacked. Its real value is keeping the pore water alkaline enough to passivate the reinforcement.

Answer: A

Converting the weak crystalline hydroxide into further calcium silicate hydrate both adds strength and densifies the paste. The cost is a slower early gain, since the reaction cannot begin until the clinker has released the hydroxide.

Answer: B

A brief burst of reaction on wetting is followed by a quiet period of a few hours during which the concrete remains workable. Acceleration then sets in and the paste stiffens and hardens before the reaction slows again.

Answer: C

Both are true and the reason gives the cause: the dormant period is what makes transport and placing possible at all, and its length is one of the things admixtures are used to adjust. Retarders lengthen it and accelerators shorten it as the job requires.

Answer: D

Correct answer: False

Setting is the loss of plasticity - the paste stiffens and can no longer be placed or compacted - and it is measured by penetration in the Vicat test. Hardening is the development of strength thereafter, and it continues for months. A mix can be set and still have almost no strength, which is exactly why the initial setting time governs how long there is to place the concrete and says nothing about when it may be loaded.

Answer: A

Correct answer: temperature differences cause cracking as the structure cools

Hydration is exothermic, and in a large pour the heat cannot escape, so the core rises well above the surface temperature. The surface then cools and contracts against a hot, expanded core, and later the whole mass contracts against its restraints - producing cracking in both phases. The remedies all attack the heat: low-heat cements, partial replacement with fly ash or slag, cooled aggregate, and pipe cooling.

Answer: B

Three hundred joules for each of 350,000 grams gives 105 million joules, which is 105 megajoules. Quantities of this order can raise the temperature of a large pour by several tens of degrees. Peak temperature is often predicted before a large pour is placed.