Why do we need Calcium hydroxide in concrete? Is it necessary there?

May 15, 2016

The calcium hydroxide is formed as the concrete hardens.

Explanation:

How concrete hardens

The dry concrete powder (Portland cement) is a mixture of several compounds.

The two most important ones are tricalcium silicate (${\text{Ca"_3"SiO}}_{5}$) and dicalcium silicate ($\text{Ca"_2"SiO"_4}$), because they are the ones that eventually contribute to the strength of the finished concrete.

When water is added to cement, these compounds undergo hydration.

For example:

underbrace("2Ca"_3"SiO"_5)_color(red)("tricalcium silicate") + "7H"_2"O" → underbrace("3CaO·2SiO"_2·4"H"_2"O")_color(red) ("calcium silicate hydrate") + "3Ca(OH)"_2 + "173.6 kJ"

The reaction releases $\text{Ca"^"2+", "OH"^"-}$, and a large amount of heat. The mixture reaches pH 11.

Eventually, the system becomes saturated, and ${\text{Ca(OH)}}_{2}$ starts to crystallize along with calcium silicate hydrate.

You end up with a thick mass of crystals interlocked with each other and all the other substances present.

The effect of ${\text{CO}}_{2}$

Carbon dioxide from the air reacts with the calcium hydroxide in concrete to form calcium carbonate.

$\text{Ca(OH)"_2 + "CO"_2 → "CaCO"_3 + "H"_2"O}$

This reaction, called carbonatation, is a slow and continuous progression from the outer surface inward.

Carbonatation increases the mechanical strength of concrete, but it also decreases the pH.

Below pH 10, the passivated surface of the steel rebar dissolves, and the rebar starts to corrode.