Carbonation

Summary

Carbonation - is a reaction that occurs between carbon dioxide from the air and calcium hydroxide in concrete. It contributes to the corrosion of steel reinforcement in the concrete.

Technical Information

Carbonation is effectively the reversal of calcination (the heating of limestone to form cement materials), and starts as soon at the concrete is poured. It increases the mechanical strength of concrete, but at the same time, lowers the pH. Because a high pH is essential for the protection of steel reinforcement from corrosion, this makes the steel reinforcement vulnerable.

Some signs that extensive carbonation has occurred in concrete are:

There may be some bright orange discolouration on the surface of the concrete if the steel has already started to erode.
Putting phenolphthalein onto the surface will show any heavily carbonated areas up.
If a sample is looked at through an optical microscope, the presence of calcite crystals and the absence of calcium hydroxide indicate that carbonation has occurred.

Carbonation also causes shrinkage and a slight increase in the mass of a concrete element.