Carbonation Shrinkage

In the presence of moisture, atmospheric carbon dioxide entering the concrete surface transforms into carbonic acid, which chemically reacts with the calcium hydroxide in the hydrated cement to form calcium carbonate and water.

The calcium carbonate produced during the reaction deposits in the concrete pores, while the released water aids the hydration of unreacted cement.

This results in the contraction of the concrete, known as carbonation shrinkage.

Carbonation starts at the concrete's surface and slowly moves inward.

Its progress depends on the concrete's permeability, moisture level, carbon dioxide content, and relative humidity of the ambient air. 

As carbonation advances, the alkaline nature of the hydrated cement paste neutralizes.

Consequently, if oxygen and moisture penetrate the concrete, corrosion of the steel reinforcement embedded within it is likely to occur.

Commonly, to assess the extent of carbonation in concrete, a freshly broken surface is sprayed with phenolphthalein.

Areas that turn pink reveal the presence of free calcium hydroxide, indicating the absence of carbonation, while the areas that retain their original color indicate carbonation.