Carbonation of concrete occurs when the carbon dioxide, in the atmosphere in the presence of moisture, reacts with hydrated cement minerals to produce carbonates, e.g. calcium carbonate.
The carbonation process is also called depassivation. Carbonation penetrates below the exposed surface of concrete extremely slowly. Carbonation is significant as it neutralizes the alkanility present in the concrete. Thus, if it reaches the reinforcements it makes them more susceptible to corrosion. Commonly termed as concrete cancer.