Carbonation of concrete

Carbonation is the reaction of atmospheric carbon dioxide with the hydrated cement compounds, mainly calcium hydroxide, in the presence of moisture:

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

• The reaction begins at the surface and moves inwards; the depth follows $d = K\sqrt{t}$ ($t$ in years).
• The pH of the pore solution falls from about 12.5 to 13.5 to about 8 to 9, which destroys the passive film on the steel and starts corrosion once the carbonation front reaches the bar.
• Rate is highest at relative humidity of 50 to 70% (dry concrete has no water; saturated concrete blocks CO$_2$ entry). It increases with high w/c, low cement content, poor curing and cracks, and high CO$_2$ concentration (cities).
• It is detected with phenolphthalein spray: the uncarbonated concrete (pH above 9.2) turns pink, carbonated concrete stays colourless.
• It causes some shrinkage (carbonation shrinkage), and slightly raises surface hardness and strength by filling pores.
• Prevent by low w/c, enough cement, proper curing, adequate cover (IS 456 Table 16), and protective coatings.