Carbonation occurs in concrete because the calcium bearing phases present are attacked by carbon dioxide of the air and converted to calcium carbonate. Cement paste contains 25-50 wt% calcium hydroxide (Ca(OH)₂), which mean that the pH of the fresh cement paste is at least 12.5. The pH of a fully carbonated paste is about 7. 

The concrete will carbonate if CO₂ from air or from water enters the concrete according to:

             Ca(OH)₂ + CO₂        ->       CaCO₃ + H₂O

When Ca(OH)₂ is removed from the paste hydrated CSH will liberate CaO which will also carbonate. The rate of carbonation depends on porosity & moisture content of the concrete.

The carbonation process requires the presence of water because CO₂ dissolves in water forming H₂CO₃. If the concrete is too dry (RH <40%) CO₂ cannot dissolve and no carbonation occurs. If on the other hand it is too wet (RH >90%) CO₂ cannot enter the concrete and the concrete will not carbonate. Optimal conditions for carbonation occur at a RH of 50% (range 40-90%).

Normal carbonation results in a decrease of the porosity making the carbonated paste stronger. Carbonation is therefore an advantage in non-reinforced concrete. However, it is a disadvantage in reinforced concrete, as pH of carbonated concrete drops to about 7; a value below the passivation threshold of steel.

Carbonation may be recognized in the field by the presence of a discolored zone in the surface of the concrete. The color may vary from light gray and difficult to recognize to strong orange and easy to recognize. Carbonation can be visualized by using phenolphthalein. 

In the optical microscope carbonation is recognized by the presence of calcite crystals and the absence of calcium hydroxide, ettringite and un-hydrated cement grains. Porosity is unchanged or lower in the carbonated zone.

Bi-carbonation - what is that?

Occasionally concrete may suffer from the so called bi-carbonation process. Bi-carbonation may occur in concrete with very high water to cement ratio due to formation of hydrogen carbonate ions at pH lower than 10. Contrary to normal carbonation, bi-carbonation results in an increase in porosity making the concrete soft and friable. Bi-carbonation may be recognized by the presence of large  "pop-corn" like calcite crystals and the highly porous paste.

Please to not hesitate to contact CXI if you have any questions regarding carbonation or any other deterioration mechanisms.