Atmospheric CO₂penetrates the concrete's pores and, in the presence of moisture, forms carbonic acid, which then reacts with calcium hydroxide in the hydrated cement, forming calcium carbonate. This process reduces the concrete's volume and is termed carbonation shrinkage.

The concrete's permeability is slightly reduced as calcium carbonate produced during the reaction fills its pores. Furthermore, its strength is slightly enhanced as the water released during the reaction facilitates the hydration of the unreacted cement. However, carbonation has a more concerning effect by neutralizing the alkaline nature of the cement paste, and the risk of corrosion increases if carbonation reaches the reinforcement steel and allows moisture and oxygen to interact with it.

Carbonation is more significant in environments where concrete is shielded from direct rainfall yet exposed to moist air, compared to that which is periodically rinsed by rain. This is because the water-filled pores slow down CO₂diffusion. Carbonation typically progresses from the surface inward, and its rate is influenced by the concrete's permeability, moisture content, CO₂levels, and ambient relative humidity.

To measure carbonation depth, a freshly broken surface of concrete is treated with a phenolphthalein indicator. The non-carbonated areas on the treated surface turn pink due to the presence of calcium hydroxide, while carbonated areas do not change their color.