Carbon dioxide () from air  and bicarbonate () or carbonate () anions dissolved in water react with the calcium hydroxide (, portlandite) produced by Portland cement hydration in concrete to form calcium carbonate () while releasing a water molecule in the following reaction: 
Exception made of the water molecule, the carbonation reaction is essentially the reverse of the process of calcination of limestone taking place in a cement kiln: 
Carbonation of concrete is a slow and continuous process of atmospheric  diffusing from the outer surface of concrete exposed to air into its mass and chemically reacting with the mineral phases of the hydrated cement paste. Carbonation slows down with increasing diffusion depth.
Carbonation has two antagonist effects for (1) the concrete strength, and (2) its durability: 
# The precipitation of calcite filling the microscopic voids in the concrete pore space decreases the concrete matrix porosity: so, it increases the mechanical strength of concrete; 
# At the same time carbonation consumes portlandite and therefore decreases the concrete alkalinity reserve buffer. Hyper-alkaline conditions (i.e., basic chemical conditions) characterized by a high pH (typically 12.5 – 13.5) are needed to passivate the steel surface of the reinforcement bars (rebar) and to protect them from corrosion. Below a pH of 10, the solubility of the iron oxides forming a protective thin coating at the surface of carbon steel increases. The thin protective oxide layer starts to dissolve, and corrosion is then promoted. As the volumetric mass of iron oxides can be as high as 6 – 7 times that of metallic iron (Fe), a detrimental consequence is the expansion of the corrosion products around the rebar. This causes the development of a tensile stress in the concrete matrix around the rebar. When the tensile strength of concrete is exceeded in the concrete cover above the rebar, concrete starts to spall. Cracks appear in the concrete cover protecting the rebar against corrosion and constitute preferential pathways for  direct ingress towards the rebar. This accelerates the carbonation reaction and in turn the corrosion process speeds up.
This explain why the carbonation reaction of reinforced concrete is an undesirable process in concrete chemistry. Concrete carbonation can be visually revealed by applying a phenolphthalein solution over the fresh surface of a concrete samples (concrete core, prism, freshly fractured bar). Phenolphthalein is a pH indicator, whose color turns from colorless at pH < 8.5 to pink-fuchsia at pH > 9.5. A violet color indicates still alkaline areas and thus non-carbonated concrete. Carbonated zones favorable for steel corrosion and concrete degradation are colorless.
The presence of water in carbonated concrete is necessary to lower the pH of concrete pore water around rebar and to depassivate the carbon steel surface at low pH. Water is central to corrosion processes. Without water, the steel corrosion is very limited and rebar present in dry carbonated concrete structures, or components, not affected by water infiltration do not suffer from significant corrosion.