Carbonation
A slow reaction in which carbon dioxide from the air neutralizes the alkalinity of concrete near the surface, removing the chemical protection that keeps embedded steel from rusting.
What does carbonation mean?
Carbonation advances inward over years and does not damage the concrete itself; the problem is that once the carbonated zone reaches the reinforcement, corrosion can begin. Rate of advance depends on permeability, moisture, and cover thickness, so dense well-cured concrete resists it. In a different and much faster form, carbon dioxide from unvented heaters can carbonate a fresh floor surface overnight, causing severe dusting.
In practice
Salamander heaters running in a closed building during a winter pour are a classic cause of a soft, dusting floor the next morning.