Civil Engineering Reference
In-Depth Information
Carbonation
Carbon dioxide gas is an acidic gas, when dissolved in water, and reacts with
the alkaline hydroxides in the concrete. Being a weak acid, the carbonic acid
does not significantly attack the cement paste, but just neutralises the alkalis
in the pore water, mainly forming calcium carbonate.
CO
2
+ H
2
O
H
2
CO
3
gas water carbonic
acid
H
2
CO
3
+ Ca(OH)
2
→
CaCO
3
+ 2H
2
O
carbonic pore calcium water
acid solution carbonate
→
There is a lot more calcium hydroxide in the concrete pores than can be
dissolved in the pore water. This helps maintain the pH at a high level as the
carbonation reaction occurs. However, eventually all the locally available
calcium hydroxide reacts, precipitating the calcium carbonate and allowing
the pH to fall to a level where steel will corrode. This is illustrated in
Figure
