Calcium Carbonate is in the equilibrium with the Carbonate ion and the calcium ion as shown below.
Here, formation of the carbonate takes place.
In the presence of Water and Carbon dioxide :
Formation of hydrogen carbonate ( ) takes place.
Medium of the solution becomes acidic, as a result the carbonate ions get protonated and bicarbonate ions start forming.
Further, it causes shifting of equilibrium and more and more formation of carbonate ions and hence after protonation, formation of bicarbonate ions takes place which then forms Calcium bicarbonate .