When the masonry has been laid, the

When the masonry has been laid, the slaked lime in the mortar slowly begins to react with carbon dioxide to form calcium carbonate (limestone) according to the reaction: Ca(OH)2 + CO2 → CaCO3 + H2O. The carbon dioxide that takes part in this reaction is principally available dissolved in rainwater rather than in gaseous form.
This process by which limestone is converted to quicklime by heating, then to slaked lime by hydration, and reverted to limestone by carbonation is known as the Lime Cycle.