The tutor explains why mortar hardens over time.

A single replacement reaction is of the form

AB + C → AC + B

The setting of mortar happens by such a reaction:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Ca(OH)₂ is an original ingredient of the mortar. After installation, it chemically absorbs CO₂, thereby transforming to calcium carbonate (CaCO₃). Water is liberated.

Calcium carbonate is about 1/100 as soluble as calcium hydroxide. Therefore, the mature mortar should be more persistent.

Source:

baldeagleconstruction.com

Mortimer, Charles E. Chemistry, 6th edition. Belmont: Wadsworth, 1986.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.