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.
Mortimer, Charles E. Chemistry, 6th edition. Belmont: Wadsworth, 1986.
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.