An important ingredient in mortar is lime.

It's an ingredient in "lime mortar."

Often, mortar is just portand cement and sand.

(Lime mortars would "set" by drying out and then SLOWLY harden by reacting
with the CO2 in the air. Portland cement hardens by reacting with water
and "setting up." Portand cement is about as strong as bricks and blocks.
Cracks tend to break the bricks and blocks. Lime mortar isn't as strong
but does "flex" . Small cracks might not be noticed and larger movements
result in the joints failing but with the bricks/blocks not being harmed.)