Until the 19th century buildings were built to breath ultimately being built in lime mortar. Lime is a living breathing material as it can move slowly with a building without cracking and respires moistures avoiding a damp build-up.
Cement is a hard, brittle and unyielding material and sets rigid which resists water penetration instead of breathing.
It is a mistake to mix the old with
the new and to replace the original flexible lime mortar with modern
cement as it begins to suffocate the building as the moisture which
used to be drawn out of the wall via the seams of lime and sand is now
trapped in the fabric by the rigid network of cement. This causes it
to be vulnerable to frost damage where the wall can no longer dry out
naturally and becomes cold and damp ultimately damaging the building.