Cement pointing reverses that relationship. It is harder than most historic brick, so the face of the brick spalls instead of the joint eroding.
How lime works
Lime mortars are permeable, allowing moisture to evaporate through the joint rather than being driven through the masonry face. They also accommodate small movement without cracking, and they can self-heal fine cracks over time.
Non-hydraulic lime putty mortars suit the softest historic work; natural hydraulic limes come in grades for more exposed positions and harder masonry.
The damage cement causes
Once cement pointing is in place, water that enters the wall can only leave through the brick. In frost, that water expands within the brick face and takes the surface off.
Removing failed cement pointing without further damaging the arris of soft bricks is slow, careful work, and it is a significant part of the cost of putting an over-pointed elevation right.
Specifying a repointing job
The mix should be matched to the masonry, the exposure and the original mortar, and the joint profile should match what was there — usually flush or slightly recessed, not a weather-struck ribbon standing proud of the brick.
Lime work is weather-dependent. It needs protection from frost and from drying too fast, which limits the season and is a legitimate reason for a contractor to programme it carefully.
Common questions
Can I point an old house with cement?
You can, but on soft historic brick or ragstone it typically leads to spalled faces and trapped moisture. Lime is the appropriate material.
Is lime mortar weaker?
It is deliberately softer than the masonry. That is the point — the joint takes the wear instead of the brick.
How long does lime pointing take to cure?
It carbonates slowly and needs protection from frost and rapid drying during that period, which is why the time of year matters.
Should old cement pointing be removed?
Where it is causing brick faces to spall, yes — carefully, by hand where the masonry is soft.