For most of the twentieth century, repointing an old masonry wall meant mixing whatever mortar was on hand, which after the mid-1900s usually meant a Portland-cement-based mix — hard, fast-curing, and far stronger than the lime mortar the wall was originally built with. That strength turns out to be exactly the wrong property for repointing historic brick or soft stone.

Older masonry, particularly soft handmade brick and many sedimentary stones, was designed to work with a mortar softer than the masonry itself, so that any movement or moisture-driven expansion is absorbed by the mortar joint rather than the brick or stone face. Portland cement mortar, being harder than the masonry around it, forces that movement into the brick instead, which is why cement-repointed historic walls so often show cracked or spalling brick faces around intact-looking mortar joints.

Cement mortar is stronger than the brick around it. On an old wall, that's a problem, not an advantage.

Lime mortar also shares the vapor permeability that's driving renewed interest in lime and clay plasters elsewhere in the industry — it lets moisture that gets into a wall migrate back out and evaporate, rather than trapping it behind an impermeable cement joint, where it eventually forces its way out through the masonry face instead.

Working with lime mortar requires different technique and patience than cement: it cures slowly through carbonation rather than a fast chemical set, needs protection from rain and frost during that cure, and behaves differently under the trowel — skills that restoration masons increasingly train for specifically, as the material has moved from a niche specialty back toward standard practice on heritage projects.

Preservation architects now specify lime mortar as close to a default on masonry restoration work, reserving cement-based mortars for genuinely new construction where the compatibility issue with an existing historic material doesn't apply.