Lime mortar is a mortar made with lime instead of, or with far less, Portland cement, producing a softer, more breathable joint historically used before roughly the 1930s and still specified for repairing old masonry.
Lime mortar is a mortar made primarily or entirely with lime instead of Portland cement, producing a softer, more breathable, more flexible joint than modern cement-based mixes. It was the standard mortar for most masonry built before roughly the 1930s, when Portland cement mortar became the industry default. Lime mortar also has a self-healing property, called autogenous healing, where fine cracks in the joint can partially reseal themselves as lime reacts with moisture and carbon dioxide over time.
That breathability matters a lot when we're repointing genuinely old brick or stone. Repointing a pre-1930s wall with hard modern Portland cement mortar traps moisture inside the brick instead of letting it wick out through the joint, and once winter hits, that trapped moisture freezes, expands, and spalls the brick face from the inside out. Matching lime mortar to lime-era masonry keeps the wall breathing the way it was built to.
More masonry vocabulary from the glossary.
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