In conservation work and like-for-like repair on older masonry, lime mortar is the only recommended material. The thick, plain or lightly punctured walls that make up most historic buildings have few concentrations of load. Calculations of stress in such cases are often needless and, subject perhaps to the check of any critical element, we can generally lay aside our concerns about mortar strength.
In contrast, the need to maintain a balance of moisture and flexibility in the body of an old wall is essential. Ignoring this will lead to the classic error of repointing old structures in brittle, impermeable Portland cement
(OPC) mortar. The mortar provides the route for evaporation from the core and should be more permeable than the brick or stone. To reverse this by sealing the joints with a hard finish can only lead to trouble.