Ensuring that a replacement mortar will be sympathetic to historic masonry is key to a successful repointing project. Our goal is to make sure that the new mortar is a sacrificial element, meaning that it, NOT the masonry units, will slowly deteriorate and can be periodically replaced. This deterioration occurs because the mortar rather than the masonry is handling the stress of building movement and moisture. Mortar should be breathable enough to transfer moisture out of the wall system and flexible enough to absorb the pressure of freeze-thaw cycles and building movement. Resultantly, the chosen mortar should be more vapor permeable and “softer” than the original mortar and the masonry units.
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New construction mason contractors are generally familiar with Type N or S mortars. These are often specified for modern wall systems because they rely on air barriers and expansion joints, neither of which historic masonry buildings feature. Historically, masonry buildings were load bearing and relied on mortar to 1) transfer vapor and moisture out of monolithic wall systems and 2) to accommodate freeze-thaw cycles and building movement. As a result, relatively rigid and impermeable mortar that is normally used successfully in new construction, is detrimental to historic masonry buildings.
Many people believe that Type N or Type O are “soft” enough for historic masonry. This misunderstanding derives from the ASTM standards for property blended (as opposed to proportion blended) which set minimum 28-day PSI requirements of 350 for Type O and 750 for Type N. However, these are *minimum* requirements and test data shows that property blended Type O and Type N can actually readily achieve strengths of 1200 and 1700 PSI respectively. These strengths are much too high for most historic buildings. Proportion blended mortars, which are not tested for performance, can easily achieve even higher strengths. Additionally, Portland cement is less vapor permeable than lime, as shown in this visual originally produced by The Construction Materials Consulting Group of Stirling, Scotland that compares lime putty, St. Astier hydraulic lime, and conventional Portland cement mortars.