The common sense answer to that question is this:
If the building was built with it, shouldn’t we restore with it?
If America was built with lime mortar, why did we ever stop using it?
The simple answer is that someone invented a mortar that got hard much more quickly, so masons everywhere began using it for that advantage. They called it portland cement.
Lime mortar takes days to get hard because it reacts with the air, while portland cement gets hard by a chemical reaction that takes a few hours and happens evenly throughout the wall.
That’s a big advantage for portland cement.
But lime mortar has several properties that make it the obvious first choice for historic home restoration.
First, lime mortar lasts longer than any other kind of mortar. Proven by time.
Why is this? Because other mortars are less flexible and less vapor permeable so they trap moisture and deteriorate.
Second, lime mortar is breathable, so it repels rain and releases ground moisture very quickly.
Why is this so important? Because of how homes were built before 1920 or so.
Homes used to be built with STRUCTURAL MASONRY WALLS. That means the outside walls were solid brick or solid stone ranging from 8″ to 24″ thick that went down into the ground to form the basement. Larger buildings and barns were even thicker than that at the foundation.
Structural masonry walls sit right on the ground. The ground is damp. The wall absorbs moisture from the ground.
Since lime mortar and lime plaster and lime stucco are all very breathable (vapor permeable), that ground moisture can escape very easily into the air within a few inches of the ground. Even paint was breathable in those days, so they could release moisture too.
This has proven to be a very long lasting building system. This is how we did things for centuries, and homes lasted with little maintenance for centuries. As long as the roof was good and there weren’t springs in the basement or constant flooding, the building lasts a long time.
Breathability is important not only for longevity and maintenance, but it also means a dry home. Few mold or mildew issues. Few problems with dampness. Basements are naturally damp but not moldy. Living spaces are dry. Humidity inside escapes easily right through the walls, and wet weather doesn’t affect the house because the walls can absorb and release moisture.
Third, lime mortar is fired at lower temperatures and absorbs carbon dioxide out of the air as it cures. It is kinder to the environment.
Lime mortar is THE BEST CHOICE for all kinds of masonry, and especially for structural masonry homes and buildings.