Historical masonry buildings are very different from modern buildings. Historical bricks were fired at lower temperatures and are much softer and more permeable than modern bricks and buildings constructed with these softer bricks were designed to absorb moisture and then release it. A key component of this design was the lime mortar historically used in masonry applications, a mortar that was also soft and readily allowed water or vapor to pass through it.
In the late 1800’s, a new mortar debuted in the United States at the height of the Industrial Revolution. Favored for all the qualities a mass-production revolution could ask for (fast-curing, inexpensive, and less work for masons), Portland cement quickly gained popularity with masons and by the early 1900’s most buildings had some Portland mortar in their masonry surfaces – usually as an additive to traditional lime mortar. By the mid-1900’s Portland was no longer used as an additive and became the predominate ingredient in mortar mixes. Historical buildings were not immune to the new technology and masonry repairs on historical buildings in the 1900’s were predominantly made with Portland mortar.
If your historic building has been re-pointed it likely was with Portland mortar. A common mistake, Portland mortar applied to historical buildings doesn’t just erode the historic fabric of the building, it causes physical damage that is often permanent. Traditional mortars worked with the softer historical masonry materials to expand and contract together as temperatures and moisture levels changed, creating a wall and masonry surface that “breathed” to expel excess moisture. Applying a Portland mortar mix to historical masonry disrupts that relationship and traps moisture in the wall and historical bricks. Moisture trapped within the walls will not easily pass through Portland cement mortar and will be forced through the soft brick instead, the path of least resistance. When the water evaporates, salt deposits are left behind that crystallize and destroy the protective shell of the bricks. Once this outer surface is damaged, the softer interior of the historical bricks rapidly disintegrates.
Portland mortar can cause problems that begin to decay masonry in a few years. The historical bricks on masonry buildings are not the only things threatened by Portland cement mortar – structural elements, interior features, and occupant health are also compromised by the moisture issues associated with Portland mortar.
Remember, historical masonry materials and mortars were designed from a construction approach that created buildings that “breathed”, allowing moisture both in and out. Modern masonry materials and mortars are designed from a watertight construction approach that aims to keep water from passing through.
Combining a material from the system designed to let a house “breathe” with a material from a system designed to prevent water from passing through is a recipe for disaster.
The truth is…historical mortar differs significantly at a molecular level from modern mortar. This difference makes modern mortar incompatible with historical masonry materials, permanently damaging historical masonry materials, and structural elements of masonry buildings, and traps moisture in walls lowering energy efficiency and endangering air quality inside the building.
