BRICK AND MORTAR BASICS
The following provides an overview of the characteristics of brick and mortar, and how they sustain unnecessary damage through improper maintenance. This is provided to assist in understanding the masonry sections of this report. A more thorough comparison of Portland mortar and hydraulic lime mortar will be found in some of our other blog posts.
Bricks that were used in the construction of the original older buildings were fired at lower temperatures than modern brick. They do have a hard exterior, but are still much more porous (absorpent to moisture) than modern bricks. If the harder face of the brick is removed by sandblasting, the much more porous and fragile interior of the brick is exposed to the weather. Sandblasting was widely accepted in the past as a way to clean dirty, painted, or stuccoed brick. It is now widely known to significantly weaken brick's resistance to damage from weather.
The original brick building walls were laid with hydraulic lime mortar, which has been in use for around 7,500 years. It is made by burning limestone, and it gets hard by going through a chemical reaction with water. It is very different from the Portland cement mortars used over the last 100 years. Portland cement is much harder, less flexible, and is completely non-porous. Portland cement mortars are now known to threaten the longevity of brick and stone walls, and can greatly accelerate damage to soft brick.
Moisture from the weather, both humidity in the air and precipitation, penetrates bricks and stones in a wall because they are naturally porous. This moisture in the bricks can cause rapid damage to the bricks if it cannot escape quickly, especially in freezing weather. Hydraulic lime mortar allows moisture to escape very quickly out of a wall. Portland cement traps moisture in the wall. Moisture that is trapped in a brick wall causes the brick faces to break off, leaving a rough, ugly, fragile exterior. It also causes moisture problems on the interior, such as plaster decay, stains, and wood rot.
Walls that are constructed of older, softer brick should be repointed with hydraulic lime mortar, never Portland cement. Walls that have been repointed with Portland-based mortar should be repointed with hydraulic lime mortar. Brick faces that have been severely deteriorated can either be repaired or replaced. Replacement of the bricks can be done by finding new bricks that match, or by removing and turning around the original bricks. Damaged bricks can be repaired by using Lithomex, a flexible, porous, lime-based material that is applied in lifts to the face of the damaged brick bringing it back to its original shape, size and color. It bonds with the damaged face of the brick, and becomes a permanent repair.
Old and damaged bricks can also be removed from the wall using special tools. Sometimes the damaged and spalling bricks can be removed, turned around and reinstalled. Others will be so badly deteriorated that they will need to be replaced entirely.
The method of removal for existing Portland-based mortar is extremely important. Using a grinder will create a lot of dust, and the spinning diamond blade very easily causes irreparable damage to the face of the old brick. We suggest using a power tool called Arbortech, along with a high quality vacuum system. It has two vibrating blades that act like scissors, cutting into the Portland-based mortar and greatly minimizing potential damage.