Entire books have been written on the very simple subject of mortar, the material used to adhere bricks together in a masonry wall. It seems so simple — just mix up some mortar and start stacking bricks. But the type of mortar used, especially in old buildings, can have a dramatic effect on how the bricks behave.
First, the bricks. In Chicago there are thousands and thousands of old brick buildings built with “Chicago Common” bricks. These were made in tremendous numbers after the Chicago Fire of 1871. They have a unique color due to the type of clay that they’re made from. They were typically made by hand and the kilns used didn’t allow for uniform firing so there’s a fair bit of irregularity. They’re also usually fairly soft, at least compared to modern bricks and that’s where the issues start.
During the late 19th century and the early part of the 20th century these bricks were turned into buildings using lime mortar. Lime mortar consists of just three ingredients: lime, sand, and water. It’s been used for thousands of years. The lime comes from limestone that’s heated to drive off carbon dioxide, then mixed with enough water to create a slurry called slaked lime, then mixed with sand to create the mortar. Gradually the mortar absorbs carbon dioxide from the atmosphere to basically turn back into limestone.
Lime mortar is fairly soft, at least compared to modern Portland cement based mortar. It’s also self-healing to some extent. As the mortar gets wet the lime dissolves just a little bit and is transported through the mortar where it sits in cracks. As the water evaporates the lime fills the cracks and “heals” the wall.
Portland cement (the names comes from a location in the English Channel, not the city in Oregon) was invented in 1824, and significant improvements in the composition and manufacturing methods occurred for several decades. It has a higher initial strength, and the common belief was that stronger is better. At first it was added to lime mortar to improve strength, and eventually it came to be the dominant mortar material used to lay up a brick wall. By about the 1930’s lime mortar wasn’t being used much.
But now the problem: Portland cement is often harder than the bricks that it’s bonding, and that means the softer bricks will become sacrificial when water moves through the wall assembly. Stronger isn’t always better. Sometimes a material needs to be able to move and flex a little bit in order to work properly. Think about the caulk in your shower. It’s flexible so that it can move a little bit but still create a water-tight joint.
So when the lime mortar of older Chicago Common brick walls finally deteriorates and needs to be replaced in the process called tuck pointing it should be done with a mortar that’s softer than the bricks – basically another batch of lime mortar. If Portland cement mortar is used that’s harder than the bricks then the bricks will start to deteriorate. This often manifests in spalling of the bricks. Spalling is when the face of the brick flakes off due to internal pressure from water and the salts carried by the water. In theory the inner part of a brick is its softest part, so when spalling exposes that inner core the brick should be covered – with a lime mortar. Again, no Portland cement mortar.
I very rarely see an old Chicago building that’s been tuck pointed with lime mortar. It’s almost always done with new Portland cement mortar which is very hard. This will cause the bricks to deteriorate as they sacrifice themselves to the harder Portland cement mortar.
Also, I often see Portland cement mortar installed in a thin layer over a large section of brick wall, often at the bottom. This thin layer of mortar is called a “parging.” This is also a bad idea. Water will be trapped inside the brick wall by this parging layer and cause deterioration to the brick and mortar. The whole idea of parging is to use a mortar that will be sacrificial, and that’s a soft lime mortar. A properly done lime mortar parging can easily last 25-30 years and will eventually deteriorate but leave the underlying wall in fine condition. But a Portland cement parging will damage the underlying bricks and mortar and often hide that damage until it’s quite extensive.
Newer bricks don’t have this problem. Newer bricks are very hard because the clay formulation and the manufacturing process allows it. So Portland cement mortar is appropriate for these new brick walls.
But for most older buildings it’s better to use lime mortar.