There are essentially two types of building mortars. Lime and cement which both work in different ways
Old houses, generally pre 1920s are constructed from brick or stone using lime mortar. Newer houses, post 1920s are constructed from brick or stone using a Portland cement mortar.
It's important to understand that historic buildings are very different from modern buildings in several key ways, these being the bricks and mortar used in the construction of the building.
First, historic bricks are a lot softer than modern bricks. This is because modern bricks are now fired at a much higher temperature than it was possible to do in the past. Second, historic masonry buildings were designed to be permeable so they could absorb water and then release it, as opposed to modern building technology using hard bricks and cement mortar which emphasizes waterproofing.
Because of these differences, it's very important to use compatible materials during restoration, as an incompatible mortar can destroy a historic masonry structure.
The effects of cement on historic soft bricks and lime built properties
The most common mistake we see is an old masonry building that has been repointed with Portland cement mortar instead of the historic lime mortar.
Modern cement mortars can do a lot of damage to a historic building that was originally pointed with lime mortar. Portland cement is generally more rigid and less permeable, so cement mortars will cause damage to the brick and/or stone during expansion and contraction. Lime mortar, on the other hand, is more accommodating, and its lower compressive strength high flexibility allows the historic masonry units to expand and contract without being damaged. You always want your mortar to be weaker than your masonry units for this reason.
Lime mortar also allows water vapor to pass in and out of the joints, which means that moisture does not get trapped in the brick or the stone. Portland cement mortars are unable to wick water out of the walls, so water gets trapped and instead escapes through the historic stone or brick. This damages the masonry units as the water pressure builds until the face of the brick or the stone pops off, exposing the inside of the unit and making it more vulnerable to continued deterioration. Trapping moisture can and will also lead to internal damp issues as well
In recent years lots of older period properties have wrongly been repointed using cement.
A recent 2017 Andrew Smith & Son property survey of a Victorian terraced house.
The front elevation has been re-pointed in the past in an inappropriate cementatious mortar and to an inferior standard, the mortar work not having been raked out first and there is already some localised failure. Cement mortar does not bond well with the original lime mortar and can trap moisture within the bricks leading to frost damage. We would recommend that this all be removed, and the brickwork carefully re-pointed using a lime mortar mixed to match the original.
There has been some isolated re-pointing to the rear elevation to a similarly poor standard but whilst the brickwork otherwise remains in fair condition we did note that to the rear elevation of the two storey addition, the mortar work is quite soft and eroded particularly to the higher levels. We would recommend that this elevation be carefully raked out and re-pointed.
Andrew Smith & Son | Lettings and Property Management