Lime Mortar vs. Portland Cement

Last week, I posted these pictures on Instagram of a repointing project at Saint Andrews Church in Roanoke.  Our masonry partner, Dominion Traditional, is doing the work.  The church has not been repointed in over 120 years, and the original lime based pointing is due for replacement.  It should last another 120 before needing to be done again.  I want to go into more depth here, because the compatibility issues between bedding mortar and pointing, and the interaction between the pointing and the stone or brick, are so important.  When there is incompatibility, permanent damage can happen to the building, both inside and out.   

Using incompatible materials is one of the most damaging things we see done to old buildings in our region.

St. Andrews Church, Roanoke, VA

Lime mortar as bedding and pointing between bricks is the sacrificial element in the wall. It is softer than the bricks, which is a good thing, and breathes, without trapping moisture.  It has high vapor permeability — it gets wet, it dries.  Portland cement does not ‘breathe’ like lime, and an old brick wall, when repointed with Portland cement, will raise the moisture content in the bricks.  With freeze / thaw cycles it will affect the bricks, turning them to dust or breaking off the faces, called spalling.  This is not a fault of the brick, but of the repointing material.  

Sometimes moisture migration will cause damage to the interior as well.  I have worked on 18th century stone houses that were Portland pointed, and the interior plaster was suffering from moisture migration to the interior.  This causes plaster and paint damage, and even wood decay if the moisture contents are high enough.    

Bottom line: old walls, if built with lime,

need to be repointed with lime.  

Lime continues to impress; if you look at a modern masonry building, they must put vertical expansion joints periodically to separate the wall sections.  On very old large brick buildings and churches like this one, the lime mortar has enough plasticity that these expansion joints are not needed.  Next time you are at a modern brick school, look at the vertical caulked joints along the walls.  You will see the engineered breaks necessary because there is virtually no movement with Portland; there is no flexibility.  

A lot of damage is done to traditional buildings by pointing them with Portland; it is a fortunate situation that this has never been done on this Roanoke church; it is simply a periodic maintenance job in this case. When you see a very old masonry wall, look closely at the mortar. Often, the mortar walls have moved somewhat, but you won’t see any cracks in a lime wall. Lime has ‘free lime’ available, and when the walls are wetted, the free lime moves into the hairline cracks. It is self healing!

Stone home being prepared for lime mortar re-pointing.