We've often come across walls which have been re-pointed in cement mortar and suffered erosion of the masonry units (bricks or stone blocks) as a result. I witnessed a particularly severe example on a church wall last week, which prompted me to write this article.
In short: if your wall was originally built with lime mortar, then it should only be re-pointed in lime mortar! Not cement mortar, not mortar with bagged lime and a bit of cement just to help it, just proper lime mortar.
Since Roman times, lime and sand were the principal ingredients for mortar, and only since the 1930s has cement mortar been widely used. The convenience and strength of cement has led to it being used for re-pointing old masonry; however its strength and water resistance is detrimental to the longevity of the wall.
What happens is that the moisture that lands on the outer face of the wall gets trapped on and behind the cement mortar, and cannot evaporate from the surface as it would with permeable lime mortar. The next frost causes this moisture to expand and the face of the brick or stone will spall away, eroding the wall on each occasion. In the example of a church wall, the soft clunch (chalk) and slightly harder limestone have been eaten away to a depth of 50-80mm and the localised action of the re-pointing is very evident. This is coupled with the slightly acidic nature of rainwater, which will dissolve the alkaline stone.
So, if your wall is more than 70 years old and needs re-pointing, you should not use any cement in your mortar. Use one part lime putty (not bagged lime) to three parts sharp sand (not masonry sand, or soft sand). For conservation-grade work, the sand should match that originally used, which may mean it is quarried locally to get the correct constituents and colouration.
If your wall has already been re-pointed, you can usually tell if cement mortar has been used as it is greyer in colour and the sand may lack the gritty particles found in sharp sand (although sometimes sharp sand will have been used). Cement mortar is harder and brittle, so it will resist scratching with a screwdriver or key, whereas lime mortar can be dug out more easily.
If the re-pointing has caused serious damage to your wall it may be worth having it examined to assess whether removing the offending material is worthwhile, but as the removal process can cause further damage to the masonry, it is best in most cases to leave it as-is.
For advice on particular cases, please contact us on firstname.lastname@example.org