Re-pointing dense impervious masonry.
Some masonry, such as granite, basalt etc. and dense impervious bricks require special consideration. Due to their very nature these materials have little, if any, moisture absorption and therefore moisture is transferred to the joints.
In these circumstances the choice of mortar and method of application and finishing is very important.
The joints are more vulnerable to the effects of wetting during placing and immediately afterwards until a full set and carbonation has taken place. Using St. Astier NHL mortars will ensure setting without having to rely completely on carbonation. Dryer mixes will be possible, avoiding free water in the joint cavity and consequent de-bonding effect. The vapour permeability of NHL mortars will ensure moisture evaporation.
Joints should be filled to flush, never recessed. Recessed joints will leave ledges for the accumulation of water that will keep the mortar joint wet for longer periods and accelerate the decay process.
Whilst feebly hydraulic limes were often used for the building of walls with impervious masonry the construction period usually left sufficient time for the joints to set up and cure before exposure to rain. Re-pointing is a much quicker process and more hydraulic materials are almost always a better option.
Joints should be raked back to approximately 25mm, thoroughly cleaned, including the top and bottom faces of the beds, ready for the new mortar. Pinning stones should not be removed, but if they are loose, they should be removed and put back during the re-pointing. Where a wall has previously been re-pointed and the pinning stones have been lost, suitable replacements should be used. The walls should be well washed to remove any dust and loose friable material making sure that the entire elevation is cleaned down to prevent staining on the walls. Impervious masonry should be dry when the work commences, however the original backing mortar should be kept damp.