Salt and Historic Masonry

Lime Repointing and Salt on Painted Masonry

Increasingly, we are finding problems when spot repointing exterior brick walls that are next to concrete walks, driveways, or slabs. (Spot repointing refers to replacing the mortar in less than 10% of that area.)

Most of us with historic brick homes have areas like this on our properties where a brick wall comes right down to concrete, and the areas have to be cleared of snow and ice for important safety reasons. Most of these problem areas involve brick walls that have been painted. Exterior painted walls are often moisture-laden, and often there are interior moisture problems in these areas.

This photograph is a prime example of what we are dealing with:

Exterior brick
Recently painted brick
Concrete walk next to the brick wall

1388864Notice the white joints in the photo. These were pointed one year ago with lime mortar, tinted red to approximate the paint color. The white in the photo is about ¼” thick of white salt crystals over the red joints!

This is a classic case of a brick wall – particularly with a concrete walk poured next to it – that is trapping excessive amounts of water. The salt eruptions around and through the new mortar are a symptom of a much bigger issue.

This photo is showing us 3 things:

1. There is always water moving through building materials, and water was/is trapped within this wall. The mortar should be the chief route of water evaporation. Besides the lime mortar shown, all other roots of evaporation have been cut off.

2. The lime mortar is so much more porous than the Portland cement or the paint. Lots of water has already evaporated through these lime mortar joints, leaving thick deposits of salt behind on the surface.

3. De-icing salts hold water. With liquid water levels high, the water moves inward, causing plaster to fail and paint to blister on the inside of the house. Also, over time the lime from the original lime mortar is dissolved out until there are just bricks supported on sand. Paint hides the deterioration.

Fortunately, there is a proper, well-understood way to treat this situation:

Center-cut the mortar joints and remove it, at least up to 3-6’. This will allow the wall to begin to dry.

Periodically vacuum off the salt hairs that will begin to grow on the surface. (If it is just allowed to fall or wash off, these salts will be wicked right back up into the wall and the process will start over, so it must be removed.)

Or to speed the process up, the salt can be removed by several applications of a clay/paper poultice using deionized water. This is an inexpensive and quite effective salt-removal process.

Once the majority of the salt has been drawn out of the wall, failed joints can be raked and repointed with an appropriate lime mortar.

The trouble for the masonry restoration mechanic comes when replastering a basement, or repointing an alley wall, because the signs of salt and water saturation are not always obvious. If the replastering or repointing is completed, salt can actually break out new pointing because it expands when it re-crystallizes.

Lancaster Lime Works cannot be held responsible for the failure of pointing due to the effects of salt in the masonry. If we see salt crystals ahead of time, we will discuss the situation with you. It may be a better decision to take steps to remove the salt before pointing or plastering.