Repointing Historic Masonry

I read that it's necessary to use "soft" mortar for repairs to historic masonry. When inquiring about repointing our 1820's brick home and stone foundation, a contractor suggested that it's fine for the brick walls, but not strong enough for the foundation. Would it be appropriate to use the hard mortar for the stone?

The mortar bedded between the bricks and stones of your home is traditionally lime-based mortar. This "soft" mortar consists primarily of sand and lime. "Hard" mortar is made with Portland cement, which was not commercially available in North America until about the mid 1870s. Even then, it was mostly used as an additive to the mortar mixture and was not a primary ingredient in mortars until around 1930.

Repointing or other repairs to historic brick walls with modern mortar, containing Portland cement, can cause extensive damage to the old bricks. The old, soft, lime-based mortar between the bricks has served as a path for any moisture within the walls to escape. Denser, less permeable Portland cement mortar applied to the joints can force moisture to migrate through the face of the bricks instead. When the moisture evaporates, it leaves soluble salt deposits in the hard "skin" of the bricks. The salts then crystallize, causing the surface of the brick to "spall." Once the soft, inner portion of the brick is exposed to the weather, it can rapidly erode.

There are other benefits to using lime-based mortar for old masonry:

Unlike stiff, brittle modern mortars, lime-based mortar's elasticity absorbs stresses, movement and vibrations. Portland cement can trap moisture in the old, soft bedding mortar, which can result in the joints swelling. Lime mortar can heal itself by perpetually dissolving lime that naturally fills small cracks and voids that may develop (picture stalactites forming).

The properties of lime-based mortar also make it a wise choice for repointing and repairs to stone walls and foundations. Although I've seen no significant damage to hard, igneous or metamorphic stones pointed with hard mortar, soft sedimentary stones can suffer the same fate as bricks. I've found some limestone and sandstone foundation so severely eroded, that the mortar joints now project well beyond the surface of the stones. Soft mortar doesn't seem to be a weakness when used in stone structures. I've visited some 700-year-old Edwardian castles that have performed quite well without the hard stuff ever being used.

That bag of lime that you might have in the shed, used for reducing the acidity of the soil in the garden, isn't the right stuff to create this old-style mortar. The garden lime is simply pulverized limestone. For repointing old masonry, you'll need limestone that was burned in a kiln to create "quicklime." The quicklime is then "slaked" with water to create lime putty, typically sold wet, in sealed buckets. Natural hydraulic lime is processed in a similar way, but can be supplied in bags as a dry powder. In the last decade, word has really spread about using appropriate mortar for historic buildings, resulting in several suppliers producing the correct lime, and even pre-mixed mortars (already blended with the sand) for restoration and preservation of our historic masonry structures.