Mortars for repointing should be softer or more permeable than
the masonry units and no harder or more impermeable than the historic
mortar to prevent damage to the masonry units. It is a common
error to assume that hardness or high strength is a measure of
appropriateness, particularly for lime-based historic mortars.
Stresses within a wall caused by expansion, contraction, moisture
migration, or settlement must be accommodated in some manner;
in a masonry wall, these stresses should be relieved by the mortar
rather than by the masonry units.
A mortar that is stronger in compressive strength than
the masonry units will not "give," thus causing stresses
to be relieved through the masonry units resulting in permanent
damage to the masonry, such as cracking and spalling, that cannot
be repaired easily. While stresses can also break the bond between
the mortar and the masonry units, permitting water to penetrate
the resulting hairline cracks, this is easier to correct in the
joint through repointing than if the break occurs in the masonry
Permeability, or rate of vapor transmission, is also critical.
High lime mortars are more permeable than denser cement mortars.
Historically, mortar acted as a bedding material not unlike
an expansion joint rather than a "glue" for the
masonry units, and moisture was able to migrate through the mortar
joints rather than the masonry units.
When moisture evaporates from the masonry it deposits any soluble
salts either on the surface as efflorescence or below
the surface as subflorescence. While salts deposited on
the surface of masonry units are usually relatively harmless,
salt crystallization within a masonry unit creates pressure that
can cause parts ofthe outer surface to spall off or delaminate.
If the mortar does not permit moisture or moisture vapor to migrate
out of the wall and evaporate, the result will be damage to the