Why use lime
mortar?
Natural Limestone based mortars,
plaster and harling were used in the construction of many pre-1914
Scottish building and boundary walls. Since then, most repair work
has used cement or concrete based materials. However, this can cause
damage to the building stone, and retain water in the building,
because the lime mortared joints are covered with a hard impermeable
layer, which does not allow moisture to escape. Also, lime mortar is
more flexible because it is softer, and hairline cracks can repair
themselves, whilst cement mortars crack and fall off the wall.
The photographs show a wall with
cracked cement covering the joints on the left, in need of
repointing. The old cement is removed, and the joints cleaned back
to the original lime mortar, usually a depth of 25mm. The fresh lime
mortar is then put in, beaten to force it into the joints and cured
for 7-10 days under damp hessian sheeting in order for it to
harden. The mortar continues to absorb moisture and carbon dioxide
for two years after the sheeting is removed, which gradually cures
it to its full hardness.
Properly cured lime mortar will
last at least 50 years before repointing, and because the mortar
used must be softer than the stone in the building, it is designed
to fail before moisture and frost damage those stones. Two of the
stones in the centre of the picture have been repaired using a
colour-matched lime based repair mortar.
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