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Re: Tuckpointing

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Mortar before the 20th century was often made with lime and sand.  Lime
mortar cures slowly and the rate at which a wall could be raised was limited
by the rate at which the mortar below hardened.  Hydraulic lime, lime
naturally containing a pozzolanic material that reacted with lime cured
faster and was used where economically available [occasionally a pozzolonic
admixture was added].  As portland cement became available, it was added to
lime and sand in mortar to speed the cure rate so that walls could be built
faster -- the use of portland cement was called gauging.  A small percentage
of portland cement was enough to produce this effect.

By 1920 in the US, the gauging with portland cement was probably standard

You can get a rough idea of what you have in the joints of your building by
a hand test.  Lime mortar can be easily eroded by rubbing with your finger.
If mortar flows from the joint as you rub it, the mortar is deteriorated and
should be replaced.  If you can remove several grains of sand with each
stroke of your finger, it is probably sound lime mortar.  If you can get
just a few grains to fall with each stroke, it is probably a gauged lime
mortar or mortar made with hydraulic lime.  If the mortar cannot be eroded
by finger strokes, it is probably a high-cement mortar.

In So. CA, it has been common to use a Type N cement-lime mortar for
pointing.  This is largely because the building departments often will not
allow use of anything that is not allowed by the building code -- this has
generally meant 50% lime, 50% portland cement [pointing generally requires a
building permit in So. CA and building departments sometimes even require
Special Inspection of pointing].  Also, design and construction
professionals are now used to portland cement mortar and anything that can
be scratched with a key is deemed suspect by many.

For long-term serviceability, 50% portland cement is too much portland
cement.  Cement does not breathe [allow moisture vapor to pass out of the
wall].  If the passage of moisture vapor is constricted, water may condense
in the wall and deterioration of stone, brick or mortar is accelerated.
Also, portland cement interferes with the self-healing process of lime
mortar: cracks, microcracks or visible cracks that form as the mortar cures
and shrinks are healed over the course of a season or two by solution and
redepositing of lime in the cracks.

Hydraulic lime is best for tuckpointing of old masonry.  It is available
from a couple of sources that I know of: Riverton on the east coast,
Transmineral USA [ ] on the west coast.  The
Transmineral staff is very helpful with evaluating your masonry and
selecting the appropriate hydraulic lime.

Nels Roselund
Structural Engineer
South San Gabriel, CA

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