Mortar formulations prior to the late-19th century used lime
as the primary binding material. Lime is derived from heating
limestone at high temperatures which burns off the carbon dioxide,
and turns the limestone into quicklime. There are three types
- magnesium, and
differentiated by the different levels of magnesium
carbonate they contain which impart specific qualities to
mortar. Historically, calcium lime was used for mortar rather
than the dolomitic lime (calcium magnesium carbonate) most often
used today. But it is also important to keep in mind the fact
that the historic limes, and other components of mortar, varied
a great deal because they were natural, as opposed to modern
lime which is manufactured and, therefore, standardized. Because
some of the kinds of lime, as well as other components of mortar,
that were used historically are no longer readily available, even
when a conscious effort is made to replicate a "historic"
mix, this may not be achievable due to the differences between
modern and historic materials.
Lime, itself, when mixed with water into a paste is very plastic
and creamy. It will remain workable and soft indefinitely, if
stored in a sealed container. Lime (calcium hydroxide) hardens
by carbonation absorbing carbon dioxide primarily from the air,
converting itself to calcium carbonate. Once a lime and sand mortar
is mixed and placed in a wall, it begins the process of carbonation.
If lime mortar is left to dry too rapidly, carbonation of the
mortar will be reduced, resulting in poor adhesion and poor durability.
In addition, lime mortar is slightly water soluble and thus is
able to re-seal any hairline cracks that may develop during the
life of the mortar. Lime mortar is soft, porous, and changes little
in volume during temperature fluctuations thus making it a good
choice for historic buildings. Because of these qualities, high
calcium lime mortar may be considered for many repointing projects,
not just those involving historic buildings.
For repointing, lime should conform to ASTM C 207, Type S,
or Type SA, Hydrated Lime for Masonry Purposes. This machine-slaked
lime is designed to assure high plasticity and water retention.
The use of quicklime which must be slaked and soaked by hand may
have advantages over hydrated lime in some restoration projects
if time and money allow.