Mortars for repointing projects, especially those involving historic
buildings, typically are custom mixed in order to ensure the proper
physical and visual qualities. These materials can be combined
in varying proportions to create a mortar with the desired performance
The actual specification of a particular mortar type should
take into consideration all of the factors affecting the life
of the building including: current site conditions, present condition
of the masonry, function of the new mortar, degree of weather
exposure, and skill of the mason. Thus, no two repointing projects
are exactly the same.
Modern materials specified for use in repointing mortar should
conform to specifications of the American Society for Testing
and Materials (ASTM) or comparable federal specifications, and
the resulting mortar should conform to ASTM C 270, Mortar for
Specifying the proportions for the repointing mortar for a specific
job is not as difficult as it might seem. Five mortar types, each
with a corresponding recommended mix, have been established by
ASTM to distinguish high strength mortar from soft flexible mortars.
The ASTM designated them in decreasing order of approximate general
- Type M (2,500 psi),
- Type S (1,800 psi),
- Type N (750 psi),
- Type O (350 psi) and
- Type K (75 psi).
The letters identifying the types are from the words MASON WORK
using every other letter.
Type K has the highest lime content of the mixes that contain
portland cement, although it is seldom used today, except for
some historic preservation projects. The designation "L"
in the accompanying chart identifies a straight lime and
sand mix. Specifying the appropriate ASTM mortar by proportion
of ingredients, will ensure the desired physical properties.
Unless specified otherwise, measurements or proportions for mortar
mixes are always given in the following order: cement-lime-sand.
Thus, a Type K mix, for example, would be referred to as 1-3-10,
or 1 part cement to 3 parts lime to 10 parts sand. Other requirements
to create the desired visual qualities should be included in the
The strength of a mortar can vary. If mixed with higher amounts
of portland cement, a harder mortar is obtained. The more lime
that is added, the softer and more plastic the mortar becomes,
increasing its workability. A mortar strong in compressive strength
might be desirable for a hard stone (such as granite) pier holding
up a bridge deck, whereas a softer, more permeable lime mortar
would be preferable for a historic wall of soft brick. Masonry
deterioration caused by salt deposition results when the mortar
is less permeable than the masonry unit. A strong mortar is still
more permeable than hard, dense stone. However, in a wall constructed
of soft bricks where the masonry unit itself has a relatively
high permeability or vapor transmission rate, a soft, high lime
mortar is necessary to retain sufficient permeability.