Admixtures are used to create specific characteristics in mortar,
and whether they should be used will depend upon the individual
Air entraining agents, for example, help the mortar to
resist freeze-thaw damage in northern climates. Accelerators
are used to reduce mortar freezing prior to setting while retarders
help to extend the mortar life in hot climates. Selection of admixtures
should be made by the architect or architectural conservator as
part of the specifications, not something routinely added by the
Generally, modern chemical additives are unnecessary and may,
in fact, have detrimental effects in historic masonry projects.
The use of antifreeze compounds is not recommended. They
are not very effective with high-lime mortars and may introduce
salts, which may cause efflorescence later.
A better practice is to warm the sand and water, and to protect
the completed work from freezing.
No definitive study has determined whether air-entraining additives
should be used to resist frost action and enhance plasticity,
but in areas of extreme exposure requiring high-strength mortars
with lower permeability, air-entrainment of 10-16 percent may
be desirable (see formula for "severe weather exposure"
in Mortar Type and Mix).
Bonding agents are not a substitute for proper joint preparation,
and they should generally be avoided. If the joint is properly
prepared, there will be a good bond between the new mortar and
the adjacent surfaces. In addition, a bonding agent is difficult
to remove if smeared on a masonry surface.