Sand is the largest component of mortar and the material that
gives mortar its distinctive color, texture and cohesiveness.
Sand must be free of impurities, such as salts or clay. The three
key characteristics of sand are:
- particle shape,
- gradation and
- void ratios.
When viewed under a magnifying glass or low-power microscope,
particles of sand generally have either rounded edges, such
as found in beach and river sand, or sharp, angular edges, found
in crushed or manufactured sand.
For repointing mortar, rounded or natural sand is preferred
for two reasons.
- It is usually similar to the sand in the historic mortar
and provides a better visual match.
- It also has better working qualities or plasticity and
can thus be forced into the joint more easily, forming a good
contact with the remaining historic mortar and the surface
of the adjacent masonry units.
Although manufactured sand is frequently more readily
available, it is usually possible to locate a supply of rounded
The gradation of the sand (particle size distribution) plays
a very important role in the durability and cohesive properties
of a mortar. Mortar must have a certain percentage of large
to small particle sizes in order to deliver the optimum performance.
Acceptable guidelines on particle size distribution may be found
in ASTM C 144 (American Society for Testing and Materials).
However, in actuality, since neither historic nor modern sands
are always in compliance with ASTM C 144, matching the same
particle appearance and gradation usually requires sieving
A scoop of sand contains many small voids between the individual
grains. A mortar that performs well fills all these small voids
with binder (cement/lime combination or mix) in a balanced manner.
Well-graded sand generally has a 30 per cent void ratio by
volume. Thus, 30 per cent binder by volume generally should
be used, unless the historic mortar had a different binder:
aggregate ratio. This represents the 1:3 binder to sand ratios
often seen in mortar specifications.
For repointing, sand generally should conform to ASTM C
144 to assure proper gradation and freedom from impurities;
some variation may be necessary to match the original size and
gradation. Sand color and texture also should match the original
as closely as possible to provide the proper color match without