More recent, 20th-century mortar has used portland cement as
a primary binding material. A straight portland cement and sand
mortar is extremely hard, resists the movement of water, shrinks
upon setting, and undergoes relatively large thermal movements.
When mixed with water, portland cement forms a harsh, stiff paste
that is quite unworkable, becoming hard very quickly. (Unlike
lime, portland cement will harden regardless of weather conditions
and does not require wetting and drying cycles.)
Some portland cement assists the workability and plasticity of
the mortar without adversely affecting the finished project; it
also provides early strength to the mortar and speeds setting.
Thus, it may be appropriate to add some portland cement to an
essentially lime-based mortar even when repointing relatively
soft 18th or 19th-century brick under some circumstances when
a slightly harder mortar is required. The more portland cement
that is added to a mortar formulation the harder it becomes
and the faster the initial set.
For repointing, portland cement should conform to ASTM C 150.
White, non- staining portland cement may provide a better color
match for some historic mortars than the more commonly available
grey portland cement. But, it should not be assumed, however,
that white portland cement is always appropriate for all historic
buildings, since the original mortar may have been mixed with
grey cement. The cement should not have more than 0.60 per cent
alkali to help avoid efflorescence.