Mortar consisting primarily of lime and sand has been used as
an integral part of masonry structures for thousands of years.
Up until about the mid-19th century, lime or quicklime
(sometimes called lump lime) was delivered to construction sites,
where it had to be slaked, or combined with water. Mixing
with water caused it to boil [sic] and resulted in a wet
lime putty that was left to mature in a pit or wooden box for
several weeks, up to a year.
Traditional mortar was made from:
- lime putty, or slaked lime, combined with
- local sand,
generally in a ratio of 1 part lime putty to 3 parts sand by
volume. Often other ingredients, such as:
- crushed marine shells (another source of lime),
- brick dust,
- natural cements,
- pigments, and even
- animal hair
...were also added to mortar, but the basic formulation for lime
putty and sand mortar remained unchanged for centuries until the
advent of portland cement or its forerunner, Roman cement, a natural,
Portland cement was patented in Great Britain in 1824 [PCM:
not 1821?]. It was named after the stone from Portland in
Dorset which it resembled when hard. This is a fast-curing, hydraulic
cement which hardens under water. Portland cement was first
manufactured in the United States in 1872, although it was
imported before this date. But it was not in common use throughout
the country until the early 20th century.
Up until the turn of the century portland cement was considered
primarily an additive, or "minor ingredient" to help
accelerate mortar set time. By the 1930s, however, most masons
used a mix of equal parts portland cement and lime putty.
Thus, the mortar found in masonry structures built between 1873
and 1930 can range from pure lime and sand mixes to a wide variety
of lime, portland cement, and sand combinations.
In the 1930s more new mortar products intended to hasten and
simplify masons' work were introduced in the U.S. These included:
- masonry cement, a premixed, bagged mortar which is
a combination of portland cement and ground limestone, and
- hydrated lime [pcm: pre-hydrated lime], machine-slaked
lime that eliminated the necessity of slaking quicklime into
putty at the site.