Question: CementWhat is Portland Cement ? |
Answer
Portland cement was patented in Great Britain in 1824. It was named
after the stone from Portland in Dorset which it resembled when hard.
This is 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.
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 re-pointing 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.
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