Concrete is a construction material composed of cement (commonly Portland cement)
as well as other cementitious materials such as fly ash and slag cement, aggregate
(generally a coarse aggregate such as gravel, limestone, or granite, plus a fine
aggregate such as sand), water, and chemical admixtures. The word concrete comes
from the Latin word "concretus", which means "hardened" or "hard". Many ancient
civilizations used forms of concrete using dried mud, straw, and other materials.
During the Roman Empire, Roman concrete was made from quicklime, pozzolanic, and
an aggregate of pumice; it was very similar to modern Portland cement concrete. The
widespread use of concrete in many Roman structures has ensured that many survive
almost intact to the present day. The Baths of Caracalla in Rome are just one example
of the longevity of concrete, which allowed the Romans to build this and similar
structures across the Roman Empire. Many Roman aqueducts have masonry cladding to
a concrete core, a technique they used in structures such as the Pantheon, Rome,
the interior dome of which is unclad concrete.
The secret of concrete was lost for 13 centuries until 1756, when the British engineer
John Smeaton pioneered the use of hydraulic lime in concrete, using pebbles and powdered
brick as aggregate. Portland cement was first used in concrete in the early 1840’s.
Recently, the use of recycled materials as concrete ingredients is gaining popularity
because of increasingly stringent environmental legislation. The most conspicuous
of these is fly ash, a by-product of coal-fired power plants. This has a significant
impact by reducing the amount of quarrying and landfill space required, and, as it
acts as a cement replacement, reduces the amount of cement required to produce a