A cement is a binder, a substance used
for construction that sets, hardens and
adheres to other materials, binding them
together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement
is used with fine aggregate to produce mortar for masonry,
or with sand and gravel aggregates to produce concrete.
Cement starts to set when mixed with water which causes a
series of hydration chemical reactions. The constituents slowly hydrate and the
mineral hydrates solidify; the interlocking of the hydrates gives cement its
strength. Contrary to popular perceptions, hydraulic cements do not set by
drying out; proper curing requires maintaining the appropriate moisture content
during the curing process. If hydraulic cements dry out during curing, the
resulting product can be significantly weakened.
Cements used in construction are usually inorganic,
often lime or calcium
silicate based, and can be characterized as being either hydraulic or non-hydraulic, depending upon the
ability of the cement to set in the presence of water.
Non-hydraulic cement will not set in wet conditions
or underwater; rather, it sets as it dries and reacts with carbon
dioxide in the air. It is resistant to attack by chemicals
after setting.
Hydraulic cements (e.g., Portland
cement) set and become adhesive due
to a chemical reaction between the dry
ingredients and water. The chemical reaction results in mineral hydrates that
are not very water-soluble and so are quite durable in water and safe from
chemical attack. This allows setting in wet condition or underwater and further
protects the hardened material from chemical attack. The chemical process for
hydraulic cement found by ancient Romans used volcanic ash (pozzolana)
with added lime (calcium oxide).