Chemistry Reference
In-Depth Information
the composite becomes increasingly hard until it sets into a strong, tough
material, stronger and tougher than its separate components.
Humans have made composite materials for thousands of years. Exam-
ples of composites that have been widely used since early antiquity include
adobe
(mud cement),
calcareous
and
gypsum cements
,
mud brick
and
burned brick
,
pottery
and most other ceramic materials, and some alloys.
A block of dry mud, for example, is easily broken by bending, that is, by
applying a mechanical force on its surface. Straw, on the other hand, is
very resistant to stretching, although very fragile and labile to creasing and
crinkling. When mud is thoroughly mixed with straw and the mixture is
left to dry, the product is
mud brick
, a strong, tough composite material,
resistant to most mechanical forces. The mixture of mud and straw also
makes an excellent building cement for joining stones or mud bricks. Other
ancient, well-known building cements such as lime and gypsum cements
are also composite materials; they consist of a coarse mixture of sand,
gravel or small stones with wet lime or gypsum (see Textboxes 33 and 34).
When any of these cements dries and sets, it becomes extremely strong
and resistant to compression and bending, much stronger than any one of
its separate components (de Wilde and Blain 1990).
Mortars
are cements used for bonding together masonry units, such as
stones or bricks. When a cement is used to conceal masonry, as a more or
less smooth covering on walls, for example, it is referred to as
plaster
. A very
fine plaster, known as
stucco
, is made of very thin sand or finely commin-
uted marble. Freshly prepared plasters and stuccos are spread on consoli-
dated masonry to form more or less uniform and smooth layers; stucco also
provides a smooth and often flat outer coating.
Concretes are cements containing a large proportion of gravel.
Hydraulic
cements
are cements that
set
(harden) in wet environments, as required when
building structures submerged in water. Like all other cements used in
ancient times, hydraulic cements were also composite materials in which one
particular component, such as
pozzolana
in ancient Rome (see text below),
endowed the cement with the property of setting in wet environments (Gani
1997; Akroyd 1962).
Mud
Probably the oldest known building cement,
mud
has been widely used since
time immemorial, and its use still continues in many areas of the world.
