CHAPTER XV

THE GEOLOGY OF CEMENTS

Derinrrion anD Groups oF CEMENTS — * Cements,”
says Desch (Chemistry and Testing of Cement, 1911, p. 1),
* may be defined as adhesive substances capable of uniting
fragments or masses of solid matter to a compact whole.”
The cements, excluding the organic such as glue, are mostly
calcareous materials which have the property of ‘ setting
hard. The name comes from the Latin caedimentum, or
“ chipped broken stone,” and was given to a mixture of
broken stone or tiles with lime; this material was really
mortar, i.e. cement mixed with inert materials.
The oldest cement was probably mud, which, like the
burnt calcareous material used in Egypt, would not last in
a wet climate. The Greeks and Romans required cement
that would withstand rain and set under water; they prepared
 hydraulic cement from volcanic tuff, and from use of
that at Pozzuoli, a suburb of Naples, it was named Pozzolana.
After the fall of Rome the art of preparing good lime was
lost until the twelfth century. Builders then began to improve
 mortar, but hydraulic cement was not rediscovered
until 1756, when Smeaton was designing the third Eddystone
Lighthouse. The cement in its predecessor had been protected
 from water by metal bands. Smeaton set himself to
prepare a cement that would resist water and be as hard as
the stones it joined; he succeeded by burning equal parts
of the earthy Liassic limestones of Glamorganshire, and of
volcanic tuff, known as Trass, from the extinct volcanoes of
the Rhine.
Cements are divided into two main groups. In the first
the action depends upon the replacement of some constituent,
usually moisture or carbon dioxide, which has been expelled
by heat; as it is replaced the cement becomes hard or * sets.”
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