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 pre- 
pared 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 im- 
prove mortar, but hydraulic cement was not rediscovered 
until 1756, when Smeaton was designing the third Eddystone 
Lighthouse. The cement in its predecessor had been pro- 
tected 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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