86 ECONOMIC GEOLOGY 
This group includes plaster of Paris and the lime cements, 
In the second group the setting is due to new compounds 
being formed at a high temperature or by mixing materials 
that are chemically unstable. This group includes hydraulic 
limes, Portland cement, and cements with amorphous or 
glassy silica. 
The simplest kind of cement is prepared by burning lime- 
stone, from which at the temperature of 1400° to 1650° F., 
the carbon dioxide is driven off and lime left. When lime 
is mixed with water it slakes to the hydrate or slaked lime 
(CaO 4+ H,0 = CaH,0,), which when mixed with sand forms 
mortar and sets firmly on drying. No chemical reaction takes 
place between the lime and sand, which prevents excessive 
shrinkage, and hardens the material. The lime absorbs 
carbon dioxide from the air, and is reconverted to carbonate. 
Pure limestone yields ‘ fat lime,” in contradistinction to 
“lean lime,” which is impure and earthy. Many of the 
older limestones have too much silica or clay to form fat 
lime, and the impurities fuse in the kiln into useless slag. 
Hyprauric anp PortraND CEMENTS—The best known 
of the cements which set under water is’ Portland cement, 
which was discovered by Aspdin of Leeds in 1824, and 
named from its resemblance to Portland stone. He began 
its manufacture in 1825, and attention was called to its 
value by its use by Brunell in 1828 in the Thames Tunnel. 
Two years later experiments at Chatham proved that the same 
type of cement could be prepared by burning an appropriate 
mixture of chalk and mud from the Medway. The same 
materials are abundant along the Thames, which became 
for a time the chief seat of the Portland cement industry. 
The cements which can be used under water include three 
groups which are not sharply defined. Hydraulic cements 
are made from clayey limestones containing from 13 to 33 
per cent. of earthy material, usually silica, silicate of alumina, 
and iron oxide. The material is burnt in a kiln; at from 
1400° to 1500° F. the carbon dioxide is given off ; at about 
1650° F. the lime begins to combine with the alumina as 
calcic aluminates; between 2000° F. and 2350° F. the silica 
combines with the lime as calcic silicate, but sufficient lime 
cemains to slake on the addition of water. 
In the South-east of England hydraulic cement was made