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148 The American Geologist. March, (77% 
I. SIMPLE CEMENTS. 
The raw materials from which cements of the present 
group are made occur in nature as hydrous calcium sulphate 
(gypsum) ; calcium carbonate (calcite, “limestone” ) ; and cal- 
cium-magnesium carbonate (dolomyte, “magnesian lime- 
stone.’’ ) 
During the processes of manufacture—of which the fund- 
amental one is simply the application of a sufficient degree of 
heat, these raw materials part with much or all of their water 
or carbon dioxide, and the resulting cements are therefore 
composed respectively of an almost anhydrous calcium sul- 
phate (“plaster-of-Paris”) ; calcium oxide (“quicklime”) or 
a mixture of calcium and magnesium oxides (magnesian lime). 
On being used in such a manner that they can more or less 
freely reabsorb the water or carbon dioxide which has been 
liberated during manufacture, the cements “set,” this “set” be- 
ing caused simply by reassumption of their original composi- 
tion. Plaster-of-Paris which has set, for example, is not chemi- 
cally distinguishable from gypsum from which it was manufac- 
tured, and, if we disregard the sand, added to counteract 
shrinkage, hardened lime mortar is nothing more or less than 
an artificial limestone. In the first subgroup of this ciass, 
water is the substance removed during manufacture and reab- 
sorbed during use; in the second, it is carbon dioxide. An in- 
termediate subgroup should really be inserted, (in order to 
make the classification theoretically complete), to include those 
cements made by driving off carbon dioxide and setting in con- 
sequence of the addition of water. Here the raw material is 
a carbonate; the set cement a hydroxide. In this subgroup 
would fall magnesia and, under certain conditions of burning, 
the magnesian limes. The definition of simple cements, given 
above, would, of course, require slight modifications. 
I. a. HYDRATE CEMENTS. 
The raw material on which all the commercial cements of 
this class are based is gypsum. By its partial dehydration is 
produced plaster-of-Paris, by far the most important member 
of the group. By the addition of relatively small amounts 
of certain other materials, and by slightly varying the processes 
of manufacture, the time of setting, hardness, and other prop- 
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