We have here the two classes into which hydraulic cements are divided.
Redgrave and Charles Spackman, Calcareous Cements (London, 1905); F.
The principles which govern the preparation and setting of the other class of calcium sulphate cements, that is, cements of the Keene class, are not fully understood, but there is a fair amount of knowledge on the subject, both empirical and scientific. The essential difference between the setting of Keene's cement and that of plaster of Paris is that the former takes place much more slowly, occupying hours instead of minutes, and the considerable heating and expansion which characterize the setting of plaster of Paris are much less marked.
Its materials are pebbles, clays and sands of various' colours from white to deep red, tinged with peroxide of iron, which sometimes cements the pebbles and sands into compact rocks.
Zinc chloride solution readily dissolves the oxide with the formation of oxychlorides, some of which are used as pigments, cements and for making artificial teeth.