simple reabsorption of the same liquid or gas and the reassump- 

 tion of original composition ; the set cement being therefore simi- 

 lar in composition to the raw material. 



Sub-grohp la. Hydrate C.ements : Setting properties due 

 to reabsorption of water. 



Sub-group Ib. Carbonate Cements : Setting properties due 

 to reabsorption of carbon dioxide. 



GROUP II. COMPLEX CEMENTS : Including all those cement- 

 ing materials whose setting properties are due to the action of 

 entirely new chemical compounds which were formed during 

 manufacture or use ; the set cement being therefore different in 

 composition from the raw material. 



Sub-group la. Silicate Cements : Setting properties due 

 largely to the formation o<f silicates. 



Sub-group lib. Oxy chloride Cements: Setting properties 

 due to the formation of oxychlorides. 



GROUP I SIMPLE CEMENTS. 



The cementing materials included in the present group are 

 those known commercially . as ''plasters," ''hard-finishing ce- 

 ments," and "limes." 



The material from which the ''plasters" and "hard-finishing 

 cements" are derived is gypsum, a hydrous calcium sulphate ; 

 while the limes are derived from limestone, which is essentially 

 calcium carbonate, though usually accompanied by greater or 

 less amounts of magnesium carbonate. 



On heating gypsum to a certain temperature, the raw mater- 

 ial parts readily with much of its water, leaving an almost 

 anhydrous calcium sulphate, known commercially as plaster-of- 

 Paris. On exposing this plaster to water, it re-hydrates, and 

 again takes the composition of the gypsum from which it was 

 derived. 



In like manner limestone, on being sufficiently heated, gives 

 off its carbon dioxide, leaving calcium oxide or "quicklime." 

 This, on exposure to moisture and air carrying carbon dioxide, 

 reabsorbs carbon dioxide and reassumes its original composi- 

 tion, calcium carbonate. 



The cementing materials included in this group, therefore, 



