while differing in composition and properties, agree in certain 

 important points. They are all manufactured by heating a 

 natural raw material sufficiently to remove much or all of its 

 water or carbon dioxide; and, in all, the setting properties of 

 the cementing material are due to the fact that, on exposure to 

 the water or carbon dioxide which has thus been driven off, the 

 cement reabsorbs the previously expelled liquid o>r gas, and re- 

 assumes the chemical composition of the raw material from 

 which it was derived. 



Plaster-of-Paris, after setting, is not chemically different from 

 the gypsum from which it was derived ; while if the sand, added 

 to avoid shrinkage, be disregarded, hardened lime- mortar is 

 nothing more or less than an artificial limestone. 



Sub-group la. Hydrate Cements. 



The materials here included are known in commerce as "plas- 

 ter-of-Paris," "cement plaster," "Keene's cement," "Parian ce- 

 ment/' etc. All of these hydrate cements are based upon one 

 raw material, gypsum. The partial dehydration of pure gyp- 

 sum produces plaster-of-Paris. By the addition of gypsum, 

 either by nature or during manufacture, of relatively small 

 amounts of other materials ; or by slight variations in the pro- 

 cesses of manufacture, the time of setting, hardness, and other 

 important technical properties of the resulting plaster can be 

 changed to a sufficient degree to warrant separate naming and 

 descriptions of the products. 



P>oth the technology and the chemistry of the processes in- 

 volved in the manufacture of the hydrate cements are simple. 

 The mineral gypsum, when pure, is a hydrous sulphaite of limie, 

 of the formula CaSO4, 2H2O, corresponding to the composi- 

 tion calcium sulphate 79.1%, water 20.9%. Gypsum, as mined, 

 rarely even approximates to this ideal composition, its impurities 

 often amounting to 25% or even more. These impurities, chiefly 

 clayey materials and fragments of quartz and limestone, often 

 exercise an appreciable effect upon the properties of the plaster 

 resulting from burning such impure gypsum. 



On burning pure gypsum at a relatively low temperature 

 (35O-4OO F.) much of its water of combination is driven off, 

 leaving a partially dehydrated lime sulphate. This, when 

 ground, is plaster of Paris, or if it either naturally or artificial- 



