4 CEMENTS, LIMES, AND PLASTERS. 



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 composition calcium carbonate. 

 Magnesite, on being heated, loses its carbon dioxide, leaving magnesium 

 oxide, or magnesia. This, under normal conditions of burning, will 

 reabsorb carbon dioxide, and set in its original form as magnesium 

 carbonate. 



The cementing materials included in this group, therefore, 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 or 

 gas, and reassumes 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 simply to avoid shrinkage, be disregarded, a thoroughly 

 hardened lime mortar is nothing more or less than an artificial lime- 

 stone. 



The principal points of difference between the two subgroups the 

 plasters and the limes may be briefly noted as follows: 



Subgroup I a. Hydrate cementing material : plasters. The materials 

 here included are known in commerce as "plaster of Paris", "cement 

 plaster", " Keene's cement", "Parian cement", etc. All of these hydrate 

 cements or plasters are based upon one raw material gypsum. The 

 partial dehydration of pure gypsum produces plaster of Paris. By 

 the addition to gypsum, either by nature or during manufacture, of 

 relatively small amounts of other materials, or by slight variations 

 in the processes of manufacture, the time of setting, hardness, and other 

 important technical properties of the resulting plaster can be changed 

 to a degree sufficient to warrant separate naming and descriptions of 

 the products. 



Both the technology and the chemistry of the processes involved 

 in the manufacture of the hydrate cements are simple. The mineral 

 gypsum, when pure, is a hydrous sulphate of lime, of the formula 

 CaS04,2H 2 0, corresponding to the composition: calcium sulphate 79.1%, 

 water 20.9%. As noted later (under the head of Cement Plasters) gyp- 

 sum, as mined, rarely even approximates to this ideal composition, its im- 

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



