The Crystalline Cements.—Eckel. 149 
erties of the plaster can be changed sufficiently to warrant sep- 
arate naming of the resulting products, some of which are 
of considerable commercial importance for special uses. 
Plaster-of-Paris. Gypsum is a hydrous sulphate of calcium 
(CaSO,, 2H,O) whose composition, when pure, corresponds 
= ~ . , . * * 
to sulphate of lime 79.1%, water 20.9%. As mixed it is usually 
far from pure, carrying at times as high as 25% or even more 
of impurities, chiefly silica, alumina, oxide of iron, and calcium 
carbonate. Certain of these foreign ingredients:seem to exer- 
cise an appreciable effect upon the rate of set of the resulting 
plaster. In addition to these variations from natural causes, 
“accelerators” and “retarders” are frequently employed. 
Upon heating to about 120°-130° C. gypsum loses three- 
fourths of its water. Plaster-of-Paris, the result of this in- 
complete dehydration, is a definite hydrous calcium sulphate 
with the formula 2CaSO,, H,O, corresponding to the compo- 
sition sulphate of lime 93.8%, water 6.2%. (Complete dehy- 
dration of gypsum, which would occur at about 170° C. would 
result in the formation of an anhydrous lime sulphate corre- 
sponding to the mineral anhydrite. This completely anhy- 
drous sulphate re-hydrates very slowly, and is consequently 
of no commercial importance. ) 
Upon the addition of water, plaster-of-Paris rapidly re- 
hydrates and “sets” reassuming the composition of gypsum. 
The rate of set of plaster is regulated by the addition of 
non-crystalline materials (blood, glue, starch, etc.) which serve 
to retard the set; or of alum or borax, which accelerate it. 
Keene’s Parian, and other hard-finishing cements are made 
by adding to plaster-of-Paris a dilute solution of borax or aium, 
and, after drying, reheating at a low red heat. 
I. b. CARBONATE CEMENTS. 
The cements of this group are oxides, derived from car- 
bonates by the application of heat, and becoming recarbonated 
upon exposure, under proper conditions, to any such source of 
carbon dioxide as the atmosphere. From the examination of 
old mortars it seems probable that a certain amount of action 
takes place between the silica of the sand and the lime, result- 
ing in the formation of lime silicates ; but this effect is of slight 
