eckel.] METHODS OF MANUFACTURE. 59 
rate of set of the cement proportionately, and appears to exert no 
injurious influence on the strength of the cement. In amounts over 
3 per cent, however, its retarding influence seems to become at least 
doubtful, while a decided weakening of the cement is noticeable. 
Sulphate of lime may be added in one of two forms, either as crude 
gypsum or as burned plaster. Crude gypsum is a natural hydrous 
lime sulphate, containing about 80 per cent of lime sulphate and 20 
per cent of water. When gypsum is calcined at temperatures not 
exceeding 400° F., most of its contained water is driven off. The 
'plaster" remaining carries about 98 per cent of lime sulphate, with 
only 7 per cent of water. 
In Portland-cement manufacture either gypsum or burned plaster 
may be used to retard the set of the cement, but g} 7 psum is univer- 
illy employed in the United States. This is merely a question of 
cost. It is true that to secure the same amount of retardation of set 
t will be necessary to add a little more gypsum than burned plaster, 
but gypsum is much cheaper than burned plaster. 
The addition of the gypsum to the clinker is usually made before it 
as passed into the ball mill, comminuter, or whatever mill is in use 
for preliminary grinding. Adding it at this point insures much more 
thorough mixing and pulverizing than if the mixture were made later 
in the process. At some of the few plants which use plaster instead 
of gypsum the finely ground plaster is not added until the clinker has 
received its final grinding and is ready for storage or packing. 
