CLINKER COOLING, GRINDING, AND STORAGE. 



545 



Methods of using gypsum. From what has been said on pre- 

 ceding pages, it is evident that in Portland-cement manufacture 

 either gypsum or burned plaster may be used to retard the set of 

 the cement. As a matter of fact, gypsum is the form almost uni- 

 versally 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 it will be necessary to add a little more of gypsum than if 

 burned plaster were used; but, on the other hand, gypsum is much 

 cheaper than burned plaster. 



The addition of the gypsum to the clinker is usually made before 

 it has passed into the ball mill, kominuter, or whatever mill is in use 

 for preliminary grinding. Adding it at this point secures 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. 



Analyses of gypsum used. The following analyses will serve to 

 illustrate the composition of the crude gypsum and of the calcined 

 plaster used at different American Portland-cement plants. 



TABLE 206. 

 ANALYSES OF GYPSUM USED IN CEMENT-PLANTS. 



TABLE 207. 

 ANALYSES OF CALCINED PLASTER USED AT CEMENT-PLANTS. 



Effect of various salts on set of cement. Experiments have been 

 made on the use of various other salts sulphates, phosphates, chlorides, 

 etc. by different chemists. Few of the results thus obtained are of any 



