57 



least 90 per cent, to pass a loo-mesh sieve; a number require 92 

 per cent. ; while a few important specifications require 95 per 

 cent. Within a few years it is probable that almost all specifi- 

 cations will go as high as this. 



Addition of gypsum. The cement produced by the rotary 

 kiln is invariably naturally so quick-setting as to require the 

 addition of sulphate of lime. This substance, when added in 

 quantities up to 2 l / 2 or 3 per cent., retards the rate of set of the 

 cement proportionately, and appears to exert no injurious influ- 

 ence on the strength of the cement. In amount over 3 per cent., 

 however, its retarding influence seems to become at least doubt- 

 ful, 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 cal- 

 cined at temperatures not exceeding 4OOF., most of its contain- 

 ed water is driven off. The "plaster" remaining carries about 

 93 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. As a mat- 

 er of fact, gypsum is the form almost universally 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 

 r.ecessary 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, komminuter, 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 the 

 iinal grinding and is ready for storage or packing. 



CONSTITUTION OF PORTLAND CEMENT. 



During recent years much attention has been paid by various 

 investigators to the constitution of Portland cement. The 

 chemical composition of any particular sample can, of course, be 



