328 CEMENTS, LIMES, AND PLASTERS. 



In the plants located near Bath and Nazareth, however, the practice 

 has been slightly different. In this particular area the cement-rock 

 quarries usually show rock carrying from 75 to 80 per cent of lime car- 

 bonate. The mills in this vicinity, therefore, require practically no 

 pure limestone, as the quarry rock itself is sufficiently high in lime 

 carbonate for the purpose. Indeed, it is at times necessary for these 

 plants to add clay or slate, instead of limestone, to their cement rock, 

 in order to reduce its content of lime carbonate to the required figure. 

 In general, however, it may be said that Lehigh practice is to mix a 

 low-carbonate cement rock with a relatively small amount of pure lime- 

 stone, and analyses of both these materials, as used at various plants 

 in the district, are given below in Tables 152 and 153. 



Character and composition of the cement rock. The cement rock is a 

 dark-gray to black, slaty limestone, breaking with an even fracture 

 into flat pieces, which usually have smooth, glistening surfaces. As 

 the percentage of lime carbonate in the rock increases i.e., as the 

 lower beds of the formation are reached the color becomes a some- 

 what lighter gray and the surfaces of the fragments lose their slaty 

 appearance. 



The range in composition of the cement rock as used at various 

 plants is well shown in the first eight columns of the above table. The 

 nearer the material from any given quarry or part of a quarry approaches 

 the proper Portland-cement composition (say 75 to 77 per cent lime 

 carbonate) the less addition of pure limestone will be necessary. In 

 by far the greater part of the district, as above noted, the cement rock 

 is apt to run about 65 to 70 per cent of lime carbonate, therefore re- 

 quiring the addition of a proportionate amount of limestone. Most 

 of the quarries near Bath and Nazareth, however, have been opened 

 on beds of cement rock running considerably higher in lime carbonate 

 and occasionally running so high (80 per cent, etc.) as to require the 

 addition of shale or clay rather than of pure limestone. 



Character and composition of the pure limestones. The pure lime- 

 stones added to the cement rock are commonly gray and break into 

 rather cubical fragments. The fracture surfaces show a finely granu- 

 lar structure quite distinct in appearance from the slaty cement rock. 



In composition the limestones commonly used will carry from 90 to 

 96 per cent of lime carbonate, with rather less magnesium carbonate 

 than is found in the cement rock. All of the cement-plants own and 

 operate their own cement-rock quarries, but most of them are com- 

 pelled to buy the pure limestone. When this is the case only very 

 pure grades of limestone are purchased, but when a cement-plant owns 



