302 CEMENTS, LIMES, AND PLASTERS. 



A glance at the tabulation above will show the relative physical 

 and chemical characters of the different raw materials. It is obvious, 

 if 75 per cent of lime carbonate will make a good cement mixture, 

 that any of the materials in the middle line (i.e., the Argillo-calcareous 

 group) could be used as a basis and its composition corrected by adding 

 either a purely calcareous material or a purely argillaceous material, 

 as might be necessary. The cement practice in the Lehigh district is 

 an example of this kind of mixing. But the same result could be ob- 

 tained by mixing any one of the materials on the first line of the 

 table (i.e., the Calcareous group) with any one of the argillaceous mate- 

 rials listed in the bottom line. This is the method followed at most plants 

 outside of the Lehigh district. There is really little to choose between 

 the two kinds of mixtures, for the final result is the main thing. In later 

 pages the few differences that do exist are pointed out and the advan- 

 tages and disadvantages of each type are mentioned. 



In previous papers the writer has grouped, under six heads, the 

 various combinations of raw materials at present used in the United 

 States in the manufacture of Portland cement. This grouping is as 

 follows : 



(1) Argillaceous hard limestone (cement rock) and pure limestone. 



(2) Pure hard limestone and clay (or shale). 



(3) Soft (chalky) limestone and clay (or shale). 



(4) Marl and clay (or shale). 



(5) Alkali waste and clay. 



(6) Slag and pure limestone. 



The relative commercial importance of these different combinations 

 is indicated by the figures tabulated on page 304. 



Examination of the statistics there given, which have been arranged 

 by the writer from figures given in the various volumes on ' ' Mineral Re- 

 sources of the United States", issued by the U. S. Geological Survey, 

 will develop several facts of interest. In the first place it will be seen 

 that the "cement-rock" type of mixture, important because of its 

 use in the Lehigh district, is slowly decreasing in relative importance, 

 having fallen from almost three fourths of the total product in 1898 to 

 only a little over half the total product in 1903. In absolute number 

 of barrels produced per year, it is of course rapidly increasing, but it 

 is no longer the only type of material to be considered. 



The use of marl as a cement material is also slowly decreasing in 

 relative importance, having reached its point of maximum output in 

 1899, when it supplied almost one fifth of all the cement made. The 

 hard limestones, on the other hand, have increased steadily in importance 



