CHAPTER XXIV. 

 ARGILLACEOUS LIMESTONE: CEMENT ROCK. 



THE term "cement rock" is here used to include all the very clayey 

 limestones carrying from 50 to 80 per cent of lime carbonate, with 

 correspondingly high percentages of argillaceous matter, and less than 

 8 per cent of magnesium carbonate. It is evident that an argillaceous 

 limestone low in magnesia, and containing approximately 75 to 77 per 

 cent of lime carbonate and 20 per cent or so of clayey materials (silica, 

 alumina, and iron oxide) would be the ideal material for use in the 

 manufacture of Portland cement; for a rock of this composition would 

 contain within itself, mixed in the proper proportions, all the ingre- 

 dients necessary for the manufacture of a good Portland. Such an 

 ideal rock would require the addition of no other raw material, but when 

 burnt alone would give a good cement. 



This ideal cement material is, of course, never realized in practice, 

 but certain deposits of clayey limestone approach it very closely in 

 composition. A limestone carrying 70 or 80 per cent of lime car- 

 bonate and 20 to 30 per cent clayey matter will require the addition 

 either of pure limestone or of clay in order to bring it to the desired 

 composition for a Portland-cement mixture. But it will be, of itself, 

 so near to the correct composition that it will need but little of the extra 

 raw material to make it absolutely perfect. Deposits of such "cement 

 rocks" possess important technologic advantages, and have been sought 

 for with great industry. Many such deposits of clayey limestones, low 

 in magnesia, occur in various parts of the United States, but few of them 

 are well located with regard to transportation routes, fuel supplies, and 

 markets. 



The most important of these argillaceous limestone, or "cement-rock", 

 deposits is at present that which is so extensively utilized in Portland- 

 cement manufacture in the "Lehigh district" of Pennsylvania and 

 New Jersey, though similar "cement rocks" occur in many other States. 

 As the Lehigh district still produces over half of all the Portland cement 

 manufactured in the United States, its raw materials will be described 

 below in some detail, after which other areas of "cement rock" will 

 be briefly noted. 



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