16 



In the majority of plants, however, most or all of the neces- 

 sary lime is furnished by one raw material, while the silica, alu- 

 mina and iron oxide are largely or entirely derived from another 

 raw material. The raw material which furnished the lime is 

 usually natural, a limestone, chalk or marl ; but occasionally 

 an artificial product is used, such as the chemically precipitated 

 lime carbonate which results as waste from alkali manufacture. 

 The silica, alumina and iron oxide of the mixture are usually 

 derived from clays, shales or slates ; but in a few plants blast- 

 furnace slag is used as the silico-aluminous ingredient in the 

 manufacture of true Portland cement. 



The various combinations of raw material which are at pres- 

 ent used in the United States in the manufacture of Portland 

 cement may be grouped under six heads. This grouping is as 

 follows : 



T. Argillaceous limestone (cement rock) and pure limestone. 



2. Pure hard limestone and clay or shale. 



3. Soft chalky limestone and clay. 



4. Marl and clay. 



5. Alkali waste and clay. 



6. Slag and limestone. 



ORIGIN AND GENERAL CHARACTERS OP LIMESTONE. 



The cement materials which are described in the four follow- 

 ing sections as argillaceous limestone or cement rock, pure hard 

 limestone, chalk, and marl, though differing sufficiently in their 

 physical and economic characters to be discussed separately 

 and under different names, agree in that they are all forms of 

 limestone. The origin, chemical composition, physical charac- 

 ters, and properties of limestone will, therefore, be briefly taken 

 up in the present chapter to serve as an introduction to the more 

 detailed statements concerning the various types of limestone to 

 be found in the succeeding chapters. 



Origin of limestones* Limestones have been formed large- 

 ly by the accumulation at the sea bottom of the calcareous re- 



*For a more detailed discussion of this subject the reader will do 

 well to consult Chapter VIII of Prof. J. F. Kemp's "Handbook of 

 Rocks." 



