PORTLAND CEMENT: PRELIMINARY STATEMENTS. 



301 



In certain favored localities deposits of argillaceous (clayey) lime- 

 stones or " cement rock" have been found in which the lime, silica, 

 alumina, and iron oxide exist in so nearly the proper proportions that 

 only a relatively small amount (say 10 per cent or so) of other material, 

 added before calcination, is required in order to make a mixture of 

 correct composition. Certain blast-furnace slags are also close in com- 

 position to the desired mixture, and are used like " cement rock". 



In the majority of plants, however, most or all of the necessary lime 

 is furnished by one raw material, while the silica, alumina, and iron 

 oxide are largely or entirely derived from another raw material. The 

 raw material which furnishes the lime is usually a natural limestone 

 either a hard limestone, a chalk, or a marl but occasionally an artificial 

 product is used, such as the chemically precipitated lime carbonate 

 which results as a waste or by-product of alkali manufacture. The 

 silica, alumina, and iron oxide of the mixture are usually derived from 

 clays or shales, more rarely from slates. 



The various raw materials available for use in Portland-cement 

 manufacture differ in composition, physical characters, and origin. As 

 to composition, they may be almost (a) purely calcareous, (6) a mixture 

 of calcareous and argillaceous elements, or (c) almost purely argilla- 

 ceous; as to physical characters they may be (a) hard and massive, 

 like the hard limestones and slates, (6) soft, like the chalks and shales, 

 or (c) granular or unconsolidated, like the marls, clays, alkali waste, 

 and granulated slag. As to origin, they may be (a) natural, like lime- 

 stones, marls, slates, clays, etc., or (6) artificial, like alkali waste and 

 furnace slag. 



TABLE 144. 

 CHARACTER OF PORTLAND-CEMENT MATERIALS. 



