296 CEMENTS, LIMES, AND PLASTERS. 



our class of Portland cements some meritorious products. But, on the 

 other hand, such a restricted definition would have decided advantages. 



There is no doubt that in theory a rock could occur containing lime, 

 silica, and alumina in such uniformly correct proportions as to always 

 give a good Portland cement on burning. Actually, however, such a 

 perfect cement rock is of extremely rare occurrence. As above stated, 

 certain brands of French and Belgian "Portland" cements are made 

 from such natural rocks without the addition of any other material; 

 but these brands are not particularly high grade, and in the better 

 Belgian cements the composition is corrected by the addition of other 

 material to the cement rock before burning. 



The following definition of Portland cement is of importance because 

 of the large amount of cement which will be accepted annually under 

 the specifications * in which it occurs. It is also of interest as being 

 the nearest approach to an official definition of the material that we 

 have in this country: 



"By a Portland cement is meant the product obtained from the 

 heating or calcining up to incipient fusion of intimate mixtures, either 

 natural or artificial, of argillaceous with calcareous substances, the 

 calcined product to contain at least 1.7 times as much of lime, by weight, 

 as of the materials which give the lime its hydraulic properties, and 

 to be finely pulverized after said calcination, and thereafter additions 

 or substitutions for the purpose only of regulating certain properties 

 of technical importance to be allowable to not exceeding 2 per cent 

 of the calcined product." 



It will be noted that this definition does not require pulverizing 

 or artificial mixing of the materials prior to burning. It seems prob- 

 able that the Belgian "natural Portlands" were kept in mind when 

 these requirements were omitted. In dealing with American-made 

 cements, however, and the specifications in question are headed "Speci- 

 fications for American Portland Cement", it is a serious error to omit 

 these requirements. No true Portland cements are at present manu- 

 factured in America from natural mixtures without pulverizing and 

 artificially mixing the materials prior to burning. Several natural- 

 cement plants, however, have placed on the market so-called Port- 

 land cements made by grinding up together the underburned and over- 

 burned materials formed during the burning of natural cements. Sev- 

 eral of these brands contain from 5 to 15 per cent of magnesia, but 

 even if that fact be disregarded, there is no warrant or excuse for con- 



* Professional Paper No. 28, Corps of Engineers, U. S. A., p. 30. 



