12 



have always been considered Portland cements by the most criti- 

 cal authorities, though all agree that they are not particularly 

 high grade Portlands. So that a definition, based upon the cri- 

 teria above named, will of necessity exclude from our class of 

 Portland cements some very meritorious products. 



There is no doubt that in theory a rock could occur, contain- 

 ing lime, silica, and alumina in such correct proportions as to 

 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 cor- 

 rected by the addition of other materials to the cement rock, be- 

 fore 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 govern- 

 ment 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 mix- 

 tures, either natural or artificial, of argillaceous with calcare- 

 ous substances, the calcined product to contain at least 1.7 times 

 as much of lime, by weight, as of the materials which give the 

 lirne 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 pulveriz- 

 ing or artificial mixing of the materials prior to burning. It 

 seems probable that the Belgian "natural Portlands" were kept 

 in mind when these requirements were omitted. In dealing 

 with American made cements, however, and the specifications 

 are headed "Specifications for American Portland Cement," it 

 is a serious error to> omit these requirements. No true Portland 

 cements are at present manufactured in America from natural 



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



