22 CEMENT MATERIALS AND INDUSTRY. [bull. 243. 
should be drawn so as to include or exclude such products is evi- 
dently largely a matter of convention; but, unlike most conventional 
issues, the decision has very important practical consequences. The 
question at issue may be stated as follows: 
If artificial mixture of the raw materials and a very high degree of 
burning are made the criteria on which to base a definition, there will 
be excluded from the class of Portland cements certain well-known 
and very meritorious products, manufactured at several points in 
France and Belgium by burning a natural rock without artificial 
mixture and at a considerably lower temperature than is attained in 
ordinary Portland-cement practice. These "natural Portlands" of 
France and Belgium have always been considered Portland cements by 
the most critical authorities, though all agree that they are not of very 
hio-h-o-rade. 
There is no doubt that there could occur a rock which would contain 
lime, silica, and alumina in such proportions as to give a good Port- 
land cement on burning. Actually, however, such a perfect cement 
rock is of extremely rare occurrence. As above noted, 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 of particular^ 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 important because 
of the large amount of cement which is accepted annually under the 
specifications" in which it occurs, and is of interest as being the nearest 
approach in this country to an official definition of the material. 
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 substi- 
tutions 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 probable 
that the Belgian u 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 "Specifications 
for American Portland Cement" — it is a serious error to omit these 
requirements. No true Portland cements are manufactured in Amer- 
ica from natural mixtures without pulverizing and artificially mixing 
the materials prior to burning. Several plants, however, have placed 
on the market so-called " Portland cements," made by grinding up 
<« Prof. Paper No. 28, Corps of Engineers, U. S. A., p. 30. 
