PART III. NATURAL-CEMENT RESOURCES OF THE UNITED 
STATES. 
INTRODUCTION. 
I On the following pages an attempt will be made to discuss the natural 
lements as a class, emphasis being laid upon the points of resemblance 
if the various brands, their many points of difference being for a time 
lisregarded. The difficulties encountered in such an attempt are greater 
pan the reader at first sight may imagine, for few engineers realize 
| T hat a heterogeneous collection of products is included under the well- 
|nown name of "natural cement." This lack of knowledge is easily 
Ixplained. Under ordinary circumstances, natural cements are too 
low in value to be shipped far from their points of production. The 
latural cement made at any given locality has usually, therefore, a 
tell-detined market area, within which it is well known and subject to 
little competition. The engineer practicing within such an area forms 
lis idea of natural cements in general from what he knows of the 
brands encountered in his work, and as all the brands from one cement- 
broducing locality are apt to resemble one another closely he is likely 
p conclude that natural cements form a homogeneous class, with many 
boints of resemblance and few of difference. On the contrary, there 
hay be as much difference in strength, rate of set, composition, etc., 
between natural cements from two different localities as between any 
fiven brand of natural cement and a Portland cement. 
DEFINITION OF NATURAL CEMENTS. 
The term "natural cements" is here used to include all cements pro- 
luced by burning a natural limestone rock without previous grinding 
>r mixing. As so used it includes the class of doubtful products com- 
nonly known as "natural Portlands," which are so largely made in 
France and Belgium. The reasons for including these " natural Port- 
ands" with the natural cements instead of with the true Portlands 
ire stated in considerable detail on pages 21-23. 
Natural cements are produced by burning a natural clayey limestone, 
containing 15 to 40 per cent of silica, alumina, and iron oxide, without 
Dreliminaiy mixing and grinding. This burning takes place at a tem- 
perature that is usually little, if any, above that of an ordinary lime- 
din. During the burning the carbon dioxide of the limestone is 
333 
