334 CEMENT MATERIALS AND INDUSTRY. [bull. 24c 
almost entirely driven off, and the lime combines with the silica 
alumina, and iron oxide, forming a mass containing silicates, alumi • 
nates, and ferrites of lime. In case the original limestone containet | 
any magnesium carbonate the burned rock will contain a corresponding 
amount of magnesia and magnesian compounds. 
The burned mass will not slake if water be poured on it. It is nee 
essary, therefore, to grind it rather line; after it is ground, if the I 
resulting powder (natural cement) be mixed with water, it will harder 
rapidly. This hardening or setting will take place either in air 01 l 
under water. 
RELATIONS OF NATURAL CEMENTS TO OTHER CEMENTS. 
Natural cements differ from ordinary limes in two very noticeable 
ways: (I) The burned mass does not slake when water is poured on it. t 
(2) Natural cement powder has hydraulic properties; i. e., if properly 
prepared it will set under water. 
Natural cements are closely related to hydraulic limes on the one 
hand and to Portland cement on the other, agreeing with both in the 
possession of hydraulic properties. They differ from hydraulic limes, 
however, in that the burned natural cement rock will not slake when 
water is poured on it. 
Natural cements differ from Portland cements in the following 
important particulars: (1) Natural cements are made by burning 
masses of natural rock, not by burning carefully prepared and finely 
ground artificial mixtures. (2) Natural cements, after burning and 
grinding, are usually }^ellow to brown and light in weight, their spec™ 
gravity being about 2.7 to 2.9; Portland cement is commonly blue to 
gray in color and heavier, its specific gravity ranging from 3.0 to 3.2. 
(3) Natural cements are always burned at a lower temperature than 
Portland, and commonly at a much lower temperature, the mass of 
rock in the kiln never being heated high enough to even approach the 
fusing or clinkering point. (1) Natural cements set more rapidly 
than Portland cement, but do not attain so high ultimate strength. 
(5) Various brands of natural cements will show very great differences 
in composition, while Portland cement is a definite product whose per- 
centages of lime, silica, alumina, and iron oxide vaiy only between 
narrow limits. 
RAW MATERIAL (NATURAL-CEMENT ROCK). 
The material used in the manufacture of natural cement is invariably 
a clayey limestone, carrying from 13 to 35 per cent of clayey material, 
of which 10 to 22 per cent or so is silica, while alumina and iron oxide 
together may vary from 4 to 16 per cent. These clayey materials give 
the resulting cement its hydraulic properties, Stress is often care- 
