10 



burnt rock would lose all of its hydraulic properties and would 

 yield simply a magnesian lime. 



This point has been emphasized because many writers on the 

 subject have either explicitly stated or implied that it is the mag- 

 nesian carbonate of the Rosendale, Akron, Louisville, Utica, 

 ar?d Milwaukee rocks that causes them to yield a natural cement 

 on burning. 



PORTLAND CEMENT. 



Portland cement is produced by burning a finely ground arti- 

 ficial mixture containing essentially lime, silica, alumina, and 

 iron oxide, in certain definite proportions. Usually this combi- 

 nation is made by mixing limestone or marl with clay or shale, 

 in which case about three times as much of the lime carbonate 

 should be present in the mixture as of the clayey materials. 

 The burning takes place at a high temperature, approaching 

 3,000 F., and must, therefore, be carried on in kilns of special 

 design and lining. During the burning, combination of the lime 

 with silica,, alumina, and iron oxide takes place. The product 

 of the burning is a semi-fused mass called clinker, and consist- 

 ing of silicates, aluminates, and ferrites of lime in certain defi- 

 nite proportions. This clinker must be finely ground. After 

 such grinding the powder ( Portland cement) will set under 

 water. 



As noted above, under the head of Natural Cements, Port- 

 land cement is blue to gray in color, with a specific gravity of 

 3.0 to 3.2, and sets more slowly than natural cements, but soon 

 attains a higher tensile strength. 



PUZZOLAN CEMENTS. 



The cementing materials included under this name are made 

 by mixing powdered slaked lime with either a volcanic ash or a 

 blast-furnace slag. The product is therefore simply a me- 

 chanical mixture of two ingredients, as the mixture is not burn- 

 ed at any stage of the process. After mixing, the mixture is 

 finely ground. The resulting powder (Puzzolan cement) will 

 set under water. 



Puzzolan cements are usually light bluish to light yellow in 

 color, and of lower specific gravity and less tensile strength 

 than Portland cement. They are better adapted to use under 

 water than to use in air. 



