CHAPTER XVII. 

 RAW MATERIAL: NATURAL-CEMENT ROCK. 



Composition of natural-cement rock. The raw material utilized for 

 natural-cement manufacture 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. 'It is the presence of these clayey materials which give 

 the resulting cement its hydraulic properties. Stress is often care- 

 lessly or ignorantly laid on the fact that many of our best-known 

 natural cements carry large percentages of magnesia, but it should at 

 this date be realized that magnesia (in natural cements at least) may be 

 regarded as being almost exactly interchangeable with lime, so far as 

 the hydraulic properties of the product are concerned. The presence 

 of magnesium carbonate in a natural-cement rock is then merely 

 incidental, while the silica, alumina, and iron oxide are essential. The 

 25 per cent or so of magnesium carbonate which occurs in the cement 

 rock of the Rosendale district, New York, could be replaced by an 

 equivalent amount of lime carbonate, and the burnt stone would still 

 give a hydraulic product. If, however, the clayey portion (silica, 

 alumina, and iron oxide) of the Rosendale rock could be removed 

 leaving only the magnesium and lime carbonates, the rock would lose 

 all of its hydraulic properties and would yield on burning simply a 

 rnagnesian lime. 



This point has been emphasized because many writers on the sub- 

 ject have either explicitly stated or implied that it is the magnesium 

 carbonate of the Rosendale, Akron, Louisville, Utica, and Milwaukee 

 rocks that causes them to yield a natural cement on burning. Even 

 a casual consideration of the subject should have recalled to mind 

 the fact that the Cumberland and Lehigh natural-cement rocks are 

 practically free from magnesium carbonate. 



A limestone containing sufficient argillaceous matter to make a 

 good natural cement can generally be recognized by the characteristic 

 clayey odor given forth when breathed on. 



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