LIME AND CEMENT INDUSTEIES 691 



Akron, Star, K,0 , ... 1.30 



Akron, Star, Xa 2 . ., 23 



Akron, Obelisk, K,0 1.6 



Akron, Obelisk, Na 2 52 



Buffalo, K 2 1.44 



Buffalo, ]STa 2 41 



These alkalis are probably derived from feldspar grains in 

 the clay. 



Richardson makes the following subdivisions of natural ce- 

 ments, based on the examination of those made in various parts 

 of the United States. 1 



1 Lime cement with only 2$ or 3$ of magnesia, 13$ to 15$ 

 of iron and aluminum oxid, and 20$ of combined silica. 



2 lime cements with as little magnesia but with less silicates 

 than class 1, and consequently less satisfactory and more fiery. 



3 Magnesia cements with a maximum of not more than 15$ 

 of magnesia, the same amount of iron and aluminum oxid and 

 15$ to 20$ of combined silica, and, in addition, considerable un- 

 combined silicates, as they are not thoroughly burned. 



4 Magnesia cement with a large amount of magnesia, viz over 

 20$, less alumina and iron and less undecomposed silicates than 

 in the preceding class. 



5 Magnesia cement deficient in alumina and iron oxids as well 

 as in combined silica. 



6 Magnesia cements thoroughly burned, made from rock hav- 

 ing a smaller amount of silicates than those of class four, with 

 only a medium per cent of magnesia and little uncombined sili- 

 cates. 



Cements of the first class set and acquire strength rapidly and 

 increase in this direction for a long period, but the final result is 

 a more brittle mortar than is obtained with the magnesia brands. 

 This class includes the lime cements of the Potomac valley. 

 According to Richardson, the second class has not as variable a 



1 Brickbuilder. Jan. 1898. p. 14. 



