LIME AND CEMENT INDUSTRIES 699 



marine work; its composition is: silica 24; alumina 2.8; ferric 

 oxid .9; lime 70. 



Schoch expresses the opposite opinion to Newberry, and con- 

 siders that alkalis act as a flux, and they can be replaced by 

 calcined soda. He also states that they are of great benefit in 

 connection with the hardening process of cement, as they con- 

 vert the silica into a soluble form, in which condition it combines 

 with the lime when wet. 



An addition of \% to j* of fluorspar is very beneficial for 

 bringing about an easy clinkering of the materials. Nearly 

 all cements contain some magnesia and sulfur, which come ori- 

 ginally either from the clay or from the fuel used. Redgrave 1 

 states that all mixtures containing 77$ of carbonate of lime 

 will, when sufficiently calcined, give Portland cement of fair 

 quality. Compounds with too much clay fuse too easily, and 

 the resulting cement is light in weight, sets quickly, has a 

 brownish color and never becomes thoroughly hard. 2 It more- 

 over crumbles when exposed to the weather. Overlimed cements, 

 that is where the part made of lime in the slurry ranges above 

 77$ or 78$, give a cement which will stand the hottest fire with- 

 out fusing. 



Such cements when burned are slow setting and hard to grind, 

 and Portland cement made from such mixtures is liable to flow 

 and swell. 



In Europe the clay is generally mixed with marl or chalk, but 

 in this country comparatively little marl is used. In this country 

 Prof. S. B. Newberry 3 gives 17 works as mixing limestone with the 

 clay, and seven using marl, and of the latter four are in New 

 York state. 



Marl is cheaper to use for the manufacture of Portland cement, 

 as it is softer and finer grained and consequently needs little 

 grinding. It always has a large percentage of moisture whicii 



must be expelled. 



- 



1 Redgrave. Calcareous cements. 



» " p. 39. 



• Mineral resources of U. S. 16th an. rep't U. S. geol. sur. 4: 545. 



