32- 



Analysis of slag used in Portland cement 'manufacture 



Silicia (SiO2) 30. 35. 



Alumina (A1203) 10. 14. 



Iron oxide (FeO) 0.2 1.2 



Lime (CaO) ; 46. 49. 



Magnesia (MgO) 0.5 3.5 



Sulphur trioxide (SOS) . 0.2 0.6 



Clays and Shales. 



Clays are ultimately derived from, the decay of older rocks, 

 the finer particles resulting from this decay being carried off and 

 deposited by streams along their channels, in lakes, or along 

 parts L- r the sea coast or sea bottom as beds of clay. In, chemi- 

 cal composition the clays are composed essentially of silica and 

 alnmina, though iron oxide is almost invariably present in more 

 or less amount, while lime, magnesia, alkalies and sulphur are 

 of frequent occurrence, though usually only in small percent- 

 ages. 



Shales are clays which have become hardened by pressure. 

 The so-called "fire-clays" of the Coal Measures arc shales, as 

 are many of the other "clays" of commerce. 



For use as Portland cement materials clays or shales should 

 be as free as possible from gravel and sand, as the silica present 

 as pebbles or grit is practically inert in the kiln unless ground 

 more finely than is economically practicable. In composition 

 they should not carry less than 55 per cent, of silica, and pre- 

 ferably from 60 to 70 per cent. The alumina and iron oxide to- 

 gether should not amount to more than one-half the percent- 

 age of silica, and the composition will usually be better the 

 nearer the ratio Al2O3+Fe2O3=SiO2 is approached. 



~T~ 



Nodules of lime carbonate, gypsum or pyrite, if present in 

 any quantity, are undesirable; though the lime carbonate is not 

 absolutely injurious. Magnesia and alkalis should be low, 

 preferably not above 3 per cent. 



Analyses of clays and shales'used in various American Port- 

 land cement plants will be found on pages 27 and 30. 



