38 CEMENT MATERIALS AND INDUSTRY. [bull. 243. 
The slag used at a German Portland cement plant has the following 
range in composition: 
Analysis of slag used in Portland-cement manufacture. 
Silica (Si0 2 ) 30. to 35. 
Alumina (A1 2 3 ) 10. to 14. 
Iron oxide (FeO) 2 to 1.2 
Lime (CaO) 46. to 49. 
Magnesium oxide ( MgO ) 5 to 3.5 
Sulphur trioxide (S0 3 ) 2 to .6 
CLAYS AND SHALES. 
Cla3 r s are ultimately derived from the decay of older rocks, the finer 
particles being carried off by streams and deposited as beds of cla} T 
along channels, in lakes, or along parts of the seacoast or sea bottom. 
In chemical composition the clays are made up essentially of silica 
and alumina, though iron oxide is almost invariably present in more 
or less amount, while lime, magnesia, alkalis, and sulphur occur fre- 
quently, though usually only in small percentages. 
Shales are clays which have become hardened by pressure. The 
so-called "fire clays" of the Coal Measures are shales, as are many of 
the other " clays" of commerce. 
For use as Portland-cement materials clays or shales should be free 
from gravel and sand, as the silica present as pebbles or grit is practi- 
cally 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 preferably from 60 to 70 per cent. The alumina 
and iron oxide together should not amount to more than one-half the 
percentage of silica, and the composition will usually be better the 
nearer the ratio Al 2 3 +Fe 2 3 = ^^ is approached. 
Nodules of lime carbonate, gypsum, or p\ 7 rite, if present in an} T 
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 Portland 
cement plants are given on pages 33 and 36. 
SLATE. 
Slate is, so far as origin is concerned, merely a form of shale in 
which a fine, even, and parallel cleavage has been developed by pres- 
sure. In composition, therefore, it varies exactly as do the shales 
considered on previous pages, and so far as composition alone is con- 
cerned, slate would not be worthy of more attention, as a Portland- 
cement material, than any other shale. 
