354 CEMENTS, LIMES, AND PLASTERS. 



more soluble ingredients of the rock are usually removed in solution 

 by surface or percolating waters, while the more insoluble portions 

 are either left behind or are carried off mechanically by streams. These 

 relatively insoluble materials when sufficiently fine grained constitute 

 the clays. When they are left as a deposit in the spot where the orig- 

 inal rock disintegrated, they are called residual clays; when they are 

 carried off by surface-waters and finally deposited in the sea or along 

 river-beds they are transported or sedimentary clays. A third class of 

 particular interest to the cement manufacturer are the glacial clays, 

 deposited under or in front of the glaciers which formerly covered most 

 of the northern states. 



Composition of clays. The residual, sedimentary and glacial clays 

 usually differ markedly in composition, owing to the different manner 

 in which they have been deposited. The residual clays, for example, 

 are apt to contain coarse fragments of any very insoluble and hard 

 material which the original rock may have contained. A residual 

 clay arising from the decay of a granite will probably contain frag- 

 ments of quartz; one derived from a limestone may contain chert or 

 flint, as well as masses of undissolved lime carbonate. A sedimen- 

 tary clay, on the other hand, having been transported by water for 

 great distances, has usually lost all its coarser material, and is a fine 

 grained and homogeneous product. The glacial clays, being formed 

 mechanically by the abrading power of the ice, show even less homo- 

 geneity than the residual clays, and are apt to contain much sand, 

 gravel, and pebbles. 



Clays used in Portland-cement manufacture. For use as Portland- 

 cement materials clays 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 pref- 

 erably 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 03+Fe 2 O3 



SiO 2 . 



= 5 is approached. 

 o 



Nodules of lime carbonate, gypsum, or pyrite, if present in any quan- 

 tity, are undesirable; though the lime carbonate is not absolutely 

 injurious. Magnesia and alkalies should be low, preferably not above 

 3 per cent. 



The clays actually used in cement plants may be separated, for con- 

 venience, into the normal clays and the limey clays. In this section the 



