eckel.] CEMENT-MAKING MATERIALS. 25 
RAW MATERIALS FOR PORTLAND CEMENT. 
For the purposes of the present section it will be sufficiently accurate 
to consider that a Portland-cement mixture, when ready for burning, 
will contain about 75 per cent of lime carbonate (CaCO ;5 ) and 20 per 
cent of silica (SiG 2 ), alumina (A1 2 3 ) and iron oxide (Fe 2 3 ) together, 
the remaining 5 per cent including any magnesium carbonate, sulphur, 
and alkalies that may be present. 
The essential elements which enter into this mixture — lime, silica, 
alumina, andiron — are all abundantly and widely distributed in nature, 
occurring in different forms in many kinds of rocks. It can therefore 
readily be seen that, theoretically, a satisfactory Portland-cement 
mixture could be prepared by combining, in an almost infinite number 
of ways and proportions, many kinds of raw material. Obviously, 
too, we might expect to find gradations in the artificial ness of the 
mixture, varying from one extreme, where a natural rock of abso- 
lutely correct composition was used, to the other extreme, where two 
or more materials, in nearly equal amounts, are required. 
The almost infinite number of raw materials which are theoretically 
available are, however, reduced to a very few under existing commer- 
cial conditions. The necessity for making the mixture as cheaply as 
possible rules out of consideration a large number of materials which 
would be considered available if chemical composition were the only 
thing to be taken into account. Some materials, otherwise suitable, 
are too scarce; some are too difficult to pulverize. In consequence, a 
comparatively few combinations of raw materials are actually used in 
practice. 
In certain localities deposits of argillaceous (clayey) limestone or 
"cement rock" occur in which the lime, silica, alumina, and iron 
oxide exist in so nearly the proper proportions that only a relatively 
small amount (say 10 per cent) of other material is required in order 
to make a mixture of correct composition. In the majority of plants, 
however, most or all of the necessary lime is furnished by one raw 
material, while the silica, alumina, and iron oxide are largely or 
entirely derived from another. The raw material which furnishes the 
lime is usually natural — a limestone, chalk, or marl — but occasionally 
it is an artificial product, such as the chemically precipitated lime car- 
bonate which results as waste from alkali manufacture. The silica, 
alumina, and iron oxide of the mixture are usually derived from clays, 
shales, or slates; but in a few plants blast-furnace slag is used as 
the, silica-aluminous ingredient in the manufacture of true Portland 
cement. 
The various combinations of raw materials which are at present used 
in the United States in the manufacture of Portland cement may be 
grouped under six heads: (1) Argillaceous limestone (cement rock) 
