ECKEL.] 
METHODS OF MANUFACTURE. 
47 
After the mixture is reduced, and when it is ready for burning, 90 
to 95 per cent of it .should pass through a LOO-mesh sieve. In the 
plants of the Lehigh district the mixture is rarely crushed as Hue as 
when limestone and clay are used. Newberry" has pointed out in 
explanation of this that an argillaceous limestone (cement rock) mixed 
with a comparatively small quantity of purer limestone, as in the 
Lehigh plants, requires less thorough mixing and less fine grinding 
than when a mixture of limestone and clay (or marl and clay) is used, 
for even the coarser particles of the argillaceous limestone will vary 
so little in chemical composition from the proper mixture as to affect 
the quality of the resulting cement but little should either mixing or 
grinding be incomplete^ accomplished. 
A very good example of typical Lehigh Valley grinding of raw 
material is afforded by a specimen examined ' v by Prof. E. D. Camp- 
bell. This specimen of raw mix ready for burning was furnished by 
one of the best of the eastern Pennsylvania cement plants. A mechan- 
ical analysis of it showed the following results: 
Mesh of sieve. 
50 
100 
200 
Per cent passing 
96. 9 
3.1 
85. (i 
14.4 
72.4 
Per cent residue 
27. 6 
The material, therefore, is so coarsely ground that only a trifle over 
85 per cent passes a 100-mesh sieve. 
SLAG- LIMESTONE MIXTURES. 
While the manufacture of Portland cement from slag and limestone 
is similar in general theory and practice to its manufacture from lime- 
stone and clay, there are certain interesting differences in the prepara- 
tion of the mixture. In the following paragraphs the general methods 
of preparing mixtures of slag and limestone for use in Portland-cement 
manufacture will first be discussed, after which certain processes pecu- 
liar to the use of this mixture will be described separately. 
General methods. — After it had been determined that puzzolanic 
cement, made by mixing slag with lime without subsequent burning, 
was not an entirely satisfactory structural material, attention was 
soon directed toward the problem of making a true Portland cement 
from such slag. The blast-furnace slags commonly available, while 
carrying enough silica and alumina for a cement mixture, are too 
low in lime to be suitable for Portland cement. Additional lime 
" Twentieth Ann. Rept. U. S. Geol. Survey, pt. 6, p. 545. 
b Jour. Am. Chem. Soc., vol. 25, p. 1106. 
