MATERIALS CAREFULLY PROPORTIONED AND 
CHEMICALLY COMEINED 
Chemists analyze the materials, determine the amount 
of each to be used, set and seal interlocking automatic scales 
which in turn feed the materials to mills for grinding. 
Before mixing, the limestone and slag or other raw material 
must be crushed, dried and ground. Day in and day out, 
huge machines crunch hungrily their diet of stone and other 
materials and pass their mechanically-mixed grist along. 
At the same time, coal crushers, driers and grinders are at 
work preparing coal dust as finely ground as the rock dust. 
After the materials are ground, proportioned and mixed, 
they pass into rotary kilns. A modern cement kiln is a 
steel tube lined with fire brick. It is as long as a residence 
lot and more than large enough in diameter for a man to 
walk through. There are twenty such kilns in the Universal 
plant at Pittsburgh, set side by side in a building as long as an 
average city block. Into one end of each kiln goes a con- 
stant stream of powdered, mixed materials while into the 
other, forced along by compressed air, goes a stream of 
coal dust. Upon its entrance to the kiln, the powdered 
coal is ignited and travels through the long kiln hissing 
and burning as from an inferno. This burning produces a 
temperature of about 2800 degrees Fahrenheit which heats 
the materials to the point of incipient fusion and combines 
them chemically. 
In this process, the powdered materials are burned into 
nodules of “clinker” ranging from the size of a small pea to 
that of a walnut. This “clinker” has no setting or binding 
qualities and can be kept indefinitely. To make it useful 
as cement, it again must be ground, this time so fine that 
789, of it will pass a 200-mesh sieve, that is, a sieve con- 
taining 200 openings per linear inch or 40.000 openings per 
square inch. 
While this fineness is considered one of the important 
qualities of portland cement, there are certain limits within 
which it must be confined in order to obtain best results in 
concrete. If not fine enough, the cement is not active, 
while if too fine, it becomes too quick in setting or too costly