48 CEMENT MATERIALS AND INDUSTRY. [bull. 243. 
must be added, usually in the form of limestone, and the slag and lime- 
stone must be well mixed and properly burned. The general methods 
for properly mixing the materials vary in details. It seems probable 
that the first method used in attempting to make a true Portland 
cement from slag was to dump the proper proportion of limestone, 
broken into small lumps, into molten slag. The idea was that both 
mixing and calcination could thus be accomplished in one stage, but 
in practice it was found that the resulting cement was variable in com- 
position and always low in grade. This method has accordingly 
fallen into disuse, and at present three different general processes of 
preparing the mixture are practiced at different European and Ameri- 
can plants: 
1. The slag and limestone are granulated, dried, and ground sepa- 
rately. The two materials are then mixed in proper proportions, the 
mixture is finely pulverized in tube mills, and the product is fed in a 
powdered state to rotary kilns. 
2; The slag is granulated, dried, and mixed with slightly less than 
the calculated proper amount of limestone, which has been previously 
dried and powdered. Sufficient powdered slaked lime (say 2 to 6 per 
cent) is added to bring the mixture up to correct composition. The 
intimate mixture and final reduction are then accomplished in ball-and- 
tube mills. About 8 per cent of water is then added, and the slurry 
is made into bricks, which are dried and burned in a dome or chamber 
kiln. 
3. Slag is granulated and mixed, while still wet. with crushed lime- 
stone in proper proportions. This mixture is run through a rotan 
calciner, heated by waste kiln gases, in which the temperature is suffi- 
cient not only to dry the mixture, but also to partly powder it and to 
reduce most of the limestone to quicklime. The mixture is then pul- 
verized and fed into rotary kilns. 
Of the three general processes above described, the second is unsuited 
to American conditions. The first and third are adapted to the use of 
the rotary kiln. The third seems to be the most economical, and has 
given a remarkably low fuel consumption in practice, but so far has 
not been taken up in the United States. 
Certain points of manufacture peculiar to the use of mixtures of 
slag and limestone will now be described. 
Composition of flic slag. — The slags adapted to Portland-cement 
manufacture are of common occurrence in iron-producing districts. 
The more basic blast-furnace slags are best suited for this use. The 
slags utilized will generally run from 30 to 40 per cent lime, but the 
higher the lime the better. The presence of over 3 per cent of mag- 
nesia renders a slag unfit for Portland-cement material, and on this 
account slags from furnaces using dolomite (magnesian limestone) as 
a flux can not be used for cement manufacture. The presence of any 
