44 CEMENT MATERIALS AND INDUSTRY. [bull. 24a 
As the chemical composition of the mixture can be more advanta- 
geously discussed after the other three subjects have been disposed of, 
it will therefore be taken up last. 
PREPARATION OF THE MIXTURE FOR THE KILN. 
In the preparation of the mixture for the kiln the raw materials 
must be reduced to a very line powder and intimately mixed. The raw 
materials are usually crushed more or less finely, then mixed, and then 
ground to powder. Two general methods of treatment, the dry and 
the wet, are in use at different plants. Unless the limy constituent 
of the mixture is a marl, already full of water, the dry method is 
almost invariably followed. In this the materials are kept in as dry a 
condition as possible throughout the entire process of crushing and 
mixing, and if they originally contained a little moisture they are dried 
before being powdered and mixed. In the wet method, on the other 
hand, the materials are powdered and mixed while in a very fluid state! 
the mixture containing 60 per cent or more of water. 
3 
PERCENTAGE OF WATER IN RAW MATERIALS. 
The percentage of water thus carried by the crude raw material w 
depend largely on the character of the material, partly on the meth 
of handling and storing it, and partly on weather conditions. 
In hard limestones, freshly quarried, the water will commonly 
range from one-half to 3 per cent, rarely reaching or exceeding the 
higher figure, except in the very wet quarries or during a rainy 
season. Such limestones, comparatively dry when quarried, are fre- 
quently sent to the grinding mills without artificial drying. 
Soft, chalky limestones, which absorb water very rapidly, usually 
contain not more than 5 per cent of water in dry weather, while pro- 
longed wet weather may necessitate the handling at the mill of mate- 
rial carrying as high as 15 to 20 per cent of water. 
The clays present much more complicated conditions. In addition 
to the hygroscopic or mechanically held water, there is also always a 
certain percentage of chemically combined water. The amount oil 
hygroscopic water will depend on the treatment and exposure of the 
clay, and may vary from 1 per cent in clays which have been stored I 
and air dried to as high as 30 per cent in fresh clays. The chemically 
combined water will depend largely on the composition of the clay., 
and may vary from 5 to 12 per cent. The hygroscopic or mechanically 
held water of clays can be driven off at a temperature of 212 ¥.. 
while the chemically combined water is lost only at a low red heat 
The total water, therefore, to be driven off from clays may range t'rou 
(3 to 12 per cent, depending on the weather, the drainage of the elaj 
pit, and the care taken to prevent unnecessary exposure of the excJ 
vated material to moisture. The average total amount of moistuJ 
will probably be about 15 per cent. 
