50 CEMENT MATERIALS AND INDUSTRY. [bill. 24,3. 
Advantages of using slag-limestone mixtures, — The manufacture of a 
true Portland cement from a mixture of slag and limestone presents 
certain undoubted advantages over the use of any other raw materials, 
while it has also a few disadvantages. 
Probably the greatest advantage is in the fact that the most impor- 
tant raw material — the slag — can usually be obtained more cheaply 
than an equal amount of rock could be quarried or mined. The slag- 
is a waste product, which is hard to dispose of and may be obtained 
at small expense to the cement plant. Another advantage is due to 
the occurrence of the lime as oxide, and not as carbonate. The heat 
necessary to drive off the carbon dioxide from an equivalent mass of 
limestone is, therefore, saved when slag forms part of the cement 
mixture, and very low fuel consumption is obtained when slag-lime- 
stone mixture is burned. 
Of the disadvantages, the toughness of the slag and the necessity 
for drying it before grinding are probably the most important. A 
third disadvantage, not always apparent at first, is the difficulty of 
procuring a proper supply of suitable slag. Unless the cement plant 
is closely connected in ownership with the furnaces from which its | 
slag supply is to be obtained this may become very serious. When J 
there is a good market for iron the furnace manager will naturally 1 
give little thought to the question of supplying slag to an independent 1 
cement plant. :l]] 
The advantages of the mixture, however, seem to outweigh its dis- J 
advantages, for the manufacture of Portland cement from slag is now I 
a large and growing industry in both Europe and America. In this | 
country two Portland-cement plants have used slag and limestone as | 
raw materials for some time; several others are in course of con- 1 
struction, and it seems probable that in the near future Alabama will 
join Illinois and Pennsylvania as an important producer of Portland i 
cement from slag. 
WET METHODS OF GRINDING AND MIXING. 
Wet methods of preparing Portland-cement mixtures date back to 
the time when millstones and similar crude grinding contrivances 
were in use. With such imperfect machinery it was almost impossible 
to grind dry materials line enough to give a good Portland-cement 
material. In this country the advent of good grinding machinery has 
practically driven out wet methods of manufacture, except in dealing 
with materials such as marls, which naturally carry a large percentage 
of water. Two plants in the United States add water to a limestone 
clay mixture, but the effect of this practice on the cost sheets of these 
remarkable plants can hardly be encouraging. »j 
The location, physical condition, and chemical composition of tin 
marls and clays used have important effects upon the cost of the we 
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