eckel.] PUZZOLAN CEMENTS. 363 
acquire sufficient momentum in its fall to solidity in a mass on the 
bottom of the trough. This method is in use at all slag-cement plants 
of the present day, being occasionally modified by the use, in addi- 
tion to the flowing stream of water in the trough, of a jet of water 
playing on the slag before it strikes the trough. 
Effects of 'granulating the slag. — The physical effect of causing hot 
slag to come in contact with cold water is to break the slag up into 
small, porous particles. As this materially aids in pulverizing the 
lag, it is probable that granulation would be practiced on this account 
alone. But as a matter of fact granulation has, in addition to its 
purely physical result, two important chemical effects. One is to 
make the slag, if it be of suitable chemical composition, energetically 
hydraulic; the other is to remove a portion of the sulphides (con- 
tained in the slag) in the form of hydrogen disulphide. 
Le Chatelier states that the hydraulic properties of granulated slag 
are due to the presence of a silico-alumino ferrite of calcium corre- 
sponding in composition to the formula 3CaO, A1 2 3 , 2Si0 2 . This 
compound appears also in Portland cements, but in them it is entirely 
inert, owing to the slow cooling it has undergone. When, however, 
as in the case of granulated slags, it is cooled with great suddenness, 
it becomes an important hydraulic agent. When so cooled "it is 
attackable by weak acids and also by alkalies. It combines particu- 
larly with hydrated lime in setting, and gives rise to silicates and 
ilurni nates of lime identical with those which are formed, by entirely 
different reactions, during the setting of Portland cement. It is upon 
this property that the manufacture of slag cements, which assumes 
daily greater importance, is based." 
DRYING THE SLAG. 
The slag as it is brought to the cement mill from the granulating 
tanks carries from 15 to over 40 per cent of water absorbed during 
granulation. As will be noted later (p. 366) attempts have been made 
to utilize this contained water in the slaking of the lime, but these 
attempts have hitherto proved unsuccessful. As the manufacture is 
at present conducted, therefore, the large percentage of water carried 
by the slag is of no service, and in order to get good results from the 
grinding machinery the water must be removed as completely as pos- 
sible before pulverization is attempted. 
Before the various types of dryers in use are described, a few words 
may be said on the general problem. The slag may carry, as above 
noted, from 15 to over 40 per cent of water, the percentage varying 
with the method of granulation, the fineness of grain, etc. As the slag 
must be reduced to extreme fineness, it is necessary that this moisture 
be reduced as much as possible. With a well-conducted rotary drier it 
