SPATS CHOLOEISM, 167 
There are several other mills on the market similar in principle to 
the Griffin mill, like the Huntington mill, the double pendulum mill using 
two rolls in place of the one of the Griffin mill, and otliers. 
As far as the grinding proper is concerned the efficiency of these 
mills is undoubtedly higher than that of the tube mill, but owing to the 
factor of centrifugal force they do not accomplish the thorough blending 
of the former. In fact since the expression for the centrifugal force 
Wv? 
G== 
gk. 
‘ involves the factor of the mass of the particles it is readily seen that there 
must be a tendency to segregation of unlike particles of different sizes and 
weights. In the manufacture of slag cement we blend two materials, slag 
and calcium hydrate, having the specific gravities 2.8 and 2.1 respectively, 
and it seems quite evident that the blending accomplished by the Griffin 
mill is bound to be inferior in quality as compared with the work of the 
tube mill. 
From the fine grinding machine the ground slag cement is con- 
veyed to the storage bins and is ready for shipment, since it is not neces- 
sary to “cure” slag cement as is the case with Portland cement. 
Testing of Slag Cements. —In first bringing out a slag cement, it 
is of great importance to subject it to the most rigid tests known to the 
manufacturer in order to determine, as far as it is possible to do so, its 
properties and ultimate behavior in use. ‘The tests employed are the 
same to which Portland cements are subjected, special attention being 
paid, however, to the tests for constancy in volume. Finely ground slag 
cement of the proper composition, containing not more than 1.3 per cent. 
of sulphur and 3 per cent. of magnesia will, as a rule, produce a sound 
cement. A peculiar color phenomenon is noticed in the hardening of 
slag cements in water, the change of the first bluish gray color to a pro- 
nounced green color, which, however, disappears in time. This is prob- 
ably due to the reaction: 
CaS+H,O0+FeO=FeS+Ca(OH).,. 
The sulphide gradually changes to calcium sulphate which, owing to 
its solubility, is brought to surface by water and appears as a white 
efflorescence. ‘The remaining calcium sulphide also oxidizes to calcium 
Sulpnaie, iormmhone CASS in wae simooe, “IS  welle olen ein 
important function as regards the durability of the cement, since on oxida- 
tion it suffers a decided increase in volume which tends to break up the 
hard mass of the cement. In water this effect of crystallization is less ap- 
parent, and hence all slag cements, especially those high in sulphur, are 
at a disadvantage when hardening in air or exposed to it after hardening, 
and show their best strength only in work under water, foundations, etc. 
They are unsuited for work exposed entirely to the air like in sidewalks. 
