bckel.] PTJZZOLAN CEMENTS. 367 
cement, but, on the other hand, they can not do as much good as an 
equal amount of lime. The third kind of material that may be present 
in lump form consists of fragments of well-burned lime which, 
through accident or carelessness, have not been well slaked. These 
umps of quicklime would, if incorporated in the cement, be actively 
injurious. 
The preceding description and discussion of the three classes of 
material which are likely to remain as lumps in the slaked lime has 
3een intentionally made detailed, in order to point out an error in 
practice committed occasionally at slag-cement plants. It has been 
*een that the materials composing these lumps are of such a char- 
icter as to be either useless or actively injurious if used in a slag 
jement. It should be obvious, therefore, that the only rational 
method of treatment is to sieve the slaked lime and to reject entirely 
ill the material failing to pass through the sieve. This is the best 
Dractice and the method usually followed. Occasionally, however, 
irged by a false idea of economy or by inaccurate reasoning, the 
manufacturer saves the material failing to pass the sieve, crushes it, 
tnd adds it to the cement at a later stage in the manufacture. 
MIXING AND GRINDING THE SLAG AND LIME. 
Prost, in consequence of his experiments with various proportions 
)f lime, advocated the proportion, to secure the best results, of from 
55 to 40 parts of lime to 100 parts of slag. He also stated that the 
imounts of lime used in actual practice for each 100 pounds of slag 
were: At Choindez, 40 to 45 pounds; at Donjeux, 40 pounds; at Bruns- 
wick, 33 pounds, and at Cleveland, 33 pounds. Mahon, in reporting 
lis experiments for the Maryland Steel Company, states that the best 
results were secured by the use of 25 parts of lime to 100 parts of slag, 
>y weight. In the manufacture of slag brick, which is in reality 
merely a branch of the slag-cement industry, the amount of lime added 
may fall as low as 10 pounds to 100 pounds of slag. 
In actual American practice the proportions are usually about 20 
founds lime to 100 pounds slag. This difference in proportions 
between the American and European plants corresponds to a difference 
in the composition of the slags used, for in this country the slags 
employed in slag-cement manufacture are usually somewhat higher in 
lime than are the slags used at European plants. 
The greatest differences in practice exist in the processes for grind- 
ing and mixing the slag and lime. The statement has been made in 
beveral publications that the differences in hardness between dry 
granulated slag and slaked lime is so great that it is impracticable to 
pulverize them together in a continuously operated mill. A number 
of plants, therefore, have installed small discontinuous mills, each of 
