STATE GEOLOGIST. ~ 191 
Some cement makers have adopted the tube mill for fine grinding. 
Though requiring more power, the tube mill can easily grind 250 barrels 
and has the advantage of doing away with the screening. It seems 
that its general adoption would be a progressive step in spite of 
the greater amount of power required, especially since it eliminates the 
troublesome attention required in the dressing of stones. The tube mill 
must invariably be preceded by an intermediate grinding machine, a dis- 
integrator or Kent mill. 
Variability of Product——In the nature of the case the rock cement 
makers labor under the great disadvantage of being compelled to work 
with natural materials whose composition cannot be controlled or regu- 
lated. If the rock happens to be of the right composition, all is well; if 
not, nothing can be done. It is idle to claim that these natural variations 
in composition can be neglected; for poor cement is liable to result, and 
will reach the market unless its character is discovered by testing. But 
the limits allowable are quite wide, owing to the pozzuolanic character, 
and some cements have never given any trouble. 
Improvement in Quality by Treatment,—Since our American ce- 
ment rocks are principally of the mono-silicate type (O.R.1:1) and fre- 
quently fall below this degree of basicity, and again since the prevailing 
reaction seems to be pozzuolanic in character, the idea is at once suggested 
that an addition of dry, hydrated lime might improve many of these 
cements, just as the hydraulicity of granulated furnace slag is increased 
by the addition of slaked lime. Following this line of reasoning, the 
writer made some experiments carried out by burning typical cement 
rocks obtained from the Rosendale, Louisville and Utica districts and 
erinding the calcined rock together with dry slaked lime in a ball-mill. 
The following results were obtained, the comparisons being made 
with the identical three samples of cement rock: 
E i Tensile pireneeh in “4 
| Kind of cement. pounds per sq. in. a 
=) 1: 1, 28 days. 5) 
Z jay 
1 Ground rock alone Ren ont aah Checked and swollen. 
NOs laplusio’/, limes. x. 230 (Burnt too long.) — 
3 Croumcdinockwmeeen cee 7 ABS 
t 
4} No.3 plus 5% lime..... | 200 
5 CrounaGrockaanee es a 100 
270 
6! No. 5 plus 5% lime..... 
The rock in each case had been overburned somewhat. But the 
increase in strength, due to the small amount of dry slaked lime, is very 
