< KMENTATION TKST. 4 J I 



CKMI.N i A i ION TKST. " 



The binding or cementing power of rock dust is such an important 

 element, in road building that much time has been spent in the endeavor 

 to devise a suitable test for determining the degree to which the vari- 

 ous rocks and gravels possess this property. Many tests have been 

 tried, but as yet only an impact test, carried on in a uniform manner 

 as described below, has given satisfactory results. 



One kilogram of the rock to be tested is broken sufficiently small 

 to pass a 5 mm but not a 1 mm mesh screen. It is then placed in a 

 ball mill and is ground for two hours and a half. This ball mill. 





FIG. 3. Ball mill. 



shown in fig. 3, contains two chilled iron balls which weigh 25 pounds 

 each, and is revolved at the rate of 2,000 revolutions per hour. It 

 was found by experiment that grinding rock thus prepared for two 

 hours and a half was sufficient to reduce it to a powder that would 

 pass through a 0.25 mm mesh. The dust thus obtained is mixed with 

 water to about the consistency of a stiff dough, and is kept in a closed 



f 'This test, and the necessary machines lor conducting it, were designed and devel- 

 oped by the writer for the Massachusetts Highway Commission and the K<>ad Mate- 

 rial Laboratory. The impact machine at present used was Imilt especially for this 

 laboratory by the Maryland ( Jeological Survey, under the direction of Mr. A. \. 

 Johnson, highway engineer to the survey, who made several useful modifications in 

 the machines. 



