14 CEMENTING POWER OF ROAD MATERIALS. 



Southwestern States, notably Alabama, occur large deposits of chert 

 and chert}- gravels which bind well and make excellent roads. In the 

 laboratory these materials gave low cementing values by the older 

 method of procedure; when, however, the chert dust was soaked and 

 kneaded the binding power rose to the values which would be expected. 

 It is well known in practical road building that some materials which 

 do not consolidate readily do improve with age upon the road, and 

 wet rolling, where possible, is the invariable remedy for the difficulty. 

 The results obtained by the old and new methods are given in Table II. 



TABLE II. Cementing values of samples of chert by the old and new methods. 



An investigation recently taken up and at present in progress 

 further illustrates the importance of the wet rolling of roads and ma} 7 

 ultimately be shown to have a direct bearing on the cementation test. 

 It consists in grinding the material with water in the ball mill and 

 immediately molding it into briquettes. Up to the present time all 

 rocks, even quartzite, show a cementing value, and in almost all cases 

 results are higher than by the dry method. At this writing, however, 

 the number of results is too small to be correlated, and no general 

 conclusions can be drawn. This investigation will be continued and 

 the results reserved for a future publication. 



CAUSES OF CEMENTING POWER. 



As was outlined in the introduction, the whole question of the bind- 

 ing power of rock dust was early recognized as being closely associ- 

 ated with the same property in clays. In fact, considered from the 

 standpoint of road materials, it is difficult to determine where the 

 classification as clay should stop; that is to say, clays pass imper- 

 ceptibly into gravels. Some gravels which contain a certain propor- 

 tion of clay base will be found to bind, while a clean quartz gravel 

 absolutely lacks this property. While this is easily understood, it 

 does not, on first examination, seem to have any bearing upon the 

 great difference in binding power which is exhibited by clean, deep- 



