12 CEMENTING POWER OF ROAD MATERIALS. 



plasticity, but the binding power, it was suspected that the prolonged 

 action of water on fine rock dust might well increase the binding 

 power at least up to some maximum value. Investigations were at 

 once begun to see if the underlying cause of the cementing power 

 could be discovered. Some of the results of this research have already 

 been published a from this laboratory. It is sufficient to say here that 

 the conclusion was reached that the rock powders which have binding 

 power contain a certain proportion of particles which are colloid in 

 their nature. Under the action of water the particles to some extent 

 soften and become adherent. On drying out, this adherent bond locks 

 the particles together. In the case of clays this bond is usually very 

 strong, while with rock dust it is extremely variable, ranging from 

 those which yield briquettes that break on the first blow to those 

 which will sustain hundreds without failure. 



After the observations had been made which led to the conclusions 

 outlined above it became evident that a modification was necessary in 

 the manner of making the briquettes for the cementation test. Since 

 the bond is developed under the prolonged action of water, it is man- 

 ifestly unfair to wet the rock dust and immediately mold the test 

 pieces. Moreover, as the action of water begins at once, and accord- 

 ing to the laws of physical chemistry is more rapid at first, slowing 

 down gradually as the point of completion of the reaction is approached, 

 it is apparent that considerable errors in the results may be occasioned 

 by small variations in time after the wetting of the dust takes place 

 until the briquette is molded. It will also be shown that with nearly 

 all materials the amount of working or kneading which is done upon 

 the wet mass will effect the softening of the particles, and hence the 

 strength of the bond. These variations were sufficient^ large in some 

 cases to account for the anomalous results in the cementation and 

 recementation tests; hence the recementation tests were immediately 

 given up as being of no value, and methods were devised by which 

 the cementation test was improved. 



The finely ground and sifted rock dusts were mixed with a sufficient 

 quantity of water, and were worked and kneaded until an even con- 

 sistency of stiff dough was obtained. From these doughs series of' 

 briquettes were made at intervals and tested. The effects produced by 

 age on doughs of three t} r pical specimens, including samples of good, 

 medium, and poor cementing values, are given in the curves shown in 

 figure 6. A study of these curves shows that there is invariably a rapid 

 rise in binding power in the newly made doughs, and that after the 

 lapse of twenty- four hours the value has, for the purpose of the test, 

 become practically constant. This effect is, of course, more apparent 

 in materials of high cementing value than in lower ones. Numerous 

 experiments have shown that with powders, such as fine sand, which 



J. Amer. Chem. Soc., 25: 5. 



