P AC IS I, METAMORPHI8M OF PORTLAND CEMENT 



191 



TABLE 3 

 Permeability of 2-inch Cubes, Age 28 Days, Subjected to 80 Ibs. Pressure 



The marked decrease in permeability resulting from the use of finer 

 cement in mortar demonstrates that in impermeability, as in strength, 

 the finest particles are the most active factors. 



Mechanical agitation irlien irate r i* added. Increased working should 

 weaken a cement after a certain maximum point is passed. In order to 

 establish this point, the effect of prolonged working was investigated. 

 It was necessary to use a mix of fluid consistency, in which, for obvious 

 reasons, the final set would not under normal conditions take place dur- 

 ing the time over which the experiments were extended. 



Two grouts were employed: one in which cement was mixed with 30 

 per cent of its weight of water, and one in which an equal weight of 

 water was used. The different tests were run respectively for periods 

 from one minute to five hours, and they were mixed in a motor-driven 

 stirring machine of the type common in chemical laboratories. 



After the stated period of stirring, the grouts were poured into glass 

 tubes and kept in a damp closet for the twenty-eight-day period. Cylin- 

 ders exactly two diameters high were cut from the specimens and crushed 

 in the compressing machine, two cylinders being crushed for each period, 

 and the average of the cornpressive strengths being recorded. 



