PROPERTIES OF PORTLAND CEMENT. PART III. 213 



At times the cement was also gauged by different methods, of mixing and by 

 various amounts of water. Owing to the heat usTially liberated by setting 

 cements, the practice of storing the test specimens in one compartment of a 

 moist air closet is very liable to cause abnormal results. To avoid this risk, we 

 placed the gauged cement in isolated compartments especially constructed for 

 this purpose. 



Experiments which produced important results were repeated from 

 time to time in order to insure their freedom from personal error and 

 from the effect of slight cha;Qges in atmospheric conditions. Unless 

 otherwise stated, only those results which were free from slight changes 

 in the working conditions are recorded. 



GENERAL OBSERVATIONS CONCERNING CHANGES IN THE RATE OF SET. 



A series of experiments showing remarkable changes which occurred 

 in the setting properties of certain cements after a few days of exposure 

 to the atmosphere have been published from this laboratory *^ and in 

 the same paper it was shown that the set of other cements, aged in the 

 same manner and at the same time, and of the same cements stored in 

 tightly covered cans, remained practically constant. 



These experiments led us to believe that a similar, but a more thorough 

 study of the behavior of many brands of the commercial product might 

 lead to definite conclusions. Accordingly, all of the cements of Table 

 A *'' were aged in three ways. They were stored in sealed bottles, in 

 open, coverless cans and in shallow pans,*® from time to time changes 

 in the plastic or setting properties were carefully noted. 



As this work progressed it soon became apparent that the variations 

 primarily depended on the manner in which the cement had been 

 stored. In this respect the data in Table XX are characteristic of the 

 whole series of experiments. 



Little or no change is shown in the setting of the cements kept in 

 sealed bottles and in no instance did a set change from quick to slow, 

 or vice - versa. Experiments in the cement laboratory of the Bureau 

 of Science demonstrate that only cements which contain considerable 

 amounts of free moisture are apt to be changed materially when aged 



"rWs Journal, Sec. A (1908), 3, 137-186. 



"Ibid. (1910), 5, 414. 



* In the latter case about 30 kilograms of cement was spread over the bottom 

 of a pan in a layer about 6 centimeters deep, the cement being thoroughly mixed 

 twice each day for three days and once daily thereafter. 



