PROPERTIES OF PORTLAND CEMENT. PART III. ^15 



in this manner. Such alterations have been reported by several other 

 investigators, but in each instance no mention is made of the amount 

 of uncombined water which was present in the cement before and after 

 the changes had occurred. 



The results obtained with cements stored in coverless cans and allowed 

 to remain undisturbed until tested, correspond -nith the information 

 recorded in Tables IX, X, and XI; the cement near the top proved 

 to be essentially different from that taken 25 millimeters or more below 

 the surface. Mixtures of cement seasoned in this manner sometimes 

 developed a considerable variation in physical properties, but it is obvious 

 that figures obtained from such mixed material are too complicated for 

 analytical treatment. 



A careful consideration of the character of the absorption curves 

 given in figures 13 and 13 shows that this difficulty can be overcome 

 to a large extent by aerating the cement in large, shallow pans. This 

 method also prevented the caking of the cement. The plastic and set- 

 ting properties of the cement treated in this manner changed decidedly, 

 and eventually became abnormal, but the alterations in this respect 

 followed no apparent rule. Some normal cements soon became decid- 

 edly quick setting. A few of these promptly again acquired a slow 

 set, but the majority remained quick setting until the cement became 

 almost inert from prolonged aeration. Others at first set more slowly 

 and then more rapidly. One sample of brand showed four distinct 

 variations. Its set changed fii'st from slow to quick, then to slow, again 

 to quick, and finall}^, back to slow. Eventually, after prolonged aera- 

 tion, all of the materials became so inert that they set very slowly. 

 This was especially true of that which had been finely ground. Such 

 cement at times failed to harden perceptibly within a week or more. 

 Then again, the set of some samples remained more constant than that 

 of others; so that an investigation was made to ascertain whether the 

 chemical composition had any bearing upon the stability of the latter. 



These results and the work already discussed in Parts I and II of 

 this paper, make it evident that changes in the physical properties of 

 Portland cement are due primarily to the absorption of water or water 

 and carbon dioxide. Therefore, the influences of chemical composition 

 presumably could definitely be ascertained, if the changes in the char- 

 acter of the set of a cement which is undergoing thorough aeriation were 

 to be considered carefully, in conjunction with the amount of water and 

 carbon dioxide absorbed, the fineness of grinding, and the amount of 

 retarder employed. 



A few examples recorded in Table XXI, demonstrated that the char- 

 acter and the stability of the set were independent of the ultimate chem- 

 ical composition, the fineness and the amount of retarder. 



