PROPERTIES OF PORTLAND CEMENT. PART III. 211 



covered bins and consequently the lime could have had very little op- 

 portunity to combine with carbon dioxide, because our previous work 

 shows that only the exposed surface of packed cement is able to do so. 

 FurtheiTQore, it requires very thorough and prolonged aeration to change 

 all of the slaked lime to calcium carbonate in the commercial products 

 usually received. 



The influences given under 7 and 8, therefore, alone remain to be 

 discussed. In carrying on this work we continued our study of the 

 test for free lime ** and by this means we have found that all changes 

 in the setting properties of Portland cement can readily be accounted 

 for. 



Our experiments on this subject have also furnished some instructive 

 data concerning the plastic and other physical properties of Portland 

 cement. We believe that they show conclusively that in the majority 

 of instances the setting properties of extremely finely ground cements 

 may be as easily controlled as when the same material is coarsely ground ; 

 and that they furnish an explanation of the beneficial effects derived 

 from grinding Portland cement in the presence of live steam. We also 

 believe that these experiments will enable the manufacturer so to control 

 the setting properties of his cement that it will remain noi-mal under all 

 ordinary conditions of storage. Unfortunately, the work which follows, 

 although presented as an abstract of the results obtained, is necessarily 

 somewhat lengthy because of the complicated nature of the subject imder 

 discussion. 



THE METHOD OF ASCERTAINING THE SETTING PROPERTIES. 



Throughout our work, several standard methods were employed for 

 determining the time of the initial and final set. The method em- 

 ploying the Vicat needle as adopted by the American Society for 

 Testing Materials was found to be the most consistent with the manner 

 in which cement is used in actiial work. It is reliable, impartial and 

 accurate. 



However, the method of the Vicat needle, like all others, has met with severe 

 criticism. For instance, Russel S. Greenman " states that the percentage of water 

 required to produce a paste of normal consistency varies from 18 to 28 per cent; 

 that the results obtained are as varied as could be imagined; that some cements 

 frequently fail to produce a required penetration of 10 millimeters unless the 

 cement is flooded with water; and so forth. 



Although our experience has often been similar to that of Green- 

 man, we believe his criticism should be applied to the cement itself, 



"This Journal, Sec. A (1910), 5, 367. 

 " Proc. Am. Soc. Test. Materiah (1906), 6, 355. 

 102594 5 



