PORTLAND CEMENT TESTING. 181 



in strength takes place after twenty-eight clays and instances are on 

 record where the strength of the briquettes weakened after three months. 

 The fifty samples illustrated in diagrams numbered 1 and 2 show only 

 an average gain (seven to twenty-eight days) by the testers, of 16.45 

 per cent for neat and 48.9 per cent for sand briquettes, whereas the 

 increase desired by the Army specifications is at least 20 and 57 per 

 cent, respectively (Table I). "Cement giving high early strength is to 

 be relied upon only in so far as it has been shown by experience that it 

 is capable of maintaining such strength." ''^ 



The fact that the early strength of this class of cement can not always 

 be relied upon is probably due to its nonuniformity in burning. Owing 

 to the fusibility of the calcium aluminate, which causes balling-up and 

 sticking together in the hot zone of the kiln,''^ thus preventing uniform 

 burning, cements high in alumina are apt to be very erratic in the 

 stability of their compounds. As a result the rapidity with which they 

 rmite with water and carbonic acid when exposed to the atmosphere 

 varies. The relative rapidity of the absorption of carbon dioxide and 

 water by cements under similar conditions would therefore indicate the 

 relative degree of low burning. 



The most important characteristic of a high-alumina cement and the 

 one that needs the most consideration is its susceptibility to become quick 

 setting by exposure to the air. It has been our universal experience 

 that Portland cements of this class containing more than 8.5 per cent of 

 alumina always gave satisfactory results if they are tested before they 

 have combined with more than 2 per cent of water and carbonic acid; 

 and that when they had combined with more than 3 per cent of volatile 

 constituents they failed to meet the setting and tensile strength require- 

 ments. 



It would seem as if there is something radically wrong with a cement 

 that will not withstand atmospheric exposure to such a slight extent 

 without developing dangerous properties, and such a cement should be 

 rejected for use, especially in this climate. A typical example, sample 

 No. 8 as recorded in Tables VII and VIII, will suffice to illustrate this. 



It is difficult perhaps to realize why such a slight difference in volatile 

 constituents should so change the quality of a cement ; and that the same 

 cement which at first set in one hour and thirty minutes (loss on igni- 

 tion=2.63 per cent) should, after a little more aeration develop such 

 rapid setting properties, and set in twenty-three minutes (loss on igni- 

 tion=3.92 per cent). 



The combination of Portland cement with water and carbonic acid 

 absorbed from the air is represented for all purposes of discussion by the 



'^Spalding, Frederick C: Ibid., 88. 

 « Meade: Chem. Eng. (1907), 5, 345. 

 71978 ,6* 



