400 



REIBLING AND REYES. 



The increase in different volatile constituents of cement is graph- 

 ically shown by figure 12. 

































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Fig. 12. — Diagram showing the characteristic effects of thorough aeration on the 

 different volatile constituents of Portland cement. 



The greatest increase is in the percentage of carbon dioxide. The 

 figures in Tables IX, X, XI and XII confirm the microscopic evidence 

 that free lime in Portland cement exposed to the atmosphere changes to 

 the carbonate soon after hydrating. 



The microscopic evidence that the change of lime to carbonate tends 

 to make the free lime in cement more inert is confirmed by the decrease 

 in the rapidity of absorption of carbon dioxide and water and the gradual 

 increase in the percentage of free moisture. 



These facts are important by reason of their bearing upon the explana- 

 tion of the changes sometimes noted in the physical properties of cements 

 which have been stored in air-tight vessels, because active, free lime in 

 cement so preserved will tend to unite with the moisture which originally 

 was present, and the slaked lime so produced may have a different effect 

 upon the physical properties of' the cement than a corresponding amount 

 of sintered or nonsintered lime. Therefore, it is our experience that 

 Portland cements which have been stored while protected from the 

 atmosphere seldom contain a high percentage of moisture. Cements that 

 do are usually high in other volatile constituents and always show little 



