174 



REIBMNG AND SALINGER. 



Portland cement, even though it does not develop quick-setting jJi'operties 

 by this exposure. Table XXIV gives a typical example : 



Table XXIV. — January 22, 1907 — mortar, 1 to 3. 

 [Specific gravity, 3.11.] 



Average _ 

 Highest _ 



7-(iay. 



171 

 195 



28-day. 



227 

 250 



Number 

 of bri- 

 quettes 

 broken. 



12 

 12 



This same cement stored in coarse canvas cloth l:)ags, twenty days and 

 three months longer, gave the following results : 



[Specific gravity, 3.03 after 3 months.] 





7-day. 



28-day. 



Number 

 of bri- 

 quettes 

 brolten. 



After 20 days: 



147 



165 



135 



146 



212 

 230 



200 

 211 



12 

 12 



16 

 16 



Highest 



After 3 months: 



Highest . ' - 





From the nature of things this loss in tensile strength is not difficult 

 to explain. It is generally understood that all cements are improved by 

 storage, but it has been proved that this is only true of those cements 

 which are either too high in lime or underburned. Aeration renders 

 part of the excess or free lime inert because of the formation of the 

 carbonate of calcium and also slakes some of it by tlie absorption of 

 water. Thus, the cause of unsoundness is removed in time, and the 

 cement is gradually improved in this respect. But "the higher in lime 

 a cement is the greater its strength is kno^^oi to be if thoroughly 

 burned," -" and "the maximum of liine is usually controlled by the 

 soimdness tests." '"' Therefore, if a cement is soimd it does not contain 

 excess or free lime and the carbonization of the lime in a sound cement 

 should reduce its tensile strength, as it lowers the percentage of active 

 lime, the carbonate of calcium being inert. 



With all due respect for the great value of Meade's work, we take 

 exception to a portion of his assertions relative to the specific gravity. 



^ Eny. News (1905), 53, 84. 

 "'Cheiii. Eng. (1907),- 5, 343. 



