PORTLAND CEMENT TESTING. 183 



affected the equilibrium maintained in the earl}' setting properties by the 

 opposing forces of the aluminates and the lime not in combination with 

 silica. Therefore, a loss in tlie activity of this lime representing 0.75 per 

 cent of the total cement of sample number 8 affects this equilibrium to a 

 degree many times greater than if the silicates would need to be taken 

 into consideration. 



Synthetic experiments also show this same phenomenon. "If much more than 

 10 per cent alumina is present the cement is almost sure to be quick setting 

 even with the addition of sulphates." " "When cement treated with sulphate of 

 lime has regained quick set, it may again be made slow set by addition of a 

 small quantity of lime." "^ 



Our belief that this cement is not of good quality is also supported by 

 universal experience. "We have already stated that this class of cements 

 gives satisfactory tests when the samples are comparatively fresh, but fails 

 to do so after seasoning. It will be noted that the percentage of aluniina 

 and silica in sample numljer 8 satisfies the limits of E. K. Meade's formula 

 for "freshly made American Portland cements which pass standard spec- 

 ifications for soundness, setting time, and tensile strength," ""^ namely : 



Per cent. 

 Silica 20-24 



Alumina 5- 9 



Iron oxide 2- 4 



Lime 60-63.5 



Sulphur trio.xide 1- 2 



However, they do not fall within the limits of Le Chatelier's formula 

 for "the limits of the amount of material usually present in good com- 

 mercial (therefore seasoned) Portland cement,'*" that is: 







Per cent. 



Silica 





21.0-24 



Alumina 





6-8 



Iron oxide 





2-4 



Lime 





60 -65 



Magnesia 





0.5- 2 



Sulphur trioxide 





0.5- 1.5 



Water and carbonic 



acid 



1 - 3 



The percentage of sulphur trioxide is also lower than that given by 

 both authors; and the loss on ignition is greater than that given by the 

 formula which considers it. 



Furthermore, Meade states that "cements should contain at least 2.5 

 times as much silica as alumina. Cements containing less than this 

 amount are apt to be quick setting or else to become quick setting on 



"Meade: Chem. Eng. (1907), 5, 345. 

 ■'Ubid., 349. 



"The Chem. Eng. (1907), 5, 349. 

 "Trans. Am. Inst. Min. Eng. (1893). 



