CHAPTERS ON THE CEMENT INDUSTRY 881 



in comparatively few cases. This is in accord with the present 

 trend of engineering practice. It would seem desirable, however, 

 that tests should be made with mortar briquets at longer time 

 periods than those given. Tests at three months and a year on a 

 few briquets from each lot would seem to be of sufficient value to 

 pay for the extra trouble incurred in carrying them out. They 

 would not, it is true, be of any service in deciding whether any 

 particular lot should be accepted or not, but they would certainly 

 give information regarding the staying qualities of each brand, 

 which would be of use in decisions regarding future shipments of 

 that brand. 



Another test which it would seem might be profitably intro- 

 duced, is the determination of the specific gravity of the cements. 

 While there are undoubtedly good Portlands which at times fail 

 to attain a specific gravity of 3.10, and while natural cements 

 and slag cements may occasionally reach that figure, the test is 

 still a good rough means of discriminating the two classes. That 

 such a method of discrimination is not absolutely unnecessary, is 

 shown by the fact that several brands, submitted as Portlands, 

 behaved in a manner which made it probable that they were really 

 natural cements, more or less carefully treated. The physical 

 properties and methods of manufacture of such doubtful mixtures 

 seem to be of some interest, and I hope soon to discuss them 

 more fully in another place. I am not alluding here to the " im- 

 proved " cements, made by some manufacturers of natural cements, 

 and sold entirely on their merits, but to so called Portlands man- 

 ufactured in much the same manner as the grappier class described 

 by Le Chatelier. 



Certain brands occasionally (and others habitually) attain in 

 seven days a strength which enables them to pass both the seven 

 day and 28 day requirements, even though the actual increase in 

 strength during the additional three weeks may be slight. In 

 some extreme cases, indeed, cements have actually shown less 

 strength at 28 days than at seven days. Whether this condition 



