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POPULAR SCIENCE MONTHLY. 



i? easy to grind and yield a cement that sets slowly and gains strength 

 over a long period. On the contrary, highly aluminous clays give a 

 fusible clinker and quick-setting cement. A high authority states that 

 the percentage of silica in the clay should be three times the percentages 

 of the alumina and iron taken together. The less iron the clay and 

 limestone contains the lighter colored will be the cement. 



The following table, No. II., also taken from Tement Industry,' 

 luige 13, gives the composition of the clays in use in several Portland 

 cement manufactories : 



Table II. 



A large part of the Portland cement manufactured in the United 

 States is made from natural cement rock, that is, from a rock that con- 

 tains both the carbonate of lime and clay, very intimately mixed in a 

 natural rock. The best cements, however, are made from an artificial 

 mixture of either limestone or marl and clay. The proportions are 

 determined after very careful chemical analysis in such manner that 

 the several ingredients that form cement shall not only be free from 

 harmful sub'stances, but shall combine to produce theoretical chemical 

 compounds in certain quantitative relations. 



Although much has been written for many years concerning the 

 chemistry of hydraulic cements, it is only within about 25 years that 

 researches have been conducted in such a manner that by constructing 

 the compounds possessing hydraulic properties from pure elementary 

 materials, much light has been thrown upon the problem. The French 

 chemist Vicat suggested an 'hydraulic index' to designate the hydraulic 

 ])roperties of diflierent cements, which is a figure representing the 

 number of parts of silica and alumina combined with 100 parts of lime 

 and magnesia. 



In 1872 Erdmenger showed that in commercial Portland cements 

 the ratio of lime to the acid constituents, silica, alumina and iron 

 oxide taken together, averaged 1.9. At about the same date, Michaelis 

 determined the ratio, as between 1.8 and 2.2, and called it the 'hydraulic 

 modulus.' These ratios no longer represent the composition of Port- 

 land cements as with improved methods of manufacture the proportion 

 of lime has steadily increased, until the 'hydraulic modulus' is no 

 longer applicable to the varying conditions and materials under which 

 the cements are now manufactured. 



