Page:Popular Science Monthly Volume 60.djvu/160

152 is 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 'Cement Industry,' page 13, gives the composition of the clays in use in several Portland cement manufactories:

A large part of the Portland cement manufactured in the United States is made from natural cement rock, that is, from a rock that contains 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 substances, 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 properties of different 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 These ratios no longer represent the composition of Portland cements as with improved methods of manufacture the proportion of lime has steadily increased, until the 'hydraulic modulus' is no longer applicable to the varying conditions and materials under which the cements are now manufactured.