Page:Popular Science Monthly Volume 38.djvu/682

664 American cements, both natural and artificial, is the excuse of the writers for the following article. The diagrams and tables given have been compiled from results obtained in an extended course of cement tests now in progress in the Engineering Department of the State University of Iowa.

Cements, such as are used for constructive purposes, may be divided into two general classes, natural and artificial. The essential ingredients, carbonate of lime, silica, and alumina, are the same in both classes, the principal difference being the proportions in which they are present, and their purity.

In the manufacture of natural cement the raw material generally used is some stone in which the carbonate of lime, silica, and alumina are present in more or less correct proportions, while in the manufacture of the artificial cement the raw material used consists of the essential ingredients, each in a comparatively pure state, thoroughly mixed in theoretically the correct proportions. It is due to this fact that artificial or Portland cement is not only much superior to natural cements, but that it is much more uniform in its quality. This feature of uniformity is perhaps the most valuable possessed by Portland cements, and one which can never be attained in the manufacture of natural cements.

The term Portland cement is now generally used to designate artificial cement, from the fact that the first artificial cement made in England, when hardened, resembled the famous Portland building-stone.

Whether the mixture of the necessary ingredients is artificial or not, it is burned almost to the point of vitrification and then ground to an extreme fineness. The fineness to which cement is ground is one of the most important points in its manufacture, for the reason that, if not finely ground, its strength may be reduced fifty or seventy-five per cent. The theory advanced by Prof. Griffin, on page 254, in regard to the setting of cement, namely, the absorption of carbon dioxide, the uniting of this gas with the lime, and the reforming of lime-stone, is simply the old lime-mortar theory, and in no way applies to the setting of cement. In regard to the changes that do take place during the setting of cement, the following quotations from an article upon the subject by Dr. L. W. Andrews and F. W. Spanutius, in The Transit for December, afford the clearest explanations:

"The setting of a cement is, in general, a complex process, partly chemical in its nature, partly mechanical. Broadly stated, the chemical changes which occur may be said rather to afford opportunity for the mechanical changes which result in hardening than themselves to cause the hardening. The chemical changes are, therefore, susceptible of wide variation without materially influencing the result. . . . In some cements, of which