664 THE POPULAR SCIENCE MONTHLY. 



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 De- 

 partment 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 pro- 

 portions in which they are present, and their purity. 



In the manufacture of natural cement the raw material gen- 

 erally 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 uni- 

 form 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 re- 

 duced 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 harden- 

 ing 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 



