is rejected and the varied masses selected are carefully mixed so as to ensure a uniform grade. The second is the difficulty of burning the rock in kilns and in large masses so uniformly as to ensure complete burning and no considerable amount of overburning. If natural cement rock is overburned or fused, it becomes a slag, and loses its hydraulic properties. It is not surprising, therefore, that a considerable lack of uniformity exists in the quality of the natural cement found upon the market. The best of them contain a considerable amount of impurity, or material that is not cement, that exists in the rock before it is burned, and also a considerable amount of unburned rock, which together serve to dilute the cement proper, as if a certain amount of sand had already been added to the cement before it is used. These impurities that are inherent in the nature of the materials from which the natural cement is made and also in the process of manufacture that is of necessity followed, result in a cement that can be made and sold at a less price than Portland cement and that is inferior to it for many purposes, while, on the other hand, for a great many purposes natural cements have been found to answer every requirement and are made and used in enormous quantities.
For Portland cements, either a very pure natural limestone or marl is selected and brought into a very finely pulverized condition. Lime- stones are selected as free as possible from every impurity except clay. ]Magnesia is never absent, and at best is an inert impurity, but the amount present should not exceed five per cent. Marl is frequently used and is generally purer than limestone. In England chalk is gen- erally used. In Germany chalk and a limestone, locally known as 'mergel,' which is soft and contains clay, are employed.
The following table, No. I., taken from 'Cement Industry,' page 1 2, gives the composition of the carbonate of lime in use in some of the leading Portland cement manufactories in the United States: .
|Limestones and Marls.||Chalk,
burg, N. J.
The clay should be highly siliceous, but should be free from grains of sand. Clays containing carbonate of lime or marl are softer and more easily mixed with the other materials. Clays containing 70%, or more, of silica stand firing without fusing, produce a clinker that