The preparation of a true Portland cement from blast furnace slag has been followed in Germany and elsewhere in Europe for several years, and recently has been introduced in the United States. As this process utilizes a waste product, its popularity is likely to increase. Whereas, for the manufacture of slag cement only the slag from gray pig iron is available, it is found that in most cases the slag from white pig iron may be used for the production of Portland cement from slag.
The method of manufacture is briefly as follows: The slag as it comes from the blast furnace is subjected to the action of a stream of water, which granulates it and changes it chemically, the water combining with the calcium sulphide, which is injurious to cement, to form lime and sulphuretted hydrogen.
1 Report of Mr. A. O. Powell, Asst. Engineer, Report Chief of Engineers, U.S. A., 1900, p. 2779.
The granulated slag is then dried, mixed with the correct proportion of dried limestone, and ground to extreme fineness. The mixture is next burned in rotary kilns, the remainder of the process being the same as that employed when ordinary raw materials are used. While a cement made from slag by this method may have some peculiarities due to the nature of the raw materials used, and should be very carefully tested before it is used in important work, it should not be confounded with slag cement, which is a mixture of granulated slag and hydrated lime subsequently ground, but not burned together.
The Michigan Alkali Company has installed at Wyandotte, Mich., a cement plant to utilize the large amount of limestone which they have as waste in the manufacture of soda products. The limestone which has served its purpose in the soda manufacture is in a finely divided and semi-fluid state; to this is added the proper percentage of clay, which has been dried and pulverized. The two are then very thoroughly mixed and ground by pug mills and tube mills, the mix corrected by proper combination of over-clayed and over-limed slurry, and finally burned in rotary kilns.