36. Slag Cement

Slag Cement is made by adding calcium hydrate to a granulated basic slag resulting from the manufacture of gray pig iron. The slag must be carefully selected as to its chemical composition; Professor Tetmajer having found by extended experiments that slags containing silica, alumina, and lime in the ratio 30 to 16 to 40 are best adapted to the purpose. As the molten slag runs from the blast furnace it is suddenly chilled by being run into water, or is partially disintegrated by being treated with a strong current of water, air, or steam. It is thus reduced to coarse particles resembling sand, or to a spongy or fibrous mass which, after drying, is readily ground to a fine powder. The process of chilling results in a certain chemico-physical change that renders the powder capable of combining more readily with the slaked lime which is subsequently added. Slag which has been allowed to cool slowly will not form an hydraulic product when mixed with the lime, although the chemical composition of the slag may be identical in the two cases. The lime is dipped into water, or treated with steam, until slaked to a fine dry powder, and is then added to the powdered slag in proportions of about one part of the former to three parts of the latter, this proportion depending upon the composition of the slag used. The powdered slag and lime are sifted, then mixed and reground together to an extreme fineness, thus insuring an intimate incorporation of the ingredients. Since there is no burning in the process, it is evident that the finished product is merely a mixture, not a chemical compound as is the case with Portland cement.

37. One of the largest mills for the manufacture of slag cement in the United States is conducted by the Illinois Steel Company, and the following description of the process is condensed from a statement of Mr. Jasper Whiting,1 manager of the cement department, and patentee of the process: Slag of the proper composition is chilled as it comes from the furnace by the action of a large stream of-cold water under high pressure. The slag is thereby broken up, about one-third of its sulphur is eliminated, and it is otherwise changed chemically. A sample of the slag thus granulated is mixed with a proportion of prepared lime, and ground in a small mill whereby actual slag cement is produced. If the tests upon this trial cement are satisfactory, the slag is dried and then ground, first in a Griffin mill and then in a tube mill, where it is mixed with the proper amount of prepared lime and the two materials ground and intimately mixed together. The resulting product is said to be so fine that but 4 per cent, is retained on a sieve having 200 meshes per linear inch. The lime is burned from a very pure limestone and stored in bins, beneath which are two screens of different mesh, the coarser at the top. A quantity of lime being drawn on the upper screen is slaked by the addition of water containing a small percentage of caustic soda. The lime passes through the two screens as it slakes and is then heated in a drier; the slaking being thus completed, the lime may be incorporated with the slag. The purpose of the caustic soda added in the above process is to render the cement quicker setting.

1 " Report of Board of Engineers on Steel Portland Cement," Appendix I.