The most common form of slag is the waste from the blast furnaces where iron is being refined. Depending on the iron ore and the ingredients added, some of it used to be ground up and used as Portland cement.

Portland cement is manufactured from lime-bearing materials, usually limestone, together with clays, shales, or blast furnace slag containing alumina and silica, in the approximate proportions of 60 percent lime, 19 percent silica, and 8 percent alumina, 5 percent iron, 5 percent magnesia, and 3 percent sulfur trioxide. Some rocks, called cement rocks, are naturally composed of these elements in approximately suitable proportions and can be made into cement without the use of large quantities of other raw materials. In general, however, cement plants rely on mixed materials.



"Cement," Microsoft(R) Encarta(R) 98 Encyclopedia. (c) 1993-1997 Microsoft Corporation. All rights reserved.