Iron and steel slag, also known as ferrous slag, is produced by adding limestone (or dolomite), lime and silica sand to blast furnaces and steel furnaces to strip impurities from iron ore, scrap and other ferrous feed materials and to lower the heat requirements of the iron- and steelmaking processes. Ferrous slag forms as a dominantly calcium silicate melt that floats on top of the molten crude iron or steel; the slag is then removed from the liquid metal.
   For typical iron ore grades (60 to 66 percent iron), blast furnace slag output will be about 0.25 to 0.30 metric tons of slag per ton of crude iron produced. Steel furnace slag production, after removal of entrained metal, will be about 10 to 15 percent of the crude steel output.
   Owing to very low unit values (a few dollars per ton) for most slag compared with the value of crude iron and steel, steel companies generally arrange for outside contractors to remove, cool and process the slag — which involves crushing and screening it and retrieving entrained metal — to prepare it for sale. 
   The market uses of ferrous slag are determined mainly by the method used to cool the slag. Air-cooled blast furnace slag and steel slag are cooled under ambient conditions, resulting in hard, dense slag that is especially suitable for use as inexpensive aggregates. Granulated blast furnace slag is formed by quenching molten slag in water to form sand-sized glassy particles. Finely ground granulated slag is useful as a partial substitute for Portland cement in concrete and, as such, commands prices in excess of $60 per metric ton. Pelletized or expanded blast furnace slag is cooled using a water jet in order to generate steam and cause air-filled vesicles to form in the slag. The material is used mainly as a lightweight aggregate. Blast furnace slag can be remelted and recooled to form mineral wool, which is suitable for use as thermal insulation.