By-product formed in smelting, welding, and other metallurgical and combustion processes from impurities in the metals or ores being treated. Slag consists mostly of mixed oxides of elements such as silicon, sulfur, phosphorus, and aluminum; ash; and products formed in their reactions with furnace linings and fluxing substances such as limestone. During smelting or refining, slag floats on the surface of the molten metal, protecting it from oxidation (see oxidation-reduction) by the atmosphere and keeping it clean. Slag cools into a coarse aggregate used in certain concretes; it is used as a road-building material, as ballast, and as a source of available phosphate fertilizer.
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