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USGS Pittsburgh East Quadrangle Low Altitude Aerial Photo 1938 Aerial Photograph 1956 Aerial Photograph 1995 Aerial Photograph
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What is slag?
Slag is a byproduct of the melt when converting iron ore or scrap steel into molten steel. Pulverized limestone is added to the melt to provide an additional source of carbon monoxide and to bind to the silicates present to prevent the formation of iron silicates. The slag with its lower density is skimmed off the top of the ladle, transferred to its own car and either poured in a waste area in molten form or after it has cooled and solidified. See diagram. Slag is formed by nucleation, with a core of calcium oxide surrounded by a layer of calcium hydroxide and a outer layer of calcium carbonate. See diagram. Other elemental concentrations vary depending on the furnace and process used, but slag generally has a high iron concentration.

What good is slag anyway?
For many years slag was treated as a waste material. However, cold poured slag is a good aggregate material with properties similar to limestone. Due to the high angularity, it forms a solid subbase upon which to build (United States Steel). Current slag production is now a revenue source for the industry.