Slag is a product of the steel making process. Once scorned as a useless byproduct, it is now accepted and, often, preferred and specified as it is known to be a valuable material with many and varied uses.
Blast Furnace Slag is formed when iron ore or iron pellets, coke and a flux (either limestone or dolomite) are melted together in a blast furnace. When the metallurgical smelting process is complete, the lime in the flux has been chemically combined with the aluminates and silicates of the ore and coke ash to form a non-metallic product called blast furnace slag. During the period of cooling and hardening from its molten state, BF slag can be cooled in several ways to form any of several types of BF slag products.
Steel Furnace Slag is produced in a (BOF) Basic Oxygen Furnace or an (EAF) Electric Arc Furnace. Hot iron (BOF) and/or scrap metal (EAF) are the primary metals to make steel in each process. Lime is injected to act a fluxing agent. The lime combines with the silicates, aluminum oxides, magnesium oxides, manganese oxides and ferrites to form steel furnace slag, commonly called steel slag. Slag is poured from the furnace in a molten state. After cooling from its molten state, steel slag is processed to remove all free metallics and sized into products.
While widespread use of Slag in its many contemporary applications is a fairly recent development, the material itself is an old as the smelting process which produces it. As early as 1589, the Germans were making cannon balls cast from iron Slag. And records are available which indicate that cast iron Slag stones were used for masonry work in Europe of the 18th century.
Roads made from Slag were first built in England in 1813 and, just seventeen years later, the first Slag road was laid in this country. By the year 1880, blocks cast of Slag were in general use for street paving in both Europe and the United States. A major city under the American flag with a long history of Slag-paved streets is San Juan, Puerto Rico. Perhaps the earliest appearance of Slag in American history came with the Pilgrims. Since Slag was commonly used as ship ballast in that era, it seems likely that the Mayflower itself carried a load of this useful material.
Even though Slag was demonstrating its versatility well before the 20th century, for a long time, its principle use in this country was as track ballast for the nation's railroads. As production grew, so did the need to find new applications. One that proved immediately valuable was in the building of military roads during World War I.
By 1918, the year the National Slag Association was formed, the nation was producing 40 million tons of pig iron a year, with a concomitant output of 20 million tons of Slag. The time had come for full recognition of Slag’s potentialities. This is what the Association has tried to achieve. Today, millions of tons of Slag aggregates are produced annually in the United States. Its future is limited only by the imagination of its users. Technical personnel of member companies, acting through the National Slag Association, are working constantly to job that imagination.
Blast furnace slag is allowed to slowly cool by ambient air, is processed through a screening and crushing plant and is processed into many sizes for use primarily as a construction aggregate
Pelletized or Expanded Slag is quickly cooled using water or steam to produce a lightweight aggregate that can be used for high fire-rated concrete masonry, lightweight fill applications, or for use to grind into a cement product.
Granulated slag is rapidly cooled by large quantities of water to produce a sand-like granule that is primarily ground into a cement commonly know as GGBS (Ground Granulated Blast Slag).
Steel slag is processed as an air-cooled material, the free metallics are magnetically separated and the material is separated and sized into construction aggregates, used as an agricultural soil amendment, as a raw ingredient in Portland cement production, as an environmental remediation material and other uses.