It was 1929 when Pit & Quarry first published a series of articles about blast furnace slag, then a relatively unknown material that was being used as a construction and concrete aggregate. Many years later and now after many millions of tons of blast furnace slag have been utilized, many are still confused about its acceptable use or are unaware of this renewable resource. Others are concerned with the environmental impact from using an industrial by-product.

However, the commercialization of iron and steel slag represents man's most successful early attempt to commercialize industry by-products. When processed as an aggregate, it is sometimes referred to as a synthetic aggregate. We prefer to call it a co-product or by-product of the steel industry.

The steel industry generates two different types of slag, each originating from a specific type of furnace. Blast Furnace Slag is the non-metallic product developed in a molten condition simultaneously with iron in a blast furnace. Steel furnace slag is the non-metallic product that is developed simultaneously with steel in open-hearth, basic oxygen (BOF) or electric arc (EAF) furnaces.

Both materials are composed of fused minerals and elements found in the raw materials (iron ore, coke, lime or limestone) used in the smelting process. Slag is poured from the furnace first as it floats on the surface while the heavier molten metal lies at the bottom of the vessel.

History

The use of slag from the iron making process dates back to Roman times when slag was used to build roads and early forms of concrete. In the Americas, blast furnace slag was available at the many iron furnaces that dotted eastern Pennsylvania because the raw materials — iron ore, limestone and charcoal from wood — were plentiful and easy to extract.

However, with the advent of the Bessemer technology, iron could be produced at large production facilities with coke derived from coal as fuel; and slag became a more plentiful material and a disposal problem for these early steel mills. Blast furnace slag began to accumulate into large stockpiles commonly known as "slag banks." In 1918, the National Slag Association was formed to unite slag producers to facilitate the promotion and recycling of this renewable resource.

Many millions of tons of slag have been sold to the construction industry during the past 88 years and we are now at a point in our history where nearly all old slag banks have been recovered. As technology continued to evolve, steel could now be produced without the more costly blast furnace by using the Electric Arc Furnace (EAF) process.

This lead to the decline in the number of blast furnaces in the U.S. and also to the decline in the amount of blast furnace slag produced and sold. The sales of blast furnace slag reached a zenith of more than 31 million tons in the decade of the 1960s, but for the past 20 years, sales have stabilized at approximately 12 million tons annually.

Since the EAF furnace also produces less slag per ton of steel production than a blast furnace, the volume of steelmaking slag available for sale is greatly reduced. Approximately 8 million tons of steel furnace slag is sold as construction aggregate. This value continues to grow by small increments annually even though global competitive pressures continue to threaten U.S. steel industry production.

Production

It is difficult to track slag production, as variations in furnaces affect production of iron and steel slag. In addition, much of the BOF slag is recycled back to the blast furnace to be used as a flux to reduce the amount of purchased limestone. These recycling practices have been going on in the steel industry for many years. Therefore, USGS only tracks slag sales as a method to measure the amount used in the United States. However, it wasn't until 1946 that slag was given its own mineral yearbook report.

The EAF furnace produces steel more cost effectively and efficiently by using recycled scrap metal. Slag produced from the process (steel furnace slag) is remarkably different from blast furnace slag. One noticeable difference between blast furnace and steel furnace slag is unit weight.