For decades the principal use of slag in the US was as rail track ballast. AS quantities grew new applications were sought, and one that proved immediately valuable was in the building of military roads in WWI. By 1918, the year the US National Association of Slag was founded, the annual production of slag had grown to 20 million tons.

The Association was formed with the objective of promoting the use of slag, and today millions of tons of slag aggregates are used in the country. The Association and its members are constantly researching new applications for slag.

Slag has been successfully used in various projects, including the Detroit Metro Airport parking garage constructed in 2000-01; “white-topping” thin concrete road overlays in Southeast Michigan; the I-70 through Colorado’s Glenwood Canyon, where the asphalt mix had to withstand harsh conditions of weather and traffic; the Chicagoland Speedway, where slag was specified because it does not shine, giving the track skid-resistance and grip that are so important when drivers are travelling at over 300 km/h; in farming as a liming agent for adjusting the pH balance in the soil.

The Federal Highway Administration, Transportation Research Board and the various state departments of transport publish technical guidance reports on the use of slag as a road aggregate.

According to the US Geological Survey, data on US slag production are unavailable, but it is estimated to have been in the range of 18 to 25 million tons in 2013. Domestic US sales in the same period amounted to an estimated 17 million tons, valued at about US$290 million (fob plant). Blast furnace slag accounted for about 45% of the tonnage sold and had a value of about US$225 million; nearly 85% of this value was from sales of granulated slag. BOS and EAF slag accounted for the remainder.

Slag was processed by nearly 30 companies servicing active iron and/or steel facilities or reprocessing old slag piles at about 120 sites in 32 states; included in this tally are a number of facilities that grind and sell ground granulated blast furnace slag (GGBFS) based on imported unground feed. Prices per ton ranged widely in 2013 from a few cents for some steel slags at a few locations to about US$100 for some GGBFS

Air-cooled iron slag and steel slag are mainly used aggregates in concrete (air-cooled iron slag only), asphaltic paving, fill, and road bases; both slag types also are used as a feed for cement kilns. Almost all GGBFS is used as a partial substitute for Portland cement in concrete mixes or in blended cements.

Pelletized slag is generally used for lightweight aggregate but can be ground into material similar to GGBFS. Owing to their low unit values, most slag types can be shipped by truck only over short distances, but rail and waterborne transportation can be longer. The much higher unit value of GGBFS allows this slag to be shipped economically over longer distances.

Slag is commonly returned to the blast furnaces as ferrous and flux feed, but data on these returns are incomplete. Entrained metal, particularly in steel slag, is routinely recovered during slag processing for return to the furnaces, but data on metal returns are unavailable.

Recent data indicate that GBFS (mostly unground) is the dominant type of ferrous slag imported, but official import data include significant tonnages of non-slag materials (such as fly-ash and silica fume) and slags or other residues of various metallurgical industries (such as copper slag). Based on official data, the principal country sources for GBFS were Canada (40%), Japan (40%), Italy (7%) and South Africa (7%).

The availability of blast furnace slag is becoming problematic because of the decline in the number of active blast furnaces in recent years, the lack of construction of new furnaces, and the depletion of old slag piles. Recent draft regulations to restrict emissions (especially of mercury) from US cement plants and to potentially reclassify fly-ash as a hazardous waste for disposal purposes have the potential to limit the supply of these cementitious materials to the market and could lead to an increase in demand for GGBFS.

Although world slag data are unavailable, the USGS estimates that world iron slag output in 2013 was of the order of 260-320 million tons and steel slag about 170-250 million tons, based on typical ratios of slag to crude iron and steel output.