While widespread use of Slag in its many contemporary applications is a fairly recent development, the material itself is as 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.
The history of slag use in road building dates back to the time of the Roman Empire, some 2000 years ago, when broken slag from the crude iron-making forges of that era were used in base construction. 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.