The NSA – Since 1918
The history of slag use in road building dates 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. Prior to the industrial revolution, there is evidence that as early as 1589, the Germans were making cannon balls cast from iron slag. 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. During the industrial revolution, records indicate that cast iron slag stones were being used for masonry work in Europe. Roads made from slag were first built in England in 1813 and, just seventeen years later, the first slag road was laid in America. 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.
Even though slag was demonstrating its versatility well before the 20th century, for a long time, its principle use in the United States was as track ballast for the nation’s railroads. Due to this material’s inherent characteristics, applications that could benefit from these properties were more widely being considered. The building of military roads during World War I is a prime example of how strength and durability was exploited to enable the construction of roadways that could withstand the wear and tear typically associated with the transportation of heavy machinery and equipment.
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 the full recognition of slag’s potential. Today, millions of tons of Slag aggregates are produced annually in the United States. Technical personnel of member companies, acting through the National Slag Association, are working constantly to engineer a product that will meet the various needs of civil and environmental engineering applications and other areas that include manufacturing, agricultural, and any other use that can be imagined.