Slag from steel industries are sometimes erroneously classified, and often looked upon, as industrial waste materials. In actually, they are valuable and extremely versatile construction materials. The history of slag use in road building dates back to the time of the Roman Empire in two thousand years ago, when broken slag from the crude iron-making forges of that era were used in base construction. Applications were quite sporadic until the last century, when large quantities began to be used for a number of purposes. In relatively recent years, the demand for maximum utilization and recycling of by-products and recovered waste materials for economic and environmental reasons has led to rapid development of slag utilization. In some areas, nearly all of the steel slags are now being used, and use is rapidly growing in many others.

Granulated blast furnace slag is produced by quickly quenching molten slag to produce a glassy, granular product. The process is quenching with water, but air or a combination of air and water may be used. Very little mineral crystallization takes place in the very rapid cooling process. The granulated slag may vary from a friable, popcorn-like structure to small, sand-size grains resembling a dense glass, depending on the chemical composition, temperature at the time of quenching, and the cooling rate. The slag glass contains the same major oxides as does Portland cement, but with considerably different proportions of lime and silica. Portland cement has excellent hydraulic properties, and with a suitable activator will set in a similar manner. Granulated slags may be crushed, graded or ground for specific applications.

Blast furnace slag had been used in road building as long ago as 1830 In the U.S. As railroad ballast since 1875, use as concrete aggregate began in the 1880's and in bituminous surfaces in 1900's. Major development of slag applications was in the construction aggregate applications, with much smaller amounts going into more specialized applications such as cement manufacture and agricultural applications. For the past 25 years it is essential all the blast-furnace slag produced has been used in the nation. This level of commercial application has been reached on a competitive basis with others material; the slag has been widely used because it can provide equal performance at a lower cost, and better performance for similar cost.

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