Slag cement, often called ground granulated blast-furnace slag (GGBFS), is one of the most consistent cementitious materials used in concrete. Slag cement is actually a byproduct of iron production. When the iron is processed using a blast furnace, slag and iron both collect at the bottom of the furnace. The molten slag must first be separated from the molten iron. The molten slag is then diverted to a granulator where it undergoes granulation. Granulation is the process where the molten slag is rapidly drenched with water until it turns into a raw material called granules. The granules are then cooled rapidly, which stops crystals from forming and instead creates a glassy, non-metallic product. These granules are then dried and ground to a suitable fineness, the result of which is slag cement.

Slag cement is increasingly being used as an ingredient in the manufacture of blended portland cement. Using slag cement to replace a portion of portland cement in a concrete mixture is an efficient way to make concrete more consistent. Among the measurable improvements are higher workability, higher finishability, lower permeability, improved resistance to aggressive chemicals, more consistent plastic and hardened properties, and higher compressive and flexural strengths.

From an environmental viewpoint, the use of slag cement in concrete serves to make concrete “greener.” Not only is slag cement can be considered a recycled material, but it can also significantly reduce energy consumption and greenhouse gasses emitted in the production of concrete raw materials.

Slag cement has actually been used in concrete projects in the United States for over a century. The earliest use of slag cement was documented in 1774, when it was combined with slaked lime and used as a mortar. Slag cement was first used commercially in Germany in the 1860s, and it was such a success that engineers in 1889 decided to build the Paris underground metro using slag-lime cement. Slag cement blended with portland cement first appeared in Germany in 1892, and four years later was introduced to workers in the United States. By the 1950s, the advances made in both the slag removal and the granulation processes resulted in slag cement being used as an entirely separate product which was added at the concrete mixer.

In the United States, the first slag cement facility was built in 1982 at Bethlehem Steel’s furnace by Atlantic Cement in Sparrows Point, Maryland. Since that time, there have been over a dozen slag cement granulation and grinding facilities in the United States. Ten American companies produce and distribute slag cement, and there are now over 80 terminals that provide outlets for distributing the material. Granules from both domestic and imported sources are currently being used at grinding mills to satisfy the growing demand for slag cement.

The last several years have seen particularly strong growth in the use of slag cement in the United States. One reason for its growing popularity is the advances in production that has occurred in the last decade. In addition, terminal capacity expansion has increased significantly in recent years. Slag cement is no longer considered a specialty product and is instead a commonly used, cemetitious material.