Slag cement, a byproduct of iron production, is a consistent and eco-friendly material used in concrete. It improves processability, finishability, and strength while reducing permeability and greenhouse gas emissions. Slag cement has been used for over a century and is now widely available in the US. Recent manufacturing advances and terminal capacity expansion have led to increased popularity.
Slag cement, often called ground granulated blast furnace slag (GGBFS), is one of the more consistent cementitious materials used in concrete. It is actually a byproduct of iron production. When iron is processed using a blast furnace, the slag and iron 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 is rapidly soaked in water until it turns into a raw material called granules. The granules are then rapidly cooled, which prevents crystal formation and instead creates a glassy, non-metallic product. These granules are then dried and ground to a fineness suitable for use as a cement.
This type of cement is increasingly being used as an ingredient in the production of blended Portland cement. Using slag cement to replace a portion of Portland cement in a concrete mix is an efficient way to make the concrete more consistent. Measurable improvements include improved processability, improved finishability, lower permeability, improved resistance to aggressive chemicals, more consistent plastic and curing properties, and higher compressive and flexural strengths.
From an environmental point of view, the use of slag cement in concrete serves to make the concrete “greener”. Not only can it be considered a recycled material, but it can also significantly reduce the energy consumption and greenhouse gases emitted in the production of concrete raw materials.
Slag concrete has actually been used in concrete projects in the United States for over a century. Its first use is documented in 1774, when it was combined with lime putty and used as mortar. This cement was first used commercially in Germany in the 1860s, and was so successful that in 1889 engineers decided to build the Paris Métro using slag-based cement. Slag cement mixed with Portland cement first appeared in Germany in 1892 and was introduced to workers in the United States four years later. In the 1950s, advances in both slag removal and granulation processes led to it being used as an entirely separate product that was added to the concrete mixer.
In the United States, the first slag cement plant was built in 1982 at the Bethlehem Steel furnace by Atlantic Cement in Sparrows Point, Maryland. Since then, over a dozen slag granulation and milling plants have been built in the United States. Ten American companies produce and distribute this cement and there are now over 80 terminals providing outlets for the material’s distribution. Granules from both domestic and imported sources are currently being used in grinding mills to meet growing demand.
Recent 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 manufacturing that have occurred over the past decade. In addition, terminal capacity expansion has increased significantly in recent years. This concrete is no longer considered a specialty product and is instead a commonly used material.
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