Slag cement, a byproduct of steel-making, enhances the traditional concrete benefits of strength and durability while offering reductions in waste, energy use and greenhouse gas emissions.
Slag cement is used in concrete for virtually any construction application, either in conjunction with traditional portland cement, part of blended cement or as a separate component. It typically replaces part of the portland cement in concrete mixes.
According to the Slag Cement Association, an industry trade group, incorporating slag cement as a supplement in concrete offers higher strength, reduced permeability and improved resistance to chemical attack. And as a recycled material, its production requires far less energy and natural resources than the conventional cement it partially replaces.
Slag cement, originally known as granulated blast-furnace slag, begins with the production of iron. The heart of the process is the blast furnace that refines iron ore into iron. The ingredients are heated to nearly 1500 degrees Centigrade to form two components: iron and molten slag.
The iron is used to produce steel, and the molten slag is converted to a cement-like material by rapidly cooling it with water. This rapid cooling, called quenching, creates glassy granules, which are then ground into the fine powder known as slag cement.
Slag cement is as old as iron-making itself. In the 1700s, it was combined with lime to make mortar. One of the first major uses of slag-lime cements was construction of the Paris underground in the late 1800s. In the U.S., blends of slag and portland cements were introduced in 1896.
Today, slag cement can account for 30% to 50% of the cement content in concrete. For applications such as mass concrete and in marine environments, that figure can rise to more than 70%.
Slag cement is used in nearly all types of concrete construction: pavements, structures and foundations, mass concrete (i.e., dams and retaining walls) and precast concrete products such as pipe and block.
Advocates point to a number of ways in which slag cement makes concrete better and more consistent:
• Easier to place and finish
• Higher strength
• Lower permeability
• Better resistance to corrosive chemicals
• Lighter color than conventional concrete (better architectural and decorative finishes)
Slag cement brings an impressive set of environmental benefits in line with the push for more sustainable construction.
First and foremost, it’s a recycled product – created from blast-furnace slag otherwise destined for disposal. The energy, emissions and raw materials required to produce slag cement is a fraction of that needed for traditional portland cement.
Replacing a portion of portland cement with slag cement – typically from 30% to 50% – significantly lowers the environmental impact of concrete.
Slag cement requires nearly 90% less energy to produce than portland cement. According to the Slag Cement Association, substituting 50% slag cement for portland cement reduces greenhouse gas emissions by more than 40% and lowers the embodied energy of concrete by more than 30%.
Another factor is slag cement’s nearly white color. Lighter-colored concrete has greater reflectance and visibility, which reduces the energy needed to light streets and parking lots at night.
Higher reflectance also mitigates the heat island effect, whereby highly developed urban areas tend to absorb heat and experience higher temperatures. Light-colored buildings and pavements reduce the energy needed for cooling and lowers ozone levels.
The Environmental Protection Agency recognizes the environmental benefits of using slag cement in concrete. It has classified slag cement as a “recovered" product under the Resource Conservation Recovery Act and has issued a procurement guideline requiring its specification on most federally-funded projects.
For more information on the EPA's Comprehensive Procurement Guidelines, visit https://www.epa.gov/smm/product-resource-guides-comprehensive-procurement-guideline-cpg-program.
The Slag Cement Association offers extensive information on applications, uses, and benefits
The Federal Highway Administration offers guidelines on the use of slag cement in pavement construction