Slag cement is a reclaimed, recyclable industrial non-metallic byproduct from an iron blast furnace. Fly ash is a byproduct of coal-fired furnaces at power generation facilities. Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys.

SCMs are proportioned within concrete and cement-based building materials as individual components or blended, interground or a combination thereof, with portland cement. Since they are recycled industrial materials, they enable the concrete industry to employ thousands of millions of tons of byproduct that would otherwise be landfilled. Moreover, their use reduces the volume of portland cement required to make concrete, thereby decreasing the amount of energy associated with cement production, lowering emissions of greenhouse gases and reducing the amount of virgin material required for the manufacture of concrete.

A life cycle inventory performed in 2006 by Construction Technology Laboratories, Skokie, IL, found that when slag cement replaces 50 percent of the portland cement in 7,500 psi concrete, the energy required to produce one cubic yard of concrete is reduced by 37 percent; carbon dioxide emissions are reduced by 46 percent; and virgin material used is reduced by 15 percent.

Characteristics of Concrete Today

Workability. In general, SCMs will enhance concrete's plastic properties such as workability and placeability. Designers like to control concrete finishes and SCMs reduce surface imperfections and segregation in stripped formwork. More importantly, SCMs enhance the hardened properties of concrete by increasing ultimate compressive strength, decreasing permeability, and enhancing long-term durability.

 

The Verdesian Opened in 2006, the LEED Platinum-certified 24-story, 299,000 ft2 Verdesian in Battery Park, New York City, designed by Pelli Clarke Pelli Architects and developed by the Albanese Organization, exceeds New York City energy efficiency standards by almost 40 percent.  Concrete contributed to the Platinum certification in two major areas: namely the recycled content and regional materials credits of the Materials and Resources (MR) category. The material costs of the concrete used in the foundation  and superstructure amounted to approximately $1.5 million; the concrete contained on average 10 percent  pre-consumer recycled  content in the form of slag cement (45 percent of the cement was replaced with ground granulated blast-furnace slag). Of the $17 million total cost of materials permanently installed in the building, the concrete component contributed about one percent of the cost. Even though one percent sounds low, it was a sizeable contribution since only 10 percent was required to earn one LEED credit. With the addition of 100 percent recycled steel for rebar and other recycled materials, such as wood used in millwork, the project received one point for the MR Recycled Content Category. In the category of Regional Materials, the concrete helped earn two points because all components of the concrete mix were manufactured and extracted within 500 miles of New York City. Photo © Jeff Goldberg@ESTO