Reducing embodied carbon
The use of supplementary cementitious materials (SCMs) as a partial replacement for Portland cement results in more durable and high-performance concrete and lowers energy consumption and greenhouse gas emissions. SCMs in concrete are used as a separate component or as a constituent of blended cement. The most common SCMs are slag (a byproduct of iron manufacturing), fly ash (a coal combustion byproduct from power plants), and silica fume (a byproduct of silicon and ferrosilicon alloy manufacturing).
Cementitious blends have many properties contributing to sustainable construction and stronger, longer-lasting concrete. They also reuse byproducts from other industries that might otherwise be disposed in landfills. Binary blends are a mixture of Portland cement and one SCM, and ternary blends are a mixture of Portland cement and two SCMs.
Considering how concrete is the world’s most widely used construction material, specifying cementitious blends can have a major impact on the environment and make a significant contribution to achieving sustainable building goals, such as those prescribed by the U.S. Green Building Council’s (USGBC) building rating system, Leadership in Energy and Environmental Design (LEED). SCMs contribute to LEED credits in the following categories: Sustainable Sites, Energy and Atmosphere, Materials and Resources, Indoor Environmental Quality, and Innovation.
EPDs and LEED v4
Environmental Product Declarations (EPDs) are registered documents that communicate the life-cycle assessment (LCA) of products based on product category rules (PCR).
EPDs must be consistent with the International Organization for Standardization (ISO) standards, such as ISO 14025, 14040, 14044, EN 15804, or ISO 21930. These standards address how to perform an LCA and what data to include in an EPD.
EPDs are important for providing transparency on products and to support design processes with innovative solutions that deliver structural integrity and eco-efficiency.
The trend of greater disclosure has been integrated into various green building standards, but particularly within the LEED v4 rating system. Within the materials and resources category, project teams can earn points for products that have verified EPDs.
SCM performance benefits
SCMs impart a wide range of exceptional performance properties. While there are many advantages of concrete in its plastic state, the greatest benefits and performance can be seen in the hardened properties.
Enhanced strength
With the addition of SCMs, flexural and compressive strengths can increase markedly at 28 days and beyond. Typically, slag cement and fly ash will lower early strengths (one to 14 days) but significantly improve long-term strengths (28 days and beyond), depending on the proportions used. Silica fume contributes primarily to strengths at 3 to 28 days.
Reduced permeability
Permeability is generally the critical factor affecting durability. SCMs significantly extend the life of concrete by reducing permeability to chlorides and other aggressive agents. Silica fume can provide as much as a five-fold reduction in permeability.
Resistance to alkali-silica reaction (ASR)
Some SCMs can prevent excessive expansion and cracking of concrete caused by ASR. In most cases, 50 percent of slag cement is sufficient with highly reactive aggregates. The amount of fly ash required typically ranges from 15 to 55 percent, depending on the calcium oxide (CaO) content of the fly ash. The lower CaO fly ashes are more efficient at mitigating ASR. Silica fume can control ASR, but the amount required generally results in poor constructability. Blends of slag cement and silica fume, as well as blends of fly ash and silica fume have a synergistic effect in mitigating expansion due to ASR.
Relevant ASTM Specifications
- ASTM C989, Standard Specification for Slag Cement for Use in Concrete
and Mortars - ASTM C618, Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete
- ASTM C1240, Standard Specification for Silica Fume Used in Cementitious Mixtures
- ASTM C150, Standard Specification for Portland Cement
- ASTM C595, Standard Specification for Blended Hydraulic Cements
- ASTM C1157, Standard Performance Specification for Hydraulic Cement
The photo caption to this story has incorrect information: the project One Dalton is located in Boston, Massachusetts – not Washington DC and it is LEED certified Silver, not platinum.
Thanks a lot for pointing that out. We have modified the caption of the photo.