Strategy #2: Use of Supplementary Cementitious Materials

Given the high contribution cement has to the carbon footprint of concrete, the most effective means to reduce the GHG emissions from concrete is to simply reduce the cement content of the mix as much as possible

There are several materials that can act as partial substitutes to Portland cement, which can lead to GHG and cost reductions. The material substitution measures discussed here include the supplementary cementitious materials (SCM) called fly ash and granulated blast furnace slag (slag). Both of these materials can act as substitutions for Portland cement and are already well known in the marketplace. Substitution rates for these materials can range as high as 50-70%. In addition, using recycled concrete as a substitute for virgin aggregate is also considered in this section as a GHG reduction measure, although the GHG savings are considerably less than cement substitution. The reductions achieved using SCM can be substantial. For example, a 50% slag substitution will result in a 40% reduction of GHG impact of a standard 4000 psi mix. The figure below shows the GHG reductions that can be achieved with varying levels of cement substitution.

Concrete

Some infrastructure or building projects specify SCM minimums to reduce the carbon impacts of the mix. Examples include the following:

Seattle Department of Transportation (SDOT) specifying that all sidewalks be constructed with a minimum of 25% SCM (Tracy, 2014 ‘add citation”).
California’s High Speed Rail project also specifies minimum SCM for specific applications (cite http://www.hsr.ca.gov/docs/programs/construction/RFP_B4-PtC1_StandrdSpecification.pdf).

Minimum SCM specifications opens up a potential perception that the owner is starting to specify means and methods. With that comes a potential liability if mix designs meeting a target do not perform as expected (Sound Transit, December 2014). Although the risk of liability seems quite low, it is still a valid consideration when writing specifications. Another downside of minimum SCM specifications is that the carbon savings are not actually quantified. As expected, with higher the SCM percentages, the carbon impact of the mix is reduced. However, without quantifying business as usual compared to the low carbon mixes, governments may have a difficult time tracking the carbon savings that result from their purchases. In an era of increasingly important Climate Action Plans, quantifying carbon savings is an important factor.

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