Concrete carbon emissions: Real challenges, real opportunities

Carbon sequestering technologies

Newer to the market are carbon-sequestering mixes. These products actively incorporate CO2 in their formulation though the specific techniques and carbon reduction impacts vary. New products on the market can directly inject CO2 into wet mix. The CO2, sourced and purified from the same captured emissions from power plants used in the beverage industry, chemically bonds the calcium oxide in the mix’s cement, creating additional strength and allowing for reductions in cement content.  While the cement and GWP reductions are small (typically three to five percent), they are still significant, and one of the advantages of this process is it can accommodate most other additional strategies outlined in this article.

Another related strategy substitutes typical portland cement for portland cement products with higher limestone content.  The two most common products are Portland-limestone cement (PLC) and limestone calcined clay cement (LC3), with the former much more common in North America (Europe has more experience with the latter). ASTM C595 guides PLC usage and increases the allowed limestone quantity in a cement mix from five percent to 15 percent, but the impacts to concrete formulation and properties are minimal. PLC replaces typical cement in the same quantity with an end product that measures and performs the same. The increase in ground limestone has a comparable 10 percent reduction in GWP reduction compared to ordinary Type I portland cement.

In Europe, some cement mixes may contain even higher PLC content (up to 35 percent of the overall cement), and LC3 combines both finely ground limestone and calcined clay to replace as much as 50 percent of the Portland cement. While calcined clay does require energy to heat and activate the material, the temperatures required are around 600 to 800 C (1112 to 1472 F) as compared to ordinary Portland cement where temperatures need to reach around 1450 C (2642 F).

LC3 also greatly reduces carbon emissions from the concrete’s chemical reactions as the clay contains very little carbon to begin with. The net effect is that LC3, as currently formulated, can impact concrete GWP reduction, approximately 30 percent.

 In the future, high-reactivity metakaolin, derived from purified kaolin clay, will further push the boundaries of curing possibilities to produce high performance, lower carbon concrete in geopolymer concretes. However, it is not yet commercially available to all markets.

Nano infused cements (NICs) are from the family of nano silica admixtures technologies. Like silica fume, NICs are highly reactive when used in concrete construction and produce very durable, strong concrete with increased hydration cycles. However, while silica fume engages with cement on the micro scale, NIC—like all nano silica admixtures—operate in the nano scale, closing capillaries and reducing porosity to the point of slabs acquiring natural moisture mitigation attributes and thus shorter mobilizations to construct concrete on the job site. NIC is an SCM that can be used as a cement replacement, but also allows for increased use of other SCMs making it easy to reduce global warming potential (GWP) on a project. Labor and material cost impacts of NIC are par for the course with traditional methods, but with reduced schedules provide the opportunity to lower overall project costs. Combining internal curing with NIC, PLC, and increased SCMs would allow for GWP reductions well beyond 30 percent in most parts of North America.

Sourcing strategies

As a locally produced and sourced product, strategies for reducing concrete’s footprint vary by region.  In some of the largest and most progressively sustainable concrete markets (e.g. San Francisco and Seattle), many suppliers have commissioned Environmental Product Declarations (EPD) to measure the GWP of their products. The most typical and useful EPDs are third-party certified and utilize product category rules (PCRs) specific to concrete, to provide a reasonably accurate and comparable life cycle assessment (LCA) of their product. While it can be somewhat costly for a supplier to generate an EPD (some programs start at $3000), as it involves an assessment of each component of a concrete mix, once they do, the study can typically apply to all their products, as mixes typically just vary the quantity of the components.

For markets with sophisticated producers with available EPDs, best practice is to directly specify a maximum GWP, as evidenced by an EPD, like any other desired performance requirement. The Embodied Carbon in Construction Calculator (EC3), a free database of construction EPDs, is quickly becoming leveraged by the industry, and concrete is among the first of the material sections to contain a large amount of product EPDs. It is quite simple to register for the free web-based database tool, search for suppliers within a geographic limit, and identify not only the compliant producers, but to report median and achievable GWP values for particular concrete strengths and other properties.

In areas with less sophisticated producers, project teams will need to be more active. It is possible to specify maximum cement content limits as a proxy for GWP. Marin County, California’s concrete regulations do this, for example. Another approach is to work directly with suppliers to measure and optimize GWP of proposed mixes. The Concrete LCA Tool, a simple concrete GWP calculator does just this using general industry LCA data from Tally (the North American GaBI LCA database) for common concrete ingredients, which allows direct comparison of proposed mixes against published NRMCA regional averages for various strength classes.

While the results are not specific to particular material or supplier, they are good enough for comparing relative impacts of mixes from a single producer.

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