Assessing the role of concrete carbonation in sustainable practice

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    Structural engineers want to do their part to reduce greenhouse gas emissions, but knowing what is the best approach is not always clear. One strategy is to substitute complementary cementing materials (CCMs) such as fly ash or slag for a portion of the cement in a concrete mix design. Recent news articles noting concrete’s potential to reduce atmospheric carbon dioxide (CO2) levels through carbonation, the process whereby cured concrete adsorbs (and absorbs) CO2 from the surrounding atmosphere, have received attention and caused confusion over the true carbon footprint of concrete. With cement production responsible for approximately 7 percent of global CO2 emissions, what a grand thing it would be if concrete could remove CO2 from the atmosphere.

    First off, how does cement production create CO2 About 40 percent of the CO2 emissions result from burning fossil fuels to produce cement clinker by heating limestone and clay to temperatures of about 2700°F. The remaining 60 percent is released from the chemical conversion of limestone (calcium carbonate or CaCO3) into calcium oxide, the principle component of cement. Known as calcination or calcining, the stochiometry of this conversion is simple: CaCO3 + heat —> CaO + CO2.

    Calcination is reversed by carbonation. This occurs when atmospheric CO2 combines with calcium hydroxide, a cement hydration product, to form calcium carbonate. CO2 enters the small pores at the surface of concrete and reacts with the calcium hydroxide, locking calcium carbonate within the pores. This blocks additional CO2 from penetrating deeper into the concrete, effectively limiting the amount of carbonation that can occur in concrete to a modest layer near the surface. Carbonation is not only surface-oriented, it is also slow. Warm, moist environments favor carbonation, but even after four years of exposure in environments conducive to carbonation, the process extends no more than 8 mm beyond the concrete surface. In some instances, carbonation does not extend deeper than 1 mm beyond the concrete surface after even longer exposures. What’s more, many modern concrete mixes incorporate CCMs; these mixes do not favor carbonation because the CCMs react with the calcium hydroxide to form secondary hydration reaction products such as calcium-silicate-hydrates that are not readily carbonated.

    Therefore, because of these limitations, we cannot rely on the carbonation as a means to capture and reduce atmospheric CO2. Using carbonation to sequester a significant amount of CO2 in concrete is not practical. Demolished concrete would have to be crushed, spread out to expose as much concrete surface as possible, and let to sit for some time to allow carbonation to occur. This is not in line with current practices. In some regions of the country, demolished concrete is still sent to the landfill. In other regions, crushed concrete is used as road base, fill material, or in limited amounts as recycled aggregate. In these applications, crushed concrete typically does not have the opportunity to carbonate. Additionally, a substantial portion of concrete is used for foundations that may stay in the ground after building demolition. Ultimately, very little of the CO2 emitted during cement production is removed from the atmosphere by carbonation.

    So where does this leave us? Despite the recent publicity about concrete’s carbonation potential, we cannot count on concrete to balance out its carbon footprint. Carbonation can remove a small amount of atmospheric CO2, though not nearly as much as cement production emits; the difference is cumulative and growing by orders of magnitude. But engineers can still address climate change concerns in their concrete design. We can limit CO2 emissions associated with each yard of concrete in our projects by specifying well-proportioned mix designs that utilize CCMs to minimize the cement content. And remember, in addition to designing efficiently, we can employ best construction practices such as specifying low-VOC formwork release agents and curing compounds. And during project schematics, we can consider and suggest dual uses of concrete elements so that they also serve as heat sinks or as the exposed finish. These are a few of the tangible ways in which engineers can achieve more sustainable concrete designs. Meanwhile, we continue to look for ways to reduce the carbon footprint of concrete structures.

    Alan Kren, S.E., LEED AP, leads the sustainability group at Rutherford and Chekene Consulting Engineers in San Francisco where he is a senior associate. Helena Meryman is the director of Technical Product Development at Clean Concrete Technologies. Sarah Vaughan, P.E., has more than 5 years of structural engineering experience. She can be reached at svaughan@stanfordalumni.org. All three are members of the SEI Sustainability Committee. The committee’s website is www.seinstitute.org/committees/sustainable.cfm