Concrete and carbon uptake

[Music] Welcome to Science Sessions, the podcast of the proceedings of the National Academy of Sciences, where we connect you with Academy members, researchers and policymakers. Join us as we explore the stories behind the science. I'm Paul Gabrielson. The production of concrete generates greenhouse gas emissions. But over its life cycle, concrete is able to absorb some carbon dioxide, partially offsetting its production-related emissions. In a recent PNAS study, Hassam Azari Jaffari of the Massachusetts Institute of Technology and Colleagues modeled the absorption of carbon dioxide by concrete in different contexts, including in different countries. The amount of absorb they found depends strongly on context, and maybe less than previously thought. Hassam, can you briefly introduce us to the recipe for concrete? We have two terms here, cement and concrete. The analogy that I usually bring is that concrete is the cake, and cement is the flour. So by using cement, you will glue together all those aggregate pieces. So an important component of that is the binder, including cement, and it can have other alternative binders or supplementary cementitious materials like fly ash, slack, or any other recycled materials. And we also have aggregates either sand or gravel. So cement is activated and hardened when it's in contact with water. How does concrete absorb carbon dioxide throughout its lifespan? And why is this important? This is where CO2 is diffused into concrete through this physical chemical reaction, when the cement is hardened, then it reacts with calcium compounds formed during the cement hydration. And it will be permanently stored in concrete and turned into stable carbonate products. This matters a lot because we know that there is a lot of CO2 released during the production of cement, especially from the chemical breakdown of limestone. The carbon uptake doesn't cancel those emissions, but it partially neutralizes them over time. And it means that concrete's climate impact is a bit lower than what you estimate by looking at production alone. We see that concrete has been cited as a carbon sink in various documents and standards and guidelines, but we don't know exactly to what extent this happens and what is the accurate estimation of this process. Tell us about the background of this study. What question were you hoping to answer? In summary, there's been a growing body of research that suggests that concrete sequesters and meaningful amounts of CO2 over its lifetime. But what you realize that these estimates vary vitally, sometimes by a factor of 2 or even more, from 3, 4% up to 55% of the emissions associated with the production of cement. We wanted to answer a simple but very important question that how much CO2 is actually sequestered by existing concrete infrastructure systems and building each year. And why do we have so much difference in the estimates? To do that, we focused on how real buildings and infrastructure are designed, used and exposed to the environment, not just how much cement is produced. Tell us about the model you and your colleagues built. We built a bottom-up national scale model that tracks carbon uptake from concrete and other cement-based products in much more detail than previous approaches. So instead of assuming all concrete and cement-based products behave the same, we account for different building and infrastructure types, surface area and geometry, the exposure conditions, whether the concrete is located in an indoor or outdoor condition, or the cement-based products is buried under the ground. The concrete strength and binder composition, whether we only use ordinary Portland cement or did we use, for example, slag or fly ash as supplementary cementitious materials. And there is another important component which is about the end of life of the concrete, where the concrete is crushed into smaller pieces and how materials are handled at the end of their life. When we crushed the concrete into smaller pieces, into aggregate pieces to recycle and reuse for infrastructure systems in the future, then we increase the speed of carbon uptake by orders of magnitude. Tell us more about your data source. So we mostly rely on the publicly available data from national databases, for example, in the US we have FEMA has a tool where you would be able to extract the geometries of the building. So we built these archetypes that represents one in use application of concrete. For example, for commercial buildings, we understand the sizes of the columns and beams and slabs on the ground and foundations. In parallel, we have data from the industry. These are some of the publicly available databases on the cement consumption in the US and in this study for Mexico and how concrete differs in different states, for example, Minnesota versus Massachusetts in terms of the composition and the quantity of the binder materials. What did you learn about carbon uptake in 2024? So in 2024, we found that the cement waste products already in place in the US sequesters on the order of 6 to 7 million metric tons of CO2, which corresponds to roughly 10 to 15% of the cement process emissions for those individual years. It's not trivial, but it's also much smaller than some earlier global estimates suggested. The carbon uptake isn't uniform across the building environment. In buildings especially residential and like commercial ones, we see that these end use applications tend to sequesters more so you to per unit of concrete because they have more exposed surface area. On the other hand, in the infrastructure like bridges, pavements, pipelines used large volume of concrete, but we have very low surface exposure relative to volume. So the carbon uptake per ton of cement consumption is relatively smaller. What differences did you find between the US and Mexico? We found that Mexico's uptake relative to the emissions associated with the cement production is slightly higher than what we observed in the US. This was mainly because we observed differences in construction practices, building typologies, climate and exposure conditions, and cement composition and those end use patterns. Carbon uptake is very context dependent. You can't reliably apply one country's estimate to another without accounting for how and where those cement based products are used. How did your analysis compare with previous estimates of concrete carbon absorption? The R estimates are generally lower than some earlier global studies, but much more tightly constrained. In the previous approaches, we observed that the authors relied on simplified assumptions about surface area and the thickness of the concrete elements or in general the exposure of the concrete and cement based products to the uptake potential that we calculated becomes more realistic. Importantly, our results are consistent with engineering standards and empirical measurements, which gives us confidence in the robustness of the outputs and the results that we have observed. What are the takeaways from this study for policymakers and the concrete industry? Carbon uptake helps, but it's not the silver bullet. It's not a climate solution by itself. We can leverage the carbon uptake. We can increase it in the future based on the levers that we have under our control, for example, the composition of the binder. Even under favorable condition, it only neutralizes a fraction of the cement emissions. This means that the largest climate benefits still come from reducing emissions at the production stage. More importantly, improving the efficiency, meaning that how much less material we can use in order to deliver the same function. And finally, carbon capture and sequestration, for example, for cement plants. Design and material choices that I mentioned earlier can modestly enhance carbon uptake, but they cannot replace these deep decarbonization levers. For policymakers, this means that carbon accounting should include carbon uptake in cement based products, but we should make sure that it is not overstated over there. What are the caveats or limitations of the study? Similar to any large-scale models in the analytical research environment, our model relies on the best available data, but there's always some uncertainty remaining, and we need to address that explicitly. So uncertainty in how alternative binders will behave over very long time. scales on certainties associated with the geometry of the building and infrastructure elements, and we definitely need to show that even at the high end carbon uptake remains a secondary mitigation pathway compared to reducing emissions at seven plants. Thanks for tuning into science sessions. You can subscribe to science sessions on iTunes, Spotify, or wherever you get your podcasts. If you like this episode, please consider leaving a review and helping us spread the word.