The Basics of Concrete and Carbon Reducing Carbon Emissions for Precast and Ready-Mix Concrete As the Northwest Regional Sustainability Engineer at Knife River Corporation, Melissa Verwest, P.E., works closely with architecture, engineering and construction (AEC) companies, educators and organizations that champion sustainability to facilitate solutions and support initiatives of sustainability as it pertains to the built environment. While knowledgeable in several construction materials, her specific focus is on reducing carbon emissions for precast and ready‑mix concrete. Acting as lead on developing the company narrative for material sustainability efforts, Melissa brings energy and excitement to this developing field. We recently had a chance to sit down with Melissa and learned more about the basics of concrete, its relationship to carbon and sustainability efforts that are on the horizon. The following are excerpts from our conversation. What is the relationship between carbon emissions and concrete production? When we talk about concrete and its carbon emissions, we need to take into consideration what we bring to the concrete plant, where it comes from, and then how and what we use to produce it. That could be energy consumption, fuel use, waste production and so on, but the biggest carbon considerations currently are the constituents that go into the concrete mix. I like to compare the constituents to making a cake. Depending on what you add to the mix, you can get either a lemon cake or chocolate cake. Still, both are cakes, but the ingredients and end result are much different. Many people think concrete is simply a giant gray block, but that couldn’t be farther from the truth. Concrete is made up of many different ingredients. The main ingredients are aggregates that add strength, cement which is the binder, water and air. There can be other admixtures included depending on specifications and weather. Also, ternary mixtures can be used. Like a cake, the number of eggs you add and the spices you dash in here and there can really change the properties of that mixture even though it may all “look the same.” When you talk about the relationship between carbon and concrete, it has to do with those constituents that have their own history of carbon consumption. By shipping, railing or trucking them to our concrete plant as ingredients and then transporting them out to the site on a mixing truck as a concrete mix, there is even more energy consumption, which equates to more carbon. How does the production of cement contribute to global CO2 emissions? Cement is the second most consumed resource in the world, following only water, which is number one. Cement is just one component of concrete, which is 12% of the weight, but 90% of the carbon. So, it’s a small add by volume, but it accelerates the carbon dioxide in the final concrete product. This is where concrete gets the black eye. Cement manufacturing accounts for 8% of the world’s carbon dioxide — but considering that the global use is approximately 20 billion tons of concrete per year — it’s not that cement is necessarily all bad, it’s that we use so much of it and that produces a lot of carbon dioxide. What steps are being taken in the concrete industry to reduce its carbon footprint? Unfortunately, we’re the downstream users of cement. Knife River does not produce cement, so there’s nothing we’re able to do in that stage of carbon contribution. What we do, and what we’ve done for a while, is use supplementary cementitious materials (SCMs). When we use ternary mixes, we can reduce our carbon consumption by 20-50% depending on that mix. If we take some cement out and replace it by adding other things in, depending on that combination, that is the reduction, and that is our best approach right now. TECHNOLOGICAL INNOVATIONS What are some emerging technologies or methods being used to create low-carbon or carbon-neutral concrete? One emerging technology is the use of cellulose nanomaterials (CN) or wood waste. In fact, the world’s most abundant polymer is cellulose. When cooked down, it looks like Elmer’s glue. When we added it to the concrete mix, again adding something new to the cake mix, we found a strength increase of 20% and greenhouse gas emissions reduction of one-third. Our Knife River plant successfully used cellulose nanomaterials in a precast concrete bridge in Yreka, California, in partnership with the U.S. Endowment for Forestry and Communities and with Oregon State University’s sustainability and concrete research. Another emerging technology that I really like is using recycled ground glass. It’s important to note that the glass is not a replacement for aggregate. Glass has different shear planes than rocks, and it would be catastrophic to take all aggregate out of the mix; rather, the glass is ground to a powder and acts as a binder. The biggest roadblock to using ground glass is the removal of contaminants before grinding. You can’t have dirty glass and expect the binding to work. Until recently, this cleaning process was very difficult, but companies on the East Coast have obtained very high purity, and we are looking forward to when this technology is readily available nationwide. 36 IDAHO ARCHITECTURE | 2025 | aiaidaho.com
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