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. 

An additional emerging and futuristic technology is one-celled marine microalgae called coccolithophores that produce calcium carbonate. Most atmospheric CO2 dissolved in seawater is rapidly converted to bicarbonate. Coccolithophores use this bicarbonate to create limestone, which can be a form of carbon capture. It takes 1-2 million acres of open pond systems to grow enough microalgae-producing limestone to meet the demand for cement production in the U.S. — that’s about 90 million tons annually. That seems like a lot, but to put it into perspective, the U.S. uses 100 million acres of land to grow corn, so we would need only 1% of that for the ponds.

This research is still being studied at a university level; that’s why it is futuristic. Last year at the National Ready Mix Concrete Association (NRMCA) annual convention, speakers from several universities talked about the potential of this growing technology. This is a good sign that this research is moving forward and that there has been a proof of concept.

How do materials like fly ash, slag or alternative binders help reduce carbon emissions in concrete production?

Simply put, such materials replace the amount of cement needed. And less cement in a concrete mix right now equals less carbon. 

Can carbon sequestration methods (e.g., injecting CO2 into concrete) be scaled for widespread use?

There are companies out there that will encapsulate CO2, bring it to your plant and inject it into your mix. But there are some drawbacks. The system that is used for CO2 injection must be integrated into every batch plant and they’re going to have to transport the CO2 to the plant. As we discussed, the carbon footprint of concrete right now is measured as the constituents and the transportation of the product to the plant. With current yields of a few percent reduction with injection, and a probable gain of carbon yield in transportation, at best, it zeroes out, and at worst, we’re going the wrong way with putting more trucks on the road. Such implementation has to be assessed on a case‑by‑case basis to best understand if there are sustainability gains to be made.

Policy and Standards

Are there government incentives or policies promoting the use of sustainable concrete?

My background is as a structural engineer. I consulted for over 15 years, and in that role, you design by building codes that govern how materials are used. You have the overall international building code, and then you have all the material codes. These codes are based on basic minimums that engineers must follow — codes that we’ve gotten to know and trust through practice and evolution of design. They keep us safe.

I’m in favor of bringing sustainability into the code system if it doesn’t compromise good engineering design and decision‑making. Lately, policies have been created and are leading the way for carbon reduction in building materials. Some of these policies do it well, while others handcuff themselves as they only provide one path to achieve sustainability with prescriptive specifications. The best policies provide multiple pathways for carbon reduction, allowing for continued good design practice.

A lot of what we’re doing now, policy-wise, is a bridging technique focused on “cradle to gate” carbon reduction, but it can’t be the end all. If you’re going to be truly sustainable, you have to take your blinders off. Sustainability should not be focused just on the front end of material supply and consumption. We should be doing a whole building or project lifecycle assessment (LCA) — when you do that, you’re looking at the project, and thus materials, “cradle to gate” and enveloping the farther reaches of sustainability. 

Challenges and Opportunities

What are the biggest barriers to adopting sustainable concrete at scale (e.g., cost, performance, availability)?

I think the biggest problem is people who are very well-meaning are looking for one solution. But there is no silver bullet right now to concrete carbon reduction. When someone asks, “Can you bring in a sample of your low-carbon concrete offering?” I reply, “The sample looks like concrete you already know, and there is rarely a physical tell to indicate it is low carbon. The indication of low-carbon concrete is in the numbers and the data that has been indicated behind its constituents. 

Concrete is regionally dependent because of the variation of constituents available. What can be done in Oregon is not necessarily what can be done in Minnesota. So as far as barriers, it’s not always cost-prohibited or performance-prohibitive so much as it’s people having a good education and understanding of how regionally dependent concrete is and what is achievable in relation to lower carbon mixes. 

What opportunities exist for architects and engineers to influence the adoption of sustainable materials?

Never shy away from an educational opportunity. As I mentioned before, there are methods being used right now to reduce carbon using SCM materials, but these are regionally dependent. The key is in the specification. That is why I strongly advocate that before you write the specifications, talk to your supplier and see what is achievable. Project owners and designers can come to the table with their carbon and project goals, and the supplier can work to help achieve them. Hold pre-procurement meetings and really dig into your goals and specifications. By working together, we can best achieve holistic sustainability in our built environment.