Many Green Cement Roads Lead Through Electric Arc Steel Furnaces

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An electric arc furnace is like a giant, high-tech kitchen appliance for melting metal. Imagine a huge, robust pot where you throw in scrap metal, like old cars or broken appliances. Instead of using gas or coal to heat things up, this “pot” uses powerful electric arcs — think of them as supercharged lightning bolts. These arcs create intense heat, enough to melt the scrap metal into liquid form.

Enter limestone, the ubiquitous soft, shallowly buried remains of long vanished seabeds. In an electric arc furnace (EAF), limestone is a multi-purpose helper. When scrap metal melts down, it often brings along unwanted guests like sulfur and phosphorus. Limestone jumps in to bond with these impurities, forming a separate layer called slag that floats to the top, making it easy to skim off and discard. This slag also acts like a protective blanket, shielding the molten metal from air and preventing oxidation. Plus, limestone helps manage the furnace’s temperature, making sure things don’t get too hot or too cold. About 50 kilograms of limestone are used per ton of steel.

Unfortunately, just as in a limestone kiln on a cement plant, this creates a bunch of carbon dioxide, about 22 kilograms per ton of steel. Interestingly for me, the carbon in the limestone isn’t used for the carbon content of the resulting steel. New steel made from iron has carbon from a variety of sources added, but the scrap steel used in EAFs already has carbon. If the carbon content needs to be adjusted, different additives that run the gamut from coal to plant-derived substances are mixed in, but the carbon in the limestone just goes up the chimney.

A ton of steel requires 400 to 500 kWh of electricity. With the US grid’s average carbon intensity per kWh, a ton of steel might have a carbon debt of about 0.2 tons of carbon dioxide per ton of steel from the electricity requirements. Obviously with a low-carbon grid, that would plummet to perhaps 0.02 tons of carbon dioxide. Many US EAF plants use 10% to 20% natural gas for additional process heat because it’s cheap, which adds roughly 0.02 tons of carbon dioxide to the mix. In a future green grid, natural gas won’t be used. That means in the future, EAF steel’s carbon dioxide emissions will actually be dominated by process emissions from limestone.

The slag is primarily made up of calcium oxide from the limestone, silicon dioxide from impurities, iron oxide from excess iron, and bits of aluminum oxide and magnesium oxide. The sharp eyes who have been wading knee deep through the articles I’m leaving behind this week of cement will have spotted the calcium oxide, silicon dioxide, and aluminum oxide and wondered why they sounded so familiar, unless they are cement nerds, in which case they are well down the path I’m leading you along at the moment.

The main ingredients in Portland cement are calcium from limestone, iron oxide, silica from sand, and alumina from clay. That should clarify why a lot of cement nerds are looking at EAF slag and EAF furnaces with appreciative eyes. This popped for me with the trigger for my personal cement week, news of a Cambridge UK academic team’s use of EAF furnaces to somehow recycle concrete. It’s taken me a while to get back to them, but today is the day. What exactly are they doing?

The Cambridge team operates under the name “Cambridge Electric Cement.” This project is part of the broader research initiatives at the University of Cambridge and involves collaboration with various industry and academic partners. The team includes researchers like Dr. Cyrille Dunant, Professor Julian Allwood, and Dr. Philippa Horton, who have been instrumental in developing and scaling this technology.

Unlike other organizations which are using the slag directly, effectively letting the EAF furnace be the limestone kiln, the Cambridge team are cutting limestone out of the equation. As noted, the carbon and extra oxygen in the limestone that turn into carbon dioxide are surplus to requirements, and all of the stuff that the EAF process needs is in the cement. Concrete is heavily recycled already, crushed to extract the steel reinforcing wire mesh or rebar, and crushed into aggregate for new concrete and construction fill to replace new gravel.

The Cambridge team inserts another couple of steps in this process. They use mechanical means to separate the aggregate from the cement, then crush the cement into a paste, which they process to remove impurities like organic material, metal fragments, plastic, and water. The resulting powder has all the ingredients of cement, but not put together the right way.

They mix this powder with the melted steel instead of limestone. When the slag forms and is cooled off, it’s effectively just chunks of activated cement that can be ground like clinker out of a clinker kiln. No limestone with its surplus-to-requirements carbon, so no additional process carbon dioxide. This process, where it could be used, would eliminate process limestone emissions from EAF steel, bringing its carbon debt down to a more manageable 0.02 tons of carbon dioxide per ton of steel. By comparison, new steel from new iron in the traditional coal-heavy process have 2 to 3 tons of carbon dioxide per ton of steel — 100 to 150 times more.

The resulting cement is very close to Portland cement, however, it’s not Portland cement exactly, notably being higher in iron oxide. As noted in the assessment of limestone calcined clay cement (LC3), Portland cement, its characteristics, and often precise compositions are written into regulations and building codes at all levels of government. Gaining approval for this EAF slag cement is far from trivial. It’s a very nice technical hack that recycles cement and displaces limestone, but a big change management process is required if it were to be used for structural elements in building and infrastructure. That’s actually a big if, and more on that later.

Further, while we scrap a lot of concrete today, the Cambridge team’s math varies from mine. They think that they could scale this process up to a billion tons a year, but as I worked out when looking at the Sublime Systems electrochemistry approach to doing exactly what the Cambridge team is doing, I could only find about two billion tons of concrete waste annually, representing about 5% of a single year’s use of concrete. It makes sense that we are building a lot more than we are demolishing. Only 10% to 15% of that is Portland cement. Assuming 10% is recovered, that’s only 200 million tons of potential resource.

There’s another challenge. While in my projection of iron and steel through 2100, we’ll be scrapping steel a lot more for new steel needs, just as the USA does today, it’s still only going to be about 1.7 billion tons made in EAF facilities. External references suggest they are still using about 50 kilograms of recycled cement as flux per ton of steel, the same amount as when limestone is used, which seems a bit odd, but I’ll go with it. If all EAF facilities used recycled cement as flux, that’s only 85 million tons of recycled cement per year.

To be clear, that would displace 85 million tons of carbon dioxide from cement manufacturing and another 37 million tons from EAF steel manufacturing, around 120 million tons or 0.3% of total global emissions of carbon dioxide. That’s 2% of cement manufacturing emissions. That’s pretty good, but it’s an order of magnitude off of what the Cambridge team is claiming. It’s unclear how they worked up those numbers.

As I noted, EAF slag is used for a lot of stuff today as well, and there are competitors for its use, like Sublime Systems. That firm’s process isn’t quite as virtuous as displacing limestone entirely, but it can be used against limestone, cement, and EAF slag, so like the comparison to Brimstone, it appears inherently more flexible. Still no idea of whether it can be seriously scaled, of course, but it does look like a very promising, low-heat, electrochemistry hack to achieve new lime for cement without any regulatory issues about Portland cement.

Carbicrete uses steel slag from EAFs as a key ingredient. They mix this slag with regular concrete ingredients and then inject carbon dioxide into the mixture in a sealed chamber. The carbon dioxide reacts with the steel slag to form a solid binding material that holds the concrete together. This not only uses up CO2 that would otherwise be in the atmosphere, but also skips the whole limestone-heating step, which is a big source of emissions in regular cement production. They create molded concrete blocks and other precast elements with this process.

About 20% to 25% of all concrete is used for precast elements, so this is a big wedge. And as a bonus, while Portland cement is specified in regulatory codes for structural elements of buildings, it typically isn’t specified for precast elements. That means that this process and the Cambridge team’s can be used for precast elements, and further the volumes will nicely fit into the supply chain for that space. It’s possible that some combination of the Carbicrete process and the Cambridge process would best work together, with recycling of cement displacing limestone and the Carbicrete process being applied on the result. Hard to say, but it’s good that there are multiple options.

The DRI-EOS project, coordinated by the FEhS Institute, brings together several partners, including Salzgitter Mannesmann Forschung GmbH, the Federal Institute for Materials Research and Testing (BAM), Friedrich Rohstoffe GmbH, Holcim Deutschland GmbH, and LOI Thermprocess GmbH. The project is working on how best to use EAF slag for cement as well. Their hack appears to be playing with the chemistry of inputs to the EAF process to adjust the slag chemistry to be more aligned with cement, then rapidly cooling the slag to make something more like clinker, so that it can be ground, mixed, and bagged directly. Still limestone, it appears, but a cleaner process in other ways.

Steel giant ArcelorMittal is doing something with this as well, but its unique approach to the question is not public, or at least I couldn’t find it. As one of the world’s larger producers of waste from steel, however, it’s in its interest to make that waste stream as valuable as possible. Currently a lot of slag ends up under railway tracks as ballast, which probably isn’t something that makes the company a lot of money.

Cement giant Halcim has been using EAF slag for years in its Envirocore Series, with about 41 million tons of other industries’ waste incorporated in its products as of 2018. As noted at the beginning of this piece, cement nerds have been looking at EAF slag for a long time.

Global integrated chemical plant giant BASF is using EAF slag as well. It doesn’t manufacture cement, but it does manufacture the admixtures that get mixed with cement, water, sand, and aggregate to make concrete. Clearly the end game for EAF slag cement startups is to have a Halcim, ArcelorMittal, or BASF acquire them, work out their golden handcuff period and retire to a cement-free beach in the Bahamas. I assume the biggies have chapter and verse on all of the different startups and research projects under way, and each have their own scoring system for potential acquisition.

I also assume that at least some EAF plant operators are getting annoyed by all of the cement nerds calling them all the time. A lot of cement decarbonization pathways run through EAF plants, so a lot of cement dust gets mixed in with the steel already.