FAQs

What is steel slag?

Steel slag is a byproduct generated during steel manufacturing. Although historically treated as a waste product, in the last few decades it has found use in several aggregate and construction applications, including as unpaved road aggregate, concrete aggregate, asphalt aggregate and as a partial substitution for cement. Nonetheless, globally, large quantities of steel slag remain in legacy piles at steel manufacturing sites or end up being landfilled. The three main types of steel slags are basic oxygen furnace (BOF), electric arc furnace (EAF) and ladle metallurgy furnace (LMF) slags, named after the types of steelmaking furnaces in which they are formed.
How much steel slag is produced every year?

Steel slag comprises approximately 10-20% of total steelmaking output. Global crude steel production totaled almost 1.9 billion metric tons in 2023. Global steel slag production is estimated to be between approximately 190 and 280 million metric tons a year. The U.S. is one of the world’s top steel producers, after China, India and Japan, and in 2023, was responsible for around 7% of global steel production.
How does steel slag remove carbon dioxide, and how permanent is this storage?

Similar to natural rocks, steel slags naturally react within their environment. The less stable minerals within steel slag break down, combine with water and atmospheric CO2, and produce stable carbonate (limestone) minerals. Carbonates are geologically long-lived with lifespans of over 100,000 years. More accurately, steel slags contain reactive minerals that dissolve upon contact with water and release calcium and magnesium ions. These ions react with dissolved CO2 from the atmosphere to form stable, inorganic carbonate minerals. Because these carbonate minerals have very low solubility, once they form, they remain in the ground and contribute towards permanent CO2 removal from the atmosphere.
How do you measure carbon dioxide removal at a project?

We aim to quantify carbon dioxide removal in our slag aggregate systems using a robust measurement, reporting, and verification (MRV) framework. Simply put, we measure the total volume of carbonate minerals after a period of time. Because steel slags have negligible carbonate content when they are produced, all measured carbonates must come from the natural geochemical reactions which remove CO2 from the atmosphere. We use a combination of methods to measure the stable and durable carbonate minerals in the slag aggregate layers. These include gas flux measurements, solid state characterization of slag aggregates with techniques such as X-ray diffraction, thermogravimetric analysis and total carbon measurements, and chemical analysis of leachate solutions that drain from the aggregate layers. This creates a quantifiable comparison of the realized carbonation from our slag deployments versus appropriate counterfactual scenarios that serve as baseline cases.
How do you ensure additionality in your carbon dioxide removal projects?

Additionality is the concept that a project provides extra environmental benefits, CO2 removal in our case, that wouldn’t otherwise exist. We ensure our projects result in additional removals in a variety of ways. Primarily, we compare our projects to “business-as-usual” scenarios. This includes using underutilized steel slag, developing proprietary material blends, and creating new deployment techniques. We also financially incentivize our partners to increase the number of total projects using steel slag. Our robust life-cycle analyses, simple MRV framework, and third-party verification ensures project additionality.

Steel slag is a byproduct generated during steel manufacturing. Although historically treated as a waste product, in the last few decades it has found use in several aggregate and construction applications, including as unpaved road aggregate, concrete aggregate, asphalt aggregate and as a partial substitution for cement. Nonetheless, globally, large quantities of steel slag remain in legacy piles at steel manufacturing sites or end up being landfilled. The three main types of steel slags are basic oxygen furnace (BOF), electric arc furnace (EAF) and ladle metallurgy furnace (LMF) slags, named after the types of steelmaking furnaces in which they are formed.
Steel slag comprises approximately 10-20% of total steelmaking output. Global crude steel production totaled almost 1.9 billion metric tons in 2023. Global steel slag production is estimated to be between approximately 190 and 280 million metric tons a year. The U.S. is one of the world’s top steel producers, after China, India and Japan, and in 2023, was responsible for around 7% of global steel production.
Similar to natural rocks, steel slags naturally react within their environment. The less stable minerals within steel slag break down, combine with water and atmospheric CO2, and produce stable carbonate (limestone) minerals. Carbonates are geologically long-lived with lifespans of over 100,000 years.

More accurately, steel slags contain reactive minerals that dissolve upon contact with water and release calcium and magnesium ions. These ions react with dissolved CO2 from the atmosphere to form stable, inorganic carbonate minerals. Because these carbonate minerals have very low solubility, once they form, they remain in the ground and contribute towards permanent CO2 removal from the atmosphere.
We aim to quantify carbon dioxide removal in our slag aggregate systems using a robust measurement, reporting, and verification (MRV) framework. Simply put, we measure the total volume of carbonate minerals after a period of time. Because steel slags have negligible carbonate content when they are produced, all measured carbonates must come from the natural geochemical reactions which remove CO2 from the atmosphere. We use a combination of methods to measure the stable and durable carbonate minerals in the slag aggregate layers. These include gas flux measurements, solid state characterization of slag aggregates with techniques such as X-ray diffraction, thermogravimetric analysis and total carbon measurements, and chemical analysis of leachate solutions that drain from the aggregate layers. This creates a quantifiable comparison of the realized carbonation from our slag deployments versus appropriate counterfactual scenarios that serve as baseline cases.
Additionality is the concept that a project provides extra environmental benefits, CO2 removal in our case, that wouldn’t otherwise exist. We ensure our projects result in additional removals in a variety of ways. Primarily, we compare our projects to “business-as-usual” scenarios. This includes using underutilized steel slag, developing proprietary material blends, and creating new deployment techniques. We also financially incentivize our partners to increase the number of total projects using steel slag. Our robust life-cycle analyses, simple MRV framework, and third-party verification ensures project additionality.

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