Electric arc furnaces could turn old concrete into zero-carbon cement

Concrete and steel production are major contributors to global CO2 emissions, but a new technology from Cambridge University offers a promising solution: recycling both materials simultaneously. This innovative process involves using old concrete in steel-processing furnaces to not only purify iron but also generate “reactivated cement” as a by-product. When powered by renewable energy sources, this method has the potential to produce entirely carbon-neutral cement, according to the research published in the journal Nature.

The Cambridge researchers’ method may represent a significant breakthrough in the quest for net-zero emissions. They propose utilising the electrically powered arc furnaces employed in steel recycling to simultaneously recycle cement, the major carbon generator in concrete production. Concrete, the second-most widely used material globally after water, is responsible for roughly 7.5 percent of human-caused CO2 emissions. Developing a scalable and cost-effective approach to reducing concrete’s environmental impact while meeting global demand is a critical challenge in global decarbonisation efforts.

The researchers’ key finding is that used cement can effectively replace lime flux, a material used in steel recycling to eliminate impurities. Traditionally, lime flux becomes slag, a waste product. However, by substituting lime with used cement, the process yields recycled cement suitable for new concrete production.

This novel cement recycling method, detailed in the journal Nature, boasts several advantages. It doesn’t significantly increase production costs for either concrete or steel, while considerably reducing emissions from both industries due to the decreased reliance on lime flux. Recent trials conducted by the Materials Processing Institute, a project collaborator, successfully produced recycled cement at scale within an electric arc furnace, a first-of- its-kind achievement. With future electric arc furnaces powered by renewable energy sources, this method has the potential to create zero-emission cement.

Concrete is the cornerstone of global construction, but its production is a particularly dirty process. Concrete production alone accounts for around 8% of total global CO2 emissions. Unfortunately, recycling concrete into a usable form for new structures has proven challenging.

Scientists have been actively exploring methods to make concrete production more environmentally friendly. These approaches include altering the concrete recipe by replacing the most polluting ingredients, particularly limestone, or designing concrete that absorbs more CO2 from the atmosphere after construction. For this new study, Cambridge researchers investigated transforming waste concrete back into clinker, the dry component of cement, for reuse.

“Prior research hinted at the possibility of crushing old concrete, removing sand and gravel, heating the cement to eliminate water, and subsequently reforming it into clinker,” explained Dr. Cyrille Dunant, the study’s lead author. “We envisioned that a bath of molten metal could facilitate this chemical reaction, and an electric arc furnace used in steel recycling seemed like a strong candidate. We felt compelled to investigate this concept.”

An electric arc furnace necessitates a “flux” material, typically lime, to purify the steel. This molten rock captures impurities, rising to the surface and forming a protective layer that shields the newly produced pure steel from air exposure. Following the process, the used flux is discarded as waste.

The Cambridge method proposes replacing the lime flux with recycled cement paste. Remarkably, this substitution not only effectively purifies the steel, but when the remaining slag is rapidly cooled in air, it transforms into new Portland cement. The resulting concrete exhibits performance comparable to conventionally produced concrete.

Significantly, the team emphasizes that this technique doesn’t incur substantial cost increases for either concrete or steel production, while significantly reducing CO2 emissions compared to traditional production methods for both materials. With electric arc furnaces powered by renewable energy sources, this approach could essentially lead to zero-emission cement.

The technology has already undergone trials in furnaces capable of producing a few dozen kilograms of cement. The researchers are currently conducting the first industrial-scale trials, aiming to produce roughly 66 tons of cement within a two-hour timeframe. They project that the process could be scaled up to generate one billion tonnes of “electric cement” annually by 2050.

“The ability to produce zero-emission cement represents a significant breakthrough,” stated Professor Julian Allwood, who led the research. “However, we must also strive to reduce the overall amount of cement and concrete we use. Concrete offers advantages in terms of affordability, strength, and widespread availability, but our reliance on it is simply excessive. We possess the potential to dramatically reduce concrete consumption without compromising safety; however, achieving this goal necessitates political will.”

“Beyond its potential to revolutionize the construction industry,” Professor Allwood continued, “we hope that Cambridge Electric Cement will serve as a beacon, prompting the government to recognise the vast opportunities for innovation on our path to net-zero emissions, extending far beyond the energy sector.” A patent application has been filed for the process, signifying the initial step towards commercialization.