Imagine a world where our cities’ very structures — the buildings, the roads, the bridges — do more than stand the test of time; they actively combat climate change. Thanks to ground-breaking research from the University of Cambridge, this vision is nearing reality with a revolutionary zero-emissions method for producing cement.

Concrete is the second-most consumed substance on Earth after water. Its primary ingredient, Portland cement, is responsible for approximately 7.5 per cent of global anthropogenic Carbon dioxide emissions. These emissions arise from two primary sources: the calcination process, where limestone (calcium carbonate) is heated to produce lime (calcium oxide), releasing CO2; and the combustion of fossil fuels to heat cement kilns to the high temperatures needed for this transformation.

The cement industry has not been idle in addressing its environmental impact. Efforts have included improving energy efficiency, reducing the use of clinker in cement mixes, and incorporating waste materials like fly ash and slag. However, these strategies, while beneficial, have not sufficed to neutralise the sector’s substantial carbon footprint. The ultimate goal, elusive until now, has been a truly sustainable form of cement that can be produced at scale without emitting any carbon dioxide.

A Paradigm Shift

The innovation presented by Cyrille F Dunant and colleagues in their paper, “Electric recycling of Portland cement at scale” is as simple in concept as it is revolutionary in application. It revolves around the integration of waste management and steel production processes to create a sustainable cycle for producing cement. The method leverages a technology familiar in steel making: the electric arc furnace (EAF). EAFs are pivotal in recycling steel, but the process typically introduces lime as a flux to remove impurities, which, like in cement production, generates considerable CO2.

The Cambridge team proposes using recovered cement paste (RCP)—cement that has been used and discarded from old buildings and roads—as a replacement for the lime traditionally used in steel recycling. Since RCP has already undergone decarbonation, it can be reintroduced into the production cycle without releasing further CO2. When added to the high-temperature environment of an EAF, RCP helps purify the steel while simultaneously being transformed back into a reactive form that can be used as the base for new cement.

The process detailed in the paper leverages the high temperatures of EAFs to facilitate the transformation of RCP into a reactive form suitable for creating Portland clinker. By mixing RCP with scrap steel in the EAF, the researchers were able to produce slag that, once cooled and ground, meets the specifications required for conventional Portland clinker. This slag can then be blended with other materials like calcined clay and limestone to produce new, environmentally friendly cement. The paper, published in Nature, shows that this new type of slag is capable of forming cementitious compounds essential for construction materials.

The paper suggests that, if powered by renewable energy sources, the process could lead to zero-emission cement. Furthermore, this approach could significantly decrease the environmental impact of the steel industry by reducing the need for lime flux, which is both cost-intensive and carbon-intensive. By integrating waste management with industrial production, this method not only addresses the issue of cement-related emissions but also enhances resource efficiency in two major industries simultaneously.

This method closes the loop of cement use and reuse, presenting a sustainable cycle that diminishes waste and emissions. The experimental results are promising: slags produced using RCP meet the specifications required for Portland clinker, the active ingredient in cement, and the final product demonstrates comparable performance to conventional cement in construction applications.

Cementing the future

Adopting this technology could significantly reduce global CO2 emissions, not just from cement but also from steel production, another major industrial emitter of carbon. The process could prove economically viable, potentially reducing the costs associated with both steel and cement production by minimising waste and the need for raw materials. Moreover, if powered by renewable energy, this method could achieve zero emissions, aligning with global carbon reduction goals.

Despite its potential, the widespread adoption of this method will require overcoming significant hurdles. Since RCP comes from diverse sources of construction and demolition waste, its chemical composition can vary widely, potentially impacting the consistency and quality of the resulting cement product. Integrating RCP into the finely tuned process of steel manufacturing requires careful adjustment of the steel recycling process to accommodate the unique properties of RCP without compromising the quality of the steel or the functionality of the EAFs. There are logistical challenges in collecting and processing RCP in sufficient quantities to meet industrial demands, as well as the need for rigorous testing and certification processes to ensure that the end products meet regulatory standards and are safe for use in construction.

The adoption of RCP in steel and cement production opens up substantial opportunities for environmental and economic advancements. This method represents a significant step towards a circular economy, where waste materials are reused, leading to reductions in landfill use and the extraction of virgin raw materials.

This could lower costs for both the steel and cement industries through reduced expenditures on raw materials and potentially lower energy costs if EAFs can operate more efficiently with RCP. Environmentally, the ability to recycle cement in a zero-emissions process (when powered by renewable energy) aligns with global initiatives aimed at reducing greenhouse gas emissions and combating climate change. This innovative recycling approach could foster new business models and create market opportunities for companies specialising in the collection, processing, and distribution of RCP, potentially spurring job creation in green technologies.

The implications of this research extend beyond the technical realms of industrial manufacturing. They touch on a vision for a sustainable future where our cities’ very skeletons—the beams, columns, and slabs—actively contribute to healing the environment. This innovative approach not only addresses the pressing issue of climate change but also reimagines waste as a valuable resource, setting a precedent for circular economies in other sectors.