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The ultimate path to zero-emission cement may be recycled cement

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The ultimate path to zero-emission cement may be recycled cement

By linking the recycling of two carbon-intensive materials—cement and steel—researchers have hit upon a breakthrough that dramatically cuts the climate impact of both.
June 4, 2024

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A newly developed method to recycle cement could yield a billion metric tons yearly by 2050, equivalent to about one-quarter of global annual production of this important building material, a new study suggests. What’s more, the method has the potential to achieve a holy grail of the green building industry: zero-emission cement.

The new study shows that large-scale cement recycling has practical potential says study team member Cyrille Dunant, an engineer at the University of Cambridge in the UK.

Cement is used to bind together sand, gravel, and water to make concrete. Concrete, in turn, is a foundation of global infrastructure: it’s the second-most-used material on the planet (behind only water).

Concrete is also responsible for about 7.5% of global greenhouse gas emissions, and is fiendishly difficult to decarbonize.

The climate problem with concrete pretty much comes down to cement. Nearly 90% of concrete emissions arise from the production of cement, which is made by crushing limestone and other materials and heating them to extremely high temperatures. This requires a lot of energy, and the consequent transformation of limestone into lime releases large amounts of carbon dioxide.

Past efforts to shrink the carbon footprint of concrete production have focused on finding materials such as fly ash to substitute for cement. But such strategies still require some use of cement, and the alternative materials aren’t available in high enough volumes for the global industry: the strategy essentially tinkers around the edges of what remains a behemoth problem.

 

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The new approach instead relies on an extremely abundant material: concrete itself. It is already possible to separate cement from the other ingredients of used concrete, but until now there has been little market for this recovered cement paste.

The spark for the new method came from the realization that recovered cement paste has a lot in common with lime flux, another carbon-intensive material made from limestone, which is floated on top of melted steel to remove impurities during steel recycling.

In conventional steel recycling, used lime flux results in a waste product called slag. The new method instead employs used cement to remove impurities in the steel, and results in recycled cement that can then be used to make new concrete.

“The performance of cement as a flux was much better than expected,” Dunant reports.

High temperatures during the steel recycling process “re-clinker” the cement, which refers to the reactivation of oxides that are key to its binding function. In a paper published in the journal Nature, Dunant and his collaborators describe using an electrically powered arc furnace, an apparatus that is already commonly used in steel recycling, to accomplish this process.

This is the second key takeaway of the study, Dunant, says: “clinker production can be electrified.” If the arc furnace is powered by renewable electricity, the result could be zero-emission cement.

By reducing the need for lime flux, the new method also reduces the carbon emissions involved in recycling steel. What’s more, the method adds little to the cost of either concrete or steel.

The recycled cement contains a higher level of iron oxide than conventional cement, but this shouldn’t affect its performance, the researchers say.

Widely implementing the process will be far from straightforward. “It’s always difficult to do co-production, particularly on a large scale, so the commercial route still needs to be understood, and what trade-offs will be required ‘in the real world’ between steel and cement production are still unknown,” Dunant says.

Still, the findings suggest the heartening conclusion that sometimes the way to solve a difficult environmental problem is to tackle two at the same time.

Source: Dunant C.F. et al.  “Electric recycling of Portland cement at scale.” Nature 2024.

Image: ©Anthropocene Magazine, AI-generated image.

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