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Researchers have developed a new method to produce carbon-neutral cement on a large scale. It consists in particular of the use of electric arc furnaces for the recovery of steel, in order to recycle it at the same time as the used cement. The resulting “reactivated cement” enables the production of high-quality concrete, the production of which would be completely carbon neutral with the use of renewable energy sources.
Composed of sand, gravel, water and cement, concrete is the most widely used building material in the world. Its production alone is responsible for 7.5% of global CO2 emissions. Although cement as a binder makes up only a small proportion of concrete, it is responsible for almost 90% of the material’s total carbon emissions. It is produced by ‘clinkering’, a very energy-intensive process in which limestone and other raw materials are crushed and heated to 1450°C.
Efforts to reduce carbon emissions from cement generally consist of either improving the energy cycle of its production (using biofuels, for example) or using alternative materials such as fly ash and slag. Fly ash is friable minerals that are formed by electrostatic or mechanical precipitation of mineral particles contained in the flue gases of boilers using pulverized coal. Slag is the solid residue from the smelting of metal ores.
However, these options do not achieve net zero emissions. Actually,
substitute materials the by-products of heavy industry with high carbon emissions are the most used. On the other hand, other substitutes still require the addition of clinker (the main component of cement) for their chemical activation. This makes it difficult to use them for the production of large quantities.
” It is a matter of quantity: we do not physically have enough of these alternatives to meet the global demand for cement, which is about four billion tons per year. », explains ua blog post from the University of Cambridge Julian Allwood, from the engineering department of that institution.
Despite the available alternatives, reducing carbon emissions from the cement industry remains one of the biggest challenges in terms of decarbonisation. Heavy industry generally represents the most difficult sectors to decarbonize not only because of high energy consumption, but also because of very long investment cycles, as we reported in a previous article.
research article.
According to Allwood, “We’ve already identified simple alternatives that help us use less cement through careful mixing, but to get to zero emissions we need to start thinking outside the box.” To achieve this, he and his colleagues are proposing a new process that would completely eliminate carbon emissions from cement production.
One billion tons of recycled cement by 2050
To develop their new process — described in the magazine
Nature — Allwood and his colleagues investigated the hypothesis that it is possible to crush used concrete and recover cement by removing sand, water, and gravel and then heating the remaining product (cement utilization). The use of a metal bath would facilitate the chemical reaction to activate the cement. They then hypothesized that the process could be carried out in electric arc furnaces commonly used for steel recycling. The chemical composition of the cement used is very similar to that of the lime used for metal alloy recycling.
The conventional process of recycling steel involves heating it by mixing it with lime. The latter attracts and removes impurities from the steel by causing them to float on the surface of the molten mixture. This creates a film that protects the purified steel from the oxygen contained in the air. Then simply remove the film after cooling and collect the remaining product.
The new study’s process involves replacing lime with used cement powder. The clinker process also requires a combination of oxides found in all types of cements used. By applying the process to concrete waste from the demolition of old buildings, the researchers discovered that the resulting clinker has almost the same chemical composition as conventional clinker. Floating above the recycled steel, the used cement powder forms a liquid slag that is virtually identical to clinker when cooled outdoors and then crushed.
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” If you get the balance right and cool the slag fast enough, you’ll get reactivated cement, with no additional costs in the steelmaking process », explains the main author of the study, Cyrille Dunant. Experts also found that recycled cement contains higher levels of iron oxide than conventional cement, but this would have little effect on its mechanical performance.
Analysis of the performance of selected clinkers produced by the new process. haveInstantaneous and cumulative heat released by high and medium alite cement and high C3A cement produced by the new process, commercial clinker crushed under the same conditions and commercial cement produced with the commercial clinker specified in the reference title. bCumulative and frequency curves of granulometric composition of cements used for strength testing. in regards toDevelopment of the resistance of cements produced according to the new process, both in the form of pure cements and LC3 mixtures. d, Slag poured from the furnace, fresh and hardened mortar bars; sample after compression failure. © Cyrille F. Dunant et al.
The process was tested with kilns capable of producing several tens of kilograms of cement. By using furnaces powered by renewable energy sources, the resulting cements would be completely carbon neutral. ” As well as representing a major step forward for the construction sector, we hope the process will also be a signal to help the government recognize that the opportunities for innovation on our journey to zero emissions extend far beyond the energy sector Allwood suggests.
The researchers said the first industrial-scale tests are planned for this month, with the goal of eventually producing nearly 66 tons of cement every two hours. A patent application was also filed with the aim of commercializing the process. Ultimately, according to experts’ estimates, this would enable the production of one billion tons of recycled cement by 2050 (i.e. a quarter of current production).