Environmentally-friendly concrete cuts carbon emissions by two thirds

A climate-friendly cement has been developed that produces up to two-thirds fewer carbon emissions during its production than conventional cement.

Researchers at the Martin Luther University Halle-Wittenberg (MLU) in Germany and the Brazilian University of Pará made use of waste material from bauxite mining as a raw material to create their new cement, which was found to be just as stable as the traditional Portland cement.

According to estimates, almost six billion tonnes of cement were produced worldwide in 2020 and it is the source of around 8 per cent of the world's carbon dioxide (CO2) emissions, according to think tank Chatham House.

“Portland cement is traditionally made using various raw materials, including limestone, which are burned to form so-called clinker,” explained MLU professor Herbert Pöllmann. “In the process, the calcium carbonate is converted into calcium oxide, releasing large quantities of carbon dioxide.”

Since CO2 is a greenhouse gas, researchers have been looking for alternatives to Portland cement for several years. One promising solution is calcium sulphoaluminate cement, in which a large portion of the limestone is replaced by bauxite.

However, bauxite is a necessary raw material in aluminium production and not available in unlimited quantities so the little sought after Belterra clay was used instead. As Belterra clay has to be removed anyway during Bauxite mining, its extraction is not carried out only for cement production.

“This layer of clay can be up to 30 metres thick and covers the bauxite deposits in the tropical regions of the earth, for example in the Amazon basin,” said Pöllmann. “It contains enough minerals with an aluminium content to ensure good quality cement. It is also available in large quantities and can be processed without additional treatment.”

Even though cement cannot be entirely produced without calcium carbonate, at least 50 to 60 per cent of the limestone can be replaced by Belterra clay. The process has another environmentally relevant advantage as the burning process only requires 1,250°C, which is 200°C less than for Portland cement.

“Our method not only releases less CO2 during the chemical conversion, but also when heating the rotary kilns”, Pöllmann said. By coupling these effects, CO2 emissions can be reduced by up to two-thirds during cement production.

In extensive laboratory tests, the mineralogists were able to prove that their alternative cement meets all the quality requirements placed on traditional Portland cement. Further research projects will now investigate whether there are also overburden sources in Germany suitable for cement production.

“Raw materials containing clay minerals with a lower aluminium content could be used particularly in construction projects where lower-grade concrete is sufficient,” Pöllmann added. “There is still huge potential here to further reduce carbon dioxide emissions.”

In June, a new technique was developed to visualise concrete’s reaction with water as it is mixed that could help scientists make it more sustainable.

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