Production of traditional Portland cement uses large amounts of limestone, which is predominantly formed of calcium carbonate (CaCO3). Burning this limestone releases huge amounts of CO2. It’s estimated that the six billion tonnes of cement produced in 2020 accounted for around eight per cent of manmade carbon emissions across the globe.
"Portland cement is traditionally made using various raw materials, including limestone, which are burned to form so-called clinker," explained Professor Herbert Pöllmann from the Martin Luther University Halle-Wittenberg (MLU) Institute of Geosciences and Geography. "In the process, the calcium carbonate is converted into calcium oxide, releasing large quantities of carbon dioxide."
Described in the journal Sustainable Materials and Technologies, the new method developed by Pöllmann and his colleagues replaces most of the limestone with Belterra clay from Brazil, which is found next to bauxite deposits. Bauxite can be used in place of limestone to make calcium sulphoaluminate cement – a greener alternative to its Portland cousin. But bauxite is also a precious commodity in its own right, so the clay acts as an inexpensive proxy that performs a similar task and which is relatively abundant.
"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, who worked with colleagues from the Brazilian University of Pará on the project. "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."
Some calcium carbonate is still required to produce cement, but at least 50 to 60 per cent of the limestone can be replaced by Belterra clay, according to the researchers. In addition, the burning process only requires temperatures of 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.
Taking both the Belterra clay and the lower temperatures together, the team estimate an overall CO2 saving of at least two-thirds. They were also able to prove that their alternative cement meets all the quality requirements placed on traditional Portland cement.