Cement process could save energy and CO2

Researchers at Karlsruhe Institute of Technology (KIT) have a developed a new way of making cement from limestone that requires significantly less energy and raw material, and produces much less carbon dioxide.

The new process takes place at 220C in an autoclave, instead of in a rotary kiln at 1450C, and could yield overall CO2 and energy reductions of 50 per cent, claimed KIT's Dr Hanns-Günther Mayer. He said that the institute has set up a joint-venture company called Celitement to commercialise the development, which has been protected by patents.

The KIT researchers analysed the chemical reactions that take place when calcium carbonate (from limestone) is first kilned into Portland cement and then hydrated to make it harden. They discovered that the traditional process is relatively inefficient, yielding three molecules of CO2 and two of calcium hydroxide for every useful calcium-silicate binder molecule produced (although the calcium hydroxide then slowly absorbs CO2, turning back into calcium carbonate over time).

By changing the process to use an autoclave and a grinder, they were able to produce a partially hydrated form of cement that contains no calcium hydroxide and needs far less water to make it set. It also requires one-third as much limestone feedstock per binder molecule produced.  

“No-one had thought of having some water already in the cement,” Mayer said. He added that although the partially hydrated powder must be kept dry, it does not need to be treated or stored any differently from Portland cement.

Because the new cement is almost pure calcium-silicate binder, it will need more added sand than Portland cement would when making concrete or mortar.  However, the researchers still need to do characterisation and testing work to determine how much more, Mayer said.

He said that Celitement is now building a pilot plant for quantity production next year, and plans to develop a family of building products based on its new binders.

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