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New formula for green cement could slash emissions

A new formula for cement could drastically reduce carbon emissions in the construction sector while improving the strength and durability of concrete, researchers say.

Cement is made by cooking a calcium-rich material (usually limestone) with a silica-rich material (usually clay) before grounding it into a powder for use in concrete. The decarbonation of limestone and extreme heat used in the cement making process is responsible for most of concrete’s carbon output.

New molecular analysis of concrete – a mixture of sand, gravel, water, and cement – by a research team at MIT and CNRS in France, however, has found that reducing the ratio of calcium to silica in cement could cut emissions from the cement by more than half and at the same time create a stronger, more durable material.

The research, published in Nature Communications, found that the optimum calcium-to-silica ratio was lower than the current industry standard. The ratio in conventional cements ranged from 1.2 to 2.2, with 1.7 the standard. The resulting molecular structures, however, had never been compared in detail.

The researchers found that 1.5 parts calcium to one part silica was the optimum, what senior research scientist Dr Roland Pellenq said was “a magical ratio”, where the material could achieve “two times the resistance of normal cement, in mechanical resistance to fracture, with some molecular-scale design”.

As well as better mechanical strength, the less crystalline, glassier concrete would also be more fracture resistant as there would be “no residual stresses in the material”.

Because concrete production was estimated to represent up to 10 per cent of industrial carbon emissions, any reduction in calcium content in cement would have a significant impact on CO2 emissions, Dr Pellenq said, and could lead to as much as a 60 per cent reduction in concrete’s carbon output.

This is big news for the world’s number one construction material – with the volume produced estimated to be three times higher than steel.

“Cement is the most-used material on the planet,” Dr Pellenq said. “There’s no other solution to sheltering mankind in a durable way – turning liquid into stone in 10 hours, easily, at room temperature. That’s the magic of cement.”

The project has only been tested at the molecular level, and now will be tested at engineering scale applications.

”We have to make sure these nanoscale properties translate to the mesoscale,” Dr Pellenq said.

The work is the culmination of five years of research by a collaborative team from MIT and CNRS, where Dr Pellenq is research director.

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