GREEN CONCRETE - IS LOW CARBON CEMENT POSSIBLE?

Worldwide, researchers are looking into ways to decarbonise concrete – is it possible to recycle cement without emitting any carbon?

The amount of concrete used in the world is second only to the quantity of water, with cement production the largest single industrial emitter of CO2?(2.8 Gtons/year of CO2?emissions).

An extremely high temperature is required to heat limestone (1,600°C) to create cement, so huge quantities of fossil fuels are burned to produce the necessary heat. Making 1kg of concrete emits nearly 1kg of CO2.

Increase in use of concrete

There are predictions that the number of buildings worldwide will double by 2060 – that is equivalent to a New York city being built every 30 days for the next 40 years (See “How to avoid a Climate Disaster ” by Bill Gates). As concrete is the most common construction material, there is potential for astronomically high carbon dioxide emissions.

Worldwide, researchers are looking into ways to decarbonise concrete. Carbon-capture technology to store trapped emissions in voids under the sea, pumping waste gas back into the concrete as it is made, or locking greenhouse gasses into buildings have all been explored. The latest decarbonising method, and with the most potential, is a way to recycle cement from demolished buildings.

Breakthrough in recycling of cement

Scientists have investigated various ways to recycle demolished concrete structures for years, but a recent breakthrough could be a giant step forward. Trials are now taking place to test the process at scale, that has the potential to reduce the carbon footprint of using concrete drastically.

The breakthrough in recycling of cement could allow new cement to be produced without emitting any CO2. The key is electric furnaces powered by green power (wind, solar or green hydrogen). The other part of the equation is not having to heat up limestone (CaCO3) to drive out the CO2 to create cement. Cambridge University scientists discovered that when used cement is exposed to high temperature, it can be reactivated and by piggy-backing onto the way steel is recycled, a green cement can be produced.

Companies such as Xeroc Ltd are pioneering concrete-to-concrete recycling. Potentially, this could work in synergy with Cambridge Electric Cement to reduce further costs, mitigate the carbon footprint of the overall process as well as providing material circularity.

The process to recycle steel requires chemicals to be floated on the surface of the molten steel to form a barrier between the molten liquid and the air. The barrier material prevents the molten steel from reacting with air which would create impurities. The floating material is called ‘slag’, and it was recognised that the barrier material which was being used has virtually the same composition as used cement.

Zero-carbon concrete

This new process of recycling cement effectively uses waste heat from recycling steel to reactivate the cement, and if the furnaces melting the steel are powered by renewables this is a zero-carbon process. The scientists are calling their re-energised cement, ‘electric cement’ and Cesla Steel, a company based in Cardiff, are about to replicate the process at full scale using their electric furnaces. This also reduces the slag having to be quarried too – so a win-win situation.

It is hoped that ‘electric’ cement will be cheaper than conventional cement, and that eventually, it could be produced in enough quantity to meet a quarter of the UK’s building needs, with the piggy-backing process to be adopted all over the world.

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