Zero-carbon cement process could slash emissions from construction

A new technique can produce cement using waste from demolished buildings, which researchers say could save billions of tonnes of carbon by 2050.

“We have definitely proved that cement can be recycled into cement,” says Julian Allwood at the University of Cambridge. “We are on course for making cement with zero emissions, which is amazing.”

Producing cement is highly polluting – responsible for 7.5 per cent of total greenhouse gas emissions – but until now there was no known way to produce it at scale without impacts on the climate.

Making cement requires “clinker”, which is made by heating a mix of raw materials, including limestone and clay, to 1450°C (2650°F). Both the heat requirements and the chemical reactions involved in making clinker result in carbon emissions, and clinker production accounts for 90 per cent of cement’s total carbon footprint.

Allwood and his colleagues have developed an alternative process to make clinker, which involves reusing cement paste from demolished buildings. This paste has an identical chemical composition to lime flux, a substance used to remove impurities from recycled steel.

As the steel melts, the flux made from old cement forms a slag that floats on the top of the recycled steel. Once ground into a powder, the slag is identical to clinker. It can then be used to make Portland cement, the most common form of cement.

If the recycled steel and cement are produced using an electric furnace, powered by renewable or nuclear energy, the process is almost entirely free of emissions. “The idea is really simple,” says Allwood.

Laboratory trials have proved the process works. It offers a “drop in” solution that could be used with conventional equipment, and a global switch to this process could save up to 3 gigatonnes of carbon dioxide a year, the team calculates.

The research team is now working on industrial trials via a spin-out company, Cambridge Electric Cement, with partners such as construction firms Balfour Beatty and Tarmac. “Within the next few weeks, we are starting a set of trials which will be producing batches of 30 tonnes per hour,” says Allwood.

Scaling up the new cement-making process depends in part on the growth of recycled steel-making, which currently accounts for about 40 per cent of global steel production. Allwood says production rates will at least double over the next 30 years, and most likely treble, as the industry decarbonises.

Yet some challenges lie ahead. The recycled cement process requires furnace temperatures of 1600 to 1750°C (2900 to 3200°F), slightly hotter than traditional cement production. This will increase power costs, says Leon Black at the University of Leeds, UK.

Other hurdles include establishing supply chains for waste cement, attracting the necessary capital investment and convincing a notoriously cautious industry to adopt a new process on a large scale.

“They have overcome one barrier in as much as they have made a material that has the same composition as Portland cement,” says Black. “The devil is in the details: the energy requirements, the logistics, the scaling up.”