Recycled cement paste offers decarbonisation opportunities

Proceedings of the National Academy of Sciences (PNAS) have published a new study, which found that recycling cement paste is one of the cheapest and most efficient methods of reducing emissions from cement production. Led by experts at Imperial College London’s Department of Civil and Environmental Engineering, this research explores how waste cement paste that has undergone CO₂ mineralisation can be a cost-effective method of cement emissions reduction.

The study found that CO₂ mineralisation has the potential to reduce emissions from cement production by 15 per cent. This is the equivalent of 0.8 per cent of the world’s total greenhouse gas emissions in 2020.

Out of the 10 CO₂ mineralisation technologies assessed in the study, researchers found that recycled cement paste made from reclaimed concrete rubble was the most efficient and cost-effective. Cement paste acts as a glue and can bind materials such as gravel and sand to create concrete. The recycled cement paste is often collected from old, demolished infrastructure.

Imperial College London’s Rupert J Myers, lead author of the study, states, “In the fight against climate change, reducing emissions from the production of cement, and the construction industry more widely, is challenging. Our findings suggest that CO₂ mineralised cement paste could be a champion technology in helping us decarbonise the sector.”

Out of the 10 different CO₂ mineralisation technologies reviewed, only two were found to be economically and scientifically viable. For most of the technologies, there was limited evidence that they can reduce CO₂ emissions in practical applications, despite companies touting their effectiveness.

It was also found that the economical CO₂ mineralisation methods were between two and five times less expensive than carbon capture and storage (CCS) technologies.

An additional benefit of CO₂ mineralisation is that it is a permanent solution that is relatively simple to manage. Building materials can store CO₂ for hundreds of years, and possibly longer if materials are ‘re-recycled’ after that.

Co-author of the study, Justin D Driver from the Department of Chemical Engineering, says, “Although these findings are promising, it is important to realise that CO₂ mineralisation is not a silver bullet solution. There is a limitation on the amount of raw materials available that can absorb CO₂, meaning the potential for the technology to reduce emissions across all sectors is also limited.”

Elina Bernard, another co-author, added, “This study shows that the bottleneck for a wider application of CO₂ mineralisation of end-of-life concrete or other calcium-based industrial waste is the limited supply of carbonatable material. However, CO₂ savings of 15 per cent in the production of useful building products are not negligible, and research should continue to focus on reducing the costs and further advancing these carbon capture and utilisation (CCU) technologies.”