Converting fly ash into environmentally friendly concrete

A new method for producing cement uses less energy and has fewer carbon and toxic emissions

Spotted: Cement is produced from a mixture of limestone and clay that is heated at very high temperatures and then ground up. The energy used in this heating is one of the reasons for cement’s massive contribution to greenhouse emissions – around eight per cent of the world’s CO2 emissions come from cement manufacturing. This is why the road to net zero will need to be paved with low-carbon cement. Now, a research team at Rice University may have a partial solution.

The team, led by chemist James Tour, started with fly ash – a powdery byproduct of coal-based electric power plants that is used in many concrete mixtures. They removed toxic heavy metals from the fly ash using a process called flash Joule heating, in which an electrical current is passed through carbon-containing materials, heating them to about 3,000 degrees Celsius in milliseconds. This is much more energy-efficient than standard processes. The purified fly ash is then added to cement.

In a recent paper, the researchers found that by replacing 30 per cent of the cement in a batch of concrete with the purified coal fly ash, the concrete became much stronger and more elastic. On top of this, it reduced greenhouse gas and heavy metal emissions by 30 per cent and 41 per cent, respectively. This is because less energy is used in the process and the vapourised heavy metals can be captured before they enter the atmosphere.

The ubiquity of cement means that any plan to achieve net zero needs to involve this material – one reason why we are seeing so much interest in improving its sustainability. Some innovations in this area we have seen recently include a carbon negative Portland cement and constructing lightweight concrete blocks using non-recyclable plastic waste.

Written By: Lisa Magloff