Researchers have found a way to take waste concrete from demolition sites and turn it into fresh new concrete that has a strength not seen before from such a product. The breakthrough could lead to significant emissions reductions in the building sector.
Concrete has been called “the most destructive material on Earth” by The Guardian thanks to the huge negative impact it has on our planet. To make concrete, minerals need to be crushed and cooked in a kiln heated to temperatures above 2,600 °F (1,427 °C) to produce a material known as klinker. The chemical processes involved in the formation of klinker creates carbon dioxide (CO2), a known greenhouse gas, as does the burning of fossil fuels needed to get the kilns hot enough to do their job. As a result, concrete production is responsible for about 8% of global CO2 emissions each year.
Yet concrete is the second-most used substance on Earth after water.
For years now, researchers have been using innovative approaches to try to make concrete more green. One technique uses electrolysis to replace the blast furnaces and create a zero carbon form of material. Another uses biochar to create concrete that absorbs more CO2 from the atmosphere than is used to create it. And still another replaces the sand used in concrete with a carbon-capturing material made from seawater.
Another popular approach to greening concrete is to use recycled materials in its creation such as discarded clay, irradiated plastic water bottles, or even old concrete itself.
The problem with recycling concrete, say researchers from Princeton University and the University of São Paulo (USP), is that the new concrete it creates is simply not as strong as the original. So, they set out to tackle this problem.
Thermoactivation
They took existing concrete waste and pulverized it, turning it into a fine powder. Then they cooked it at 932 °F (500 °C), which is about a third the temperature needed to create the original batch of concrete. They found that this temperature was high enough to dehydrate the cement powder while not being so hot that it destroyed the carbonate components it contained, which would cause the release of CO2.
When they used the new “thermoactivated” cement to make new concrete, they found that it was simply not strong enough due to the fact that it was more porous than regular cement. The solution, they found, was to add about 20% of finely ground fresh Portland cement or limestone to the mix. This filled in the pores and created an end product on par with current industry standards.
“Previously, if you only used thermoactivated recycled cement, it didn’t perform well enough to be an acceptable replacement,” said study co-author, Claire White from Princeton. “But by lowering the surface area and optimizing the packing of particles in the material’s microstructure, we get something that behaves quite comparably to Portland cement.”
The researchers estimate that the new material could cut emissions from the cement industry by up to 61%. They note that their cement produces between 198 and 320 kilograms of carbon dioxide per metric ton, resulting in emissions up to 40% lower than even those of limestone calcined clay cement (LC3), a commercially available low-carbon alternative.
“Construction waste typically ends up either in a landfill, or, if it’s recycled, will be used in low-grade applications such as in pavements or in soils,” said research leader Sérgio Angulo, from USP. “It’s exciting to show that we can, in fact, recycle this recovered cement waste into a high-quality application.”
The team sees its technique having a role in a more circular economy in concrete production where old concrete is harvested, treated, and used to create new buildings. To do that though, the researchers say that thought needs to be given to revamping building practices and changing the way construction waste is sorted after a demolition project.
Their study has been published in the journal ACS Sustainable Chemistry & Engineering.
Source: Princeton Engineering