Greener concrete made with waste materials

Globally, around three tonnes of concrete is used per person annually, a quantity that takes a dramatic toll on the environment.

Traditional concrete is composed of water, cement and a filler material such as sand. The cement industry alone is responsible for about 8 per cent of the world’s carbon dioxide emissions and production of concrete consumes 10 per cent of the world’s industrial water.

A team at the National University of Singapore (NUS) has been trying to develop greener, more sustainable concretes with the aim of reducing its immense impact.

They have shown that they can drastically reduce the amount of sand needed in the concrete mixture by using a common clay material that can easily be obtained as a waste from excavation works.

The researchers first obtained excavated waste clay from construction sites in Singapore. The waste clay was heated to 700° Celsius to ‘activate’ it and enhance its bonding ability in concrete.

The activated clay was used to replace up to half the fine sand powder typically used in concrete. The researchers were then able to produce ultra-high performance concrete (UHPC) - an incredibly strong type of concrete that can reduce the size of the structural elements, and potentially reduce the amount of concrete used.

Replacing the fine sand powder is especially advantageous as this material is expensive, has a large carbon footprint and is carcinogenic with prolonged exposure since it contains silica.

In addition, the NUS team also found that replacing part of the sand filler with activated waste clay did not have a significant effect on the strength of the UHPC.

Tunnelling and foundation works, which are common in Singapore, generate a large amount of excavation waste materials. Disposing of this waste clay is problematic as land-scarce Singapore has limited available space for landfill.

“Our discovery not only reduces the consumption of valuable resources, but also promotes a circular economy with the utilisation of waste clay. It opens an avenue to transform this waste into a potential resource,” said associate professor Pang Sze Dai.

This is the first time low-grade waste clay has been used as filler in concrete.

“Globally, low-grade clay is abundant. Its multi-faceted utilisation in concrete as fillers can not only help curtail the carbon footprint of concrete, but also reduce the cost of concrete production,” Pang added.

The team is now looking into using waste clay for more concrete applications and their latest study showed that the use of waste clay resulted in substantial improvement in the durability of concrete.