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Tyres transformed into graphene for tougher concrete

Scientists at Rice University have demonstrated a process to convert waste from old rubber tyres into graphene which can, in turn, be used to strengthen concrete.

The environmental benefits of strengthening concrete – such as by adding graphene – are well established: “Concrete is the most-produced material in the world, and simply making it produces as much as nine per cent of the world’s carbon dioxide emissions,” said nanotechnology expert Professor James Tour. “If we can use less concrete in our roads, buildings, and bridges, we can eliminate some of the emissions at the very start.”

The unrivalled properties of concrete as a construction material makes it challenging to build without this material, so efforts to cut the carbon footprint associated with construction could involve making stronger forms of the material to extend its lifetime and reduce the quantity that must be produced.

Graphene has been shown to strengthen cementitious materials including concrete at the molecular level. Graphene-reinforced concrete has greater compressive strength and water resistance (due to the low permeability of graphene). It also minimises the alkali-silica reaction: the swelling resulting from moisture infiltration which can lead to cracking.

Now, Tour and his colleagues have optimised a process for converting waste from rubber tyres into graphene for strengthening concrete. Recycled tyre waste is already used as a component of Portland cement; reclaiming even a fraction of the 16 per cent of tyres which end up in landfills rather than incinerators could “keep millions of tyres from reaching landfills,” Tour said.

Tour and his colleagues developed a “flash” process in 2020, which can be used to convert food waste, plastic, and other carbon sources by exposing them to a jolt of electricity which purifies the sample. This leaves behind a “turbostratic” graphene structure – with layers slipped out of alignment – which is more soluble than graphene produced via exfoliation from graphene. This makes it easier to use in composite materials.

Rubber was more challenging to turn into graphene. The scientists optimised the process by using commercial pyrolysed waste rubber from tyres. After useful oils are extracted from the rubber, this carbon residue has until now had minimal value. Tyre-derived carbon black can be flashed into graphene.

The laboratory flashed tyre-derived carbon black with pulses lasting between 0.3 and 1 seconds, and found that they could turn approximately 70 per cent of the material into graphene. When flashing shredded rubber tyres mixed with carbon black for conductivity, about 47 per cent was converted to graphene. The electricity used in the process would cost about $100 per ton of starting carbon.

Because turbostratic graphene is soluble, it can be added easily to cement for stronger, more environmentally friendly concrete.

Tour and his colleagues tried blending minute quantities of tyre-derived graphene (0.1 weight percent for tyre-derived carbon black and 0.05 weight percent for carbon black and shredded tyres) with Portland cement. They used this to produce concrete cylinders. After curing for seven days, the cylinders showed gains of 30 per cent or more in compressive strength. After curing for 28 days, 0.1 weight per cent of graphene was sufficient to give both versions strength gain of at least 30 per cent.

“This increase in strength is in part due to a seeding effective of 2D graphene for better growth of cement hydrate products, and in part due to a reinforcing effect at later stages,” said Rouzbeh Shahsavari of C-Crete Technologies.

Earlier this year, researchers found the rare mineral aluminous tobermorite – credited with allowing marine concrete barriers built during the Roman empire to survive more than 2,000 years – in the thick concrete walls of a decommissioned nuclear power plant in Japan. The mineral boosted the strength of the plant walls by more than three times their design strength, following years of full operation.

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