A process that turns waste from rubber tyres into graphene can be used to strengthen concrete, claim scientists at Rice University in Texas.
According to Rice chemist James Tour, there are clear environmental benefits of adding graphene to concrete.
“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,” he said in a statement. “If we can use less concrete in our roads, buildings and bridges, we can eliminate some of the emissions at the very start.”
Recycled tyre waste is already used as a component of Portland cement, but graphene has been proven to strengthen cementitious materials, concrete among them, at the molecular level.
Most of the 800 million tyres discarded annually are burned for fuel or ground up for other applications, but 16 per cent still wind up in landfills.
“Reclaiming even a fraction of those as graphene will keep millions of tyres from reaching landfills,” Tour said.
The ‘flash’ process introduced by Tour and his colleagues in 2020 has been used to convert food waste, plastic and other carbon sources by exposing them to a jolt of electricity that removes everything but carbon atoms from the sample.
Those atoms reassemble into turbostratic graphene, which has misaligned layers that are more soluble than graphene produced via exfoliation from graphite. That makes it easier to use in composite materials.
Rubber proved more challenging than food or plastic to turn into graphene, but the lab is said to have optimised the process by using commercial pyrolysed waste rubber from tyres. After useful oils are extracted from waste tyres, this carbon residue has until now had near-zero value, Tour said.
Tyre-derived carbon black or a blend of shredded rubber tyres and commercial carbon black can be flashed into graphene. Because turbostratic graphene is soluble, it can easily be added to cement to make more environmentally friendly concrete.
The research led by Tour and Rouzbeh Shahsavari of C-Crete Technologies is detailed in the journal Carbon.
The Rice lab flashed tyre-derived carbon black and found about 70 per cent of the material converted to graphene. When flashing shredded rubber tyres mixed with plain carbon black to add conductivity, about 47 per cent converted to graphene. Elements besides carbon were vented out for other uses.
The electrical pulses lasted between 300 milliseconds and one second. The lab calculated electricity used in the conversion process would cost about $100 per ton of starting carbon.
The researchers blended minute amounts of tyre-derived graphene – 0.1 weight/percent (wt%) for tyre carbon black and 0.05wt% for carbon black and shredded tyres – with Portland cement and used it to produce concrete cylinders. Tested after curing for seven days, the cylinders showed gains of 30 per cent or more in compressive strength. After 28 days, 0.1wt% of graphene was enough to give both products a strength gain of at least 30 per cent.
“This increase in strength is in part due to a seeding effect of 2D graphene for better growth of cement hydrate products, and in part due to a reinforcing effect at later stages,” Shahsavari said.