Scientists have optimized a process to convert waste from rubber tires into graphene that can, in turn, be used to strengthen concrete. Concrete is the most-produced material in the world and simply making it produces as much as 9 percent of the world’s carbon dioxide emissions. If less concrete can be used in roads, buildings, and bridges, some of the emissions can be eliminated at the start.
Recycled tire waste is already used as a component of but graphene has been proven to strengthen cementitious materials, such as concrete, at the molecular level. While the majority of the 800 million tires discarded annually are burned for fuel or ground up for other applications, 16 percent of them wind up in landfills.
The flash process 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 scientists optimized the process by using commercial pyrolyzed waste rubber from tires. After useful oils are extracted from waste tires, this carbon residue has until now had near-zero value. Tire-derived carbon black or a blend of shredded rubber tires 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 scientists flashed tire-derived carbon black and found about 70 percent of the material converted to graphene. When flashing shredded rubber tires mixed with plain carbon black to add conductivity, about 47 percent converted to graphene.
The electrical pulses lasted between 300 milliseconds and 1 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 tire-derived graphene — 0.1 weight/percent (wt%) for tire carbon black and 0.05 wt% for carbon black and shredded tires — with Portland cement and used it to produce concrete cylinders.
Tested after curing for seven days, the cylinders showed gains of 30 percent or more in compressive strength. After 28 days, 0.1 wt% of graphene sufficed to give both products a strength gain of at least 30 percent.