Bolts of lightning could make concrete recyclable

It’s strong, versatile and — after water — the world’s most widely used material. But concrete also comes with two major downsides: it’s a major contributor to global greenhouse gas emissions (anywhere from 8 to 15 percent of carbon dioxide emissions) and it’s near-impossible to efficiently recycle.

Solve the second problem, and we could make a big dent in addressing the first. And new research by scientists in Germany might have the answer: lightning.

Using electrodynamic fragmentation — a method developed by Russian researchers in the 1940s and then abandoned — scientists at the Fraunhofer Institute for Building Physics have been able to break down one ton of concrete into its component parts (aggregate and cement stone) in one hour.

“To work efficiently, our goal is a throughput rate of at least 20 tons per hour,” said researcher Volker Thome. He estimates a market-ready technology could be as little as two years away.

Concrete has been hard to truly recycle because there’s been no easy way to separate the cement portion from the aggregate, which is a mix of stone particles of varying sizes. Instead, waste concrete today — if it’s reused at all — is shredded, which produces stony fragments that are suitable only for a few uses, such as in sub-base for roads. This means lots of waste concrete is never reused; Germany alone produced nearly 130 million tons of concrete construction waste in 2010.

If all the original aggregate could be separated from the cement, though, it could be reused to make new concrete.

“The recovery of valuable aggregate from waste concrete would multiply the recycling rate by a factor of around ten and thereby increase it to 80 percent,” Thome said.

The new process for separating concrete into its components works like this: aim a lightning-like bolt of electricity at concrete under water. (This ensures that the bolt “prefers” to travel through a solid, rather than its typical preference for air or water.) As it passes through the concrete, the lightning follows the path of least resistance — the boundaries between the cement and the aggregate. This weakens the bond between the two.

When the pre-discharge of electricity reaches a counter-electrode, a plasma channel then forms in the concrete and spreads from the inside out like a pressure wave.

“The force of this pressure wave is comparable with a small explosion,” said Thome. The result: the concrete is torn apart and broken down into its basic components.

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