Waste doesn’t always look like discarded soda bottles or piles of unwanted newspaper. Humans are constantly altering their environments to serve a new purpose or accommodate a new need. We tear down, erect, and renovate buildings constantly, and the result is millions of tons of building rubble. Until recently, the only way to recycle concrete waste was to smash it up and use it as a base layer for roads. Now, researchers from the Fraunhofer Institute for Building Physics have developed a better way: use lightning to break separate concrete debris into its most basic, and reuseable, parts.
Concrete is extremely versatile, which is why it’s the world’s most popular building material. Created by mixing cement, water and aggregate, and a mixture of stone particles such as gravel or limestone grit in various sizes, concrete is cheap and easy to use. Unfortunately these positive aspects have a big, dirty downside. According to the Fraunhofer Institute, “the production of one ton of burned cement clinker of limestone and clay releases 650 to 700 kilograms of carbon dioxide.” This means that every year 8 to 15 percent of global CO2 production is attributable to concrete manufacturing.
The key to recycling concrete, and curbing some of these harmful carbon emissions, is efficiently reducing concrete rubble into ingredients that can then be mixed into new concrete. The process developed by the Fraunhofer researchers uses electrodynamic fragmentation, very short pulses (less than 500 nanoseconds) of induced lightning, to separate gravel from cement materials in concrete.
When the lightning strikes the concrete debris, it runs along the path of least resistance which is the boundaries between the components, i.e. between the gravel and the cement stone. The initially generated impulses, the pre-discharges, first weaken the material mechanically. “The pre-discharge which reaches the counter-electrode in our fragmentation plant at first, then causes an electrical breakdown,” explains Volker Thome from the Fraunhofer Institute for Building Physics IBP at the Concrete Technology Group in Holzkirchen. “At this instant a plasma channel is formed in the concrete which grows within a thousandth of a second, like a pressure wave from the inside outwards.” The force of the explosion quickly and efficiently breaks down concrete in a fraction of the time it would take for traditional methods. The researchers have set a goal of 20 tons per hour which they say could be reached in just two years’ time.