From downcycling to recycling: Using lighting to separate cement particles from stone

Oct 01, 2012
Researchers have developed a method of breaking down concrete into its constituent parts. Credit: Fraunhofer IBP

Every year several millions of tons of building rubble are produced. An efficient way of recycling concrete – the building material of the 20th and 21st century – does not yet exist. Researchers are working on new recycling methods, and with the aid of lightning bolts, they can break down the mixture of cement and aggregate into its components.

Whether the Pantheon in Rome or the German concrete canoe regatta, whether ultra-light or decorative: concrete is unbelievably versatile and is the world's most widely used material – next to water. It is made of cement, water and aggregate, a mixture of stone particles such as gravel or limestone grit in various sizes. However, the CO2 emissions, which are mainly the result of , are problematic: 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. And when it comes to recycling waste concrete, there is no ideal solution for closing the materials loop. In Germany alone the quantity of amounted to almost 130 million tons in 2010.

"This is an enormous material flow, but at the moment there is no effective recycling method for concrete rubble" explains Volker Thome from the Fraunhofer Institute for Building Physics IBP from the Concrete Technology Group in Holzkirchen. The current method is to shred the concrete, which produces huge amounts of dust. At best, the stone fragments end up as sub-base for roads. "This is downcycling," explains Thome, in other words, simply the reutilization of raw materials, the quality of which deteriorates from process to process. On the other hand, if it were possible to separate the stone particles from the cement stone, the gravel could easily be reused as an aggregate in new cement – a first decisive step in the direction of recycling waste 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," says Thome. If it were also possible to obtain a cement substitute from waste concrete, the cement industry's CO2 emissions would be considerably reduced. To achieve these goals Thome revived a method that Russian scientists already developed in the 1940s then put on ice: electrodynamic fragmentation. This method allows the concrete to be broken down into its individual components – aggregate and cement stone.

Recycling valuable components

Using this approach, the researchers in Holzkirchen are unleashing a veritable storm of lightning bolts. "Normally, lightening prefers to travel through air or water, not through solids," says Thomas. To ensure the bolt strikes and penetrates the concrete, the expert uses the Russian scientists' expertise. More than 70 years ago they discovered that the dieletric strength, i.e. the resistance of every fluid or solid to an electrical impulse, is not a physical constant, but changes with the duration of the lightning. "With an extremely short flash of lightning – less than 500 nanoseconds – water suddenly attains a greater dielectric strength than most solids," explains Thome. In simple terms, this means that if the concrete is under water and researchers generate a 150 nanosecond bolt of lightning the discharge runs preferably through the solid and not through the water." That is the essence of the method," says Thome. In the concrete the lightning then runs along the path of least resistance which is the boundaries between the components, i.e. between the gravel and the 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 Thome. 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 this pressure wave is comparable with a small explosion," says Thome. The concrete is torn apart and broken down into its basic components. With the laboratory fragmentation plant the researchers can currently process one ton of waste per hour. "To work efficiently, our goal is a throughput rate of at least 20 tons per hour," says Thome. In as little as two years' time, an appropriate installation could be ready for market-launch.

Explore further: Lifting the brakes on fuel efficiency

add to favorites email to friend print save as pdf

Related Stories

Recipe for success: Recycled glass and cement

Feb 21, 2012

(PhysOrg.com) -- Michigan State University researchers have found that by mixing ground waste glass into the cement that is used to make concrete, the concrete is stronger, more durable and more resistant ...

Green Ideas: Making Concrete from Rice

Jul 21, 2009

(PhysOrg.com) -- Concrete accounts for about 5% of all human-related CO2 emissions. The fact that we use so much cement in building could mean that the issue becomes even more pronounced in the future. Bu ...

Novel experiments on cement yield concrete results

Apr 02, 2007

Using a brace of the most modern tools of materials research, a team from the National Institute of Standards and Technology and Northwestern University has shed new light on one of mankind’s older construction materials—cement.

Recommended for you

Lifting the brakes on fuel efficiency

Apr 18, 2014

The work of a research leader at Michigan Technological University is attracting attention from Michigan's Governor as well as automotive companies around the world. Xiaodi "Scott" Huang of Michigan Tech's ...

Large streams of data warn cars, banks and oil drillers

Apr 16, 2014

Better warning systems that alert motorists to a collision, make banks aware of the risk of losses on bad customers, and tell oil companies about potential problems with new drilling. This is the aim of AMIDST, the EU project ...

User comments : 0

More news stories

Ex-Apple chief plans mobile phone for India

Former Apple chief executive John Sculley, whose marketing skills helped bring the personal computer to desktops worldwide, says he plans to launch a mobile phone in India to exploit its still largely untapped ...

A homemade solar lamp for developing countries

(Phys.org) —The solar lamp developed by the start-up LEDsafari is a more effective, safer, and less expensive form of illumination than the traditional oil lamp currently used by more than one billion people ...

NASA's space station Robonaut finally getting legs

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...