Urban mining is increasingly being taken seriously by industry because it gives access to materials—such as expensive metals used in electronics—that are buried in waste tips and landfills. However, there is a new kid on the block—literally. Concrete buildings, when demolished, can serve as an excellent source of new building materials. “Instead of transporting aggregates from far away, we can use local buildings as a source for aggregates,” says Francesco Di Maio, a researcher in waste separation and recycling technology at the Technical University of Delft in the Netherlands. He is also the coordinator of the EU-funded project called C2CA.

Concrete consists mainly of two components: cement, in the form of calcium silicates, and an aggregate, which can consist of sand, pebbles and small rocks of up to about 2 centimetres across. The recycling of concrete involves recuperating these two components. "The main aim of C2CA is to recycle the aggregates contained in the concrete into new aggregates that can be used to make new mortar and new concrete," Di Maio tells youris.com. The second component to be recycled, cement, is fed back into the kiln. The advantage of recycling the cement component is that it does not release CO₂ into the atmosphere. "When you reintroduce the calcium silicates into the kiln, the CO₂ is already released," says Di Maio.

Although this scheme looks simple, there are still obstacles to the recycling of concrete. One is the cost of separating the aggregates from the concrete. "We wanted to develop a process, which is attractive economically, [as] washing is too expensive," says Di Maio. The technique they developed, called Advanced Dry Recovery, separates the aggregates into three components using a ballistic effect. "The materials are subjected to an acceleration of up to 1,000 G, which separates the sand and dust from the stones, with the heaviest stones flying the furthest," explains Di Maio. 

Ironically, their biggest competitor in the Netherlands is the very cheap virgin aggregate obtained from the dredging of the Maas river. Also, doubts about the purity of the aggregate are still an obstacle for the acceptance of the recycled material. "Our customers would not like to see a small piece of wood sticking out of a concrete wall," Di Maio tells youris.com. Although they are perfecting the detection of impurities using optical techniques, tests have shown that the concrete made from recycled aggregate has better mechanical properties than concrete made using virgin aggregates, explains Di Maio. 

Besides lab experiments, the technology developed by the project has now been tested on a construction project in Groningen, the Netherlands. The concrete in a parking garage built on the site contains 20% of the aggregate obtained from two demolished high-rise buildings on the same site, reports Di Maio.

The building industry is increasingly taking on the so-called cradle to cradle concept. "Establishing a profile of what went into the building’s construction so that sixty, seventy years down the line those materials can be properly reclaimed, is a big issue," says Ben Dezark, a certification officer at the Cradle to Cradle Products Innovation Institute in San Francisco, USA.

Technically, the approach by the project researchers is not an issue. "They have sufficient technical analysis of performance; they are not going to have a problem in that area," says Douglas Mulhall, a researcher at the Environmental Protection Encouragement Agency (EPEA) in Hamburg, Germany. "The fact that they have combined the recovery of the concrete using an advanced demolition technique is important in itself. That is going in the right direction." He believes the project approach is not, as yet, getting widespread attention from industry. 

But concerning how this technology will develop in the future, there still remains one question, according to Mulhall: "What happens when you scale up the technology a hundred fold, and integrate it with a design of disassembly of concrete buildings with prefabrication?" And here there is the caveat. "As more and more buildings are required to be designed for disassembly, it will be important for the concrete industry to demonstrate that they can compete, otherwise they will be replaced by steel and other materials that are easier to use for disassembly,” Mulhall tells youris.com. He concludes: “This will be a question of survival for the concrete industry.”

Image credits to: Grufnik