Recycling fine rubble

Aerated concrete made of brick (rear); aerated concrete made of sand-lime stones (front).

Sand is becoming scarce in many countries because of the global construction boom. Humungous quantities of concrete, which consists mostly of sand and gravel, are used to build hotels, shopping malls and condominiums. Dubai apparently had to import Australian sand to erect the Burj Khalifa since its own desert sand was ill-suited to this purpose.

Aro und the world, some 40 billion tonnes of sand and gravel are used every year. At the same time, demolished buildings and infrastructure around the world leave behind several million tonnes of fine-grained building rubble, which fill up landfills and are sometimes used in road construction.

The Fraunhofer Institutes for Building Physics (IBP), for Material Flow and Logistics (IML), for Environmental, Safety and Energy Technology (UMSICHT), and for Optronics, Systems Technology and Image Analysis (IOSB) came together to launch a project called BauCycle to recycle construction waste. The aim of this three-year project, costing €3.3 million, is to convert this mix of minerals into a sustainable resource and demonstrate potential applications in construction. Specifically, recycling particles of mineral construction waste less than 2 mm in diameter is a key target. “The conventional method of treating rubble is to crush it. Components of less than 2 mm are sieved out and end up in the landfill. If this fine-grained building rubble were to be recycled, this could redress the sand shortage over the long term,” says IBP scientist Volker Thome, the project’s manager.

The first step is to sort the rubble’s heterogeneous content. The researchers have developed an opto-pneumatic detector that enables fine fractions to be separated on the basis of colour, brightness and chemical composition; it can even distinguish sulphates from silicates. “The waste components are placed on a conveyor belt that transports them past an infrared camera equipped with special filters to detect the various fine fractions. The particles drop off the end of the belt in free fall, past nozzles that shoot the main components into different containers with targeted blasts of compressed air,” says Thome. Thome’s team has succeeded in differentiating 1 mm particles. This technology can achieve a throughput of 1.5 tonnes per hour.

In t he best case scenario, four clean aggregates may be recycled and reused to produce aerated concrete, a light building material with good thermal insulation properties. Tests have shown that aggregates of concrete and sand-lime bricks are also recyclable and can serve as a secondary raw material to make competitive grades of aerated concrete. The researchers achieved the best results with a mix of 80 per cent sand-lime bricks and 20 per cent reclaimed concrete.

Anot her project finding was that a combination of bricks and recovered concrete can be used to make geopolymers, a cement-free building material that is strong and acid-resistant, much like concrete. The researchers made various components to demonstrate the recycling potential of construction waste.