What the future is made of

New demand is also arising in other areas. The last thing a layperson would expect is a gypsum shortage − after all, we see this construction material everywhere. The German industry alone requires 10 million tons a year. However, 60 percent of this gypsum comes from coal-fired power plants that are set to close in 2040. Based on present requirements, that would mean an annual shortfall of 6 million tons of gypsum.

“Recycling gypsum from building rubble would close that gap,” says Dr. Volker Thome, Head of Department at the Fraunhofer Institute for Building Physics IBP. A team of researchers is working on that very issue in ENSUBA, a discovery project on the desulphation of building rubble. To date, the fine fraction of rubble, i.e. anything smaller than 2 millimeters, has been sent to landfill. Amounting to 5  million tons, or a volume 160 times that of the Berlin TV Tower, the quantities involved would definitely be worthwhile. Around 10 percent of this fine fraction is gypsum, contained in chemical compounds.

Using a wet chemistry process, the Fraunhofer IBP researchers can remove the gypsum from the fine fraction in a cost-effective way. “First, we mix the rubble with ammonium carbonate, also known as hartshorn salt, a kind of baking powder used in German ginger-bread cookies,” explains Dr. Thome. This results in chalky, sulphate-free rubble, which can be processed directly at cement factories, and ammonium sulfate. If you mix chalk and ammonium sulfate, the gypsum is precipitated. Cement factories would be pos-sible recipients for both the end products. Researchers in a large pilot project by the Central Innovation Programme for small and medium-sized enterprises aim to upscale the process and resolve some teething problems. “The method is relatively simple − and the interest in it is cor-respondingly high,” says Dr. Thome.

With this process, 500,000 tons of gypsum could be recovered every year − a lot, but not enough. That’s why Fraunhofer IBP researchers are linking up with Fraunhofer IML, Fraunhofer UMSICHT and Fraunhofer IOSB in the BauCycle joint project to develop and optimize a process for identifying larger pieces of building rubble by electro-optical means and separating them from the rest with compressed air. This would make it possible to recover fragments between 2 and 8 millimeters in size with close to full purity.

Sand: A scarce commodity even in Dubai

The sands are running out − even for construction sand. In Dubai, for example, there’s a major shortage of construction sand, because desert sand is not fit for purpose, and all their construction sand has to be imported from Australia. With construction sand also at a premium elsewhere, pirates are mining sand illegally off the coast of Africa, causing irreparable ecological damage. Meanwhile, 20 Indonesian islands have vanished entirely from the face of the earth due to excessive sand mining. However, if we could recover construction sand from old con-crete, this would create a gigantic source of the material. Because, after water, concrete is the most used substance in the world. However, this is no small endeavor. As yet, sand cannot be separated from its surrounding matrix. “Up to now, concrete has just been crushed mechanically. However, by using electrodynamic fragmentation, we’ve developed the first separation method that can be used to recover sand, gravel and steel fibers from concrete,” reports Dr. Thome of Fraunhofer IBP. In the process, ultrashort pulses are generated underwater; these pulses tend to travel along the phase boundaries in the solid objects, thus breaking the concrete down into its constituent materials. “When the first predischarge alone reaches the counter electrode, it results in pressures in the region of a TNT detonation,” relates Dr. Thome. While this process allows researchers to extract and reuse the gravel in pure form and unchanged quality, and even to isolate steel fibers cleanly, some further research is still required when it comes to sand. The process still produces a mix-ture of sand and cement stone. Scientists are currently working to separate the sand cleanly, but until then, even the mixture can be reused − to manufacture insu-lation for example. The researchers have already cleared some significant hurdles in the recycling process. “In a collaborative project with a generator manufacturer, we managed to reduce the energy requirement for electrodynamic fragmentation to a tenth, so it’s now within the region of mechanical processing,” enthuses Dr. Thome. The team has also resolved the problem of throughputs, which had been very low until now. By the end of the year, they are aiming to have a plant running with a capacity of 3 tons an hour − previous plants could only manage one.

The process could also be very useful in recycling the ashes from waste incineration. Iron, non-iron metals such as aluminum foil, bottle caps, glass, stones and ceramic with sizes of over 2 millimeters can be fully separated and reused, thus reducing the ash volume to 50 to 60 percent. Fire-proof ceramics would be another field of application for electrodynamic fragmentation; among other things, these ceramics are used in iron and steel manu-facturing or in waste incineration plants. These must be replaced once a year, with the old ceramics ending up in landfill. However, as China has a high demand for these ceramics itself, the price for ceramic aggregates is cur-rently skyrocketing. Take high-purity bauxite for example − it now costs 800 to 1200 euros per ton. By comparison, concrete only costs 6 to 14 euro per ton. “Using electrodynamic fragmentation, we can separate out these aggregates with unchanged quality levels, so we already patented the process back in 2017,” relates Dr. Thome. In FAVRE, an ongoing Fraunhofer project aimed at developing a fragmentation plant for recycling composite mate-rials and funded by the Fraunhofer-Zukunftsstiftung (Fraunhofer Future Foundation), Fraunhofer IBP researchers are upscaling this technology. This will open up a new source of raw materials like bauxite, hopefully toppling prices and guaranteeing their availability in the long term. It’s just one more area where reusable materials are becoming an ever more valuable resource.