Bulky waste : From rubble to resource: How sensor-based sorting is transforming demolition waste into high-grade recycled concrete

At SORTAG in Rümlang, near Zurich, between 35,000 and 40,000 tonnes of mixed construction and demolition waste and bulky goods pass through the facility every year. SORTAG is part of the KIBAG Group — one of the largest producers of building materials in Switzerland — which operates 25 concrete plants and covers the entire building materials cycle: from construction projects to dismantling and material recovery, right back to its own concrete production.

The ambition at SORTAG goes well beyond mere diversion from landfill. The facility aims to produce recycled mineral aggregates that meet the stringent requirements of Swiss concrete standards — not downcycled material destined for road base, but a recovered product that feeds directly back into the Group’s own concrete production.

“KIBAG is a very large concrete producer and so we need RC [editor’s note: recycled] building materials,” says Benjamin Rickli, Head of Waste Management at KIBAG and SORTAG. “SORTAG can deliver this in perfect quality.”

The input material arriving at SORTAG is a heterogeneous mixture of minerals, wood, plastics, insulation, composite materials, steel, scrap and non-ferrous metals. The target is a mineral fraction with a purity of around 98 per cent. Separating metals is comparatively straightforward. The harder challenge lies elsewhere: plaster.

Used in almost every building — as wet plaster or drywall — gypsum-based materials are ubiquitous in demolition arisings. Their density is similar to that of concrete and brick, ruling out separation by wind sifting. When crushed, plaster disintegrates into fine particles distributed throughout the stone-based output. Once there, gypsum triggers sulphate reactions in cementitious products that cause long-term material degradation.

Swiss standards reflect this risk with particular rigour: the maximum impurity content in concrete granulate is 0.3 per cent by mass. The European standard EN 206 limits the sulphate content in recycled aggregates to 0.2 per cent, and studies from Belgium, France and Canada indicate that most industrially available recycled aggregates exceed this value. Plaster cannot be reliably separated by conventional mechanical means precisely because its density is too close to that of concrete and brick.

“The RC components used to be mixed with plaster,” says Rickli. “But today, we can say that: we are plaster-free.”

To achieve this result, SORTAG relies on an integrated sorting concept from STEINERT that combines magnetic separation and sensor-based sorting in a single process, using six machines to cover the full workflow.

After mechanical pre-sorting and screening by particle size, metal removal begins. A STEINERT UME self-cleaning overhead suspension magnet and a STEINERT MOR magnetic pulley extract ferrous metals from the flow; a STEINERT EddyC separator removes non-ferrous metals. With metals cleared, downstream processing equipment is protected and the material stream is ready for finer separation.

The next stage deploys a UniSort PR EVO 5.0, which uses near-infrared technology to separate wood, plastics and other non-mineral components from the stone-based fraction. Equipped with a hyperspectral NIR camera, the machine delivers more reliable detection even with complex and composite materials.

The decisive step in achieving final purity is taken by two STEINERT KSS XT | CLI combination sensor sorting machines. These units combine camera, laser, induction and X-ray technology. It is here that gypsum is identified and sorted out, and impurities are detected that no single sensor technology could reliably catch in isolation. The combination of multiple sensors within one machine also gives the facility its flexibility: the sorting programme can be adjusted to respond to changing input compositions without requiring any hardware changes.

“The focus is clearly on product quality,” says Rickli. “And so we have a good product at the end of the day.”

One of the operational realities of handling demolition waste is that its composition is not constant. It varies by season, by construction activity and by the nature of each demolition project. A feed that is predominantly concrete-based one day may arrive the next as a heavily mixed stream with elevated proportions of wood and insulation. Most construction and demolition sorting facilities are calibrated to a specific material flow; if the composition shifts, sorting quality drops or manual reworking becomes necessary.

SORTAG’s approach addresses this variability directly. Rickli describes day-to-day operations in sober terms: “We basically have two to three standard programmes with which we can sort throughout the day and produce consistently good quality. In special cases, STEINERT’s sorting technology can be used to change waste streams and sort out other products.”

The ability to switch programmes quickly has tangible practical value. “That’s the flexibility we didn’t have before, but we do today,” says Rickli.

Switzerland sits among the countries with the most demanding requirements for recycled building materials. The SIA 2030 standard and the Federal Office for the Environment (FOEN) directive define precise limit values, and public-sector clients are increasingly specifying RC concrete and monitoring its quality accordingly. Meanwhile, landfill capacity is shrinking and natural gravel reserves are diminishing — pressures that are not unique to Switzerland.

“Globally, we see ourselves in a pioneering position,” says Rickli. “Because Switzerland has a very high standard of concrete. And it simply needs top quality, otherwise it can no longer be sold.”

The gypsum challenge, however, is not a Swiss peculiarity. Any operator seeking to produce mineral fractions for bonded applications — and who must therefore keep sulphate content within regulatory limits — faces the same fundamental sorting problem. Conventional mechanical separation cannot resolve it reliably. That is what makes sensor-based sorting of direct relevance to facilities well beyond the Alpine context.

“In the end, it’s always about the output fraction and its quality,” says Rickli. “That’s the be-all and end-all.”