Plaster-free: how sensor-based sorting secures the mineral fraction for recycled concrete

Sortag, part of the Kibag Group, is one of the largest producers of building materials in Switzerland. Using Steinert sorting technology, Sortag recycles demolition waste to deliver mineral aggregates that meet tough Swiss standards for reinforced concrete (RC).

If sorted material goes straight into production, contamination can follow, so whatever passes through the initial sorting ends up in the final product. Therefore, the quality requirements placed on Sortag are particularly demanding. The facility must sort the mineral fraction in such a way that it consistently meets the standards for RC, which in Switzerland are among the strictest in the world.

Sorting 40 000 tonnes

This is exactly what happens at Sortag in Rümlang, near Zurich. As part of the Kibag Group, the company operates within a network of 25 concrete plants and covers the entire building materials cycle. This ranges from construction projects to dismantling and recycling, and back into its own concrete production.

Each year, the facility processes between 35 000 and 40 000 tonnes of mixed demolition waste and bulky goods. The goal is clear: produce recycled mineral aggregates that meet Swiss concrete standards. Rather than downcycling material into road base, the output flows directly back into the group’s own concrete production.

‘Kibag is a very large concrete producer and so we need RC building materials,’ says Benjamin Rickli, head of waste management at Kibag and Sortag. ‘Sortag can deliver this in perfect quality.’

What goes in must come out

The input material is a complex mixture of minerals, wood, plastics, insulation, composite materials, steel, scrap and non-ferrous metals. As a result, the process must deliver a mineral fraction with a purity of around 98%.

While metals are comparatively easy to separate, the real challenge is plaster. Plaster is used in almost every building, whether as surface coating or drywall. Consequently, it is present in nearly all demolition waste streams. The difficulty is that plaster has a density similar to concrete and bricks. It cannot be separated by wind sifting and, when crushed, disintegrates into fine grains that spread throughout the mineral fraction.

This creates a serious issue. Gypsum triggers sulphate reactions in concrete, which can damage the material over time. In Switzerland, the maximum impurity content in concrete granulate is limited to 0.3% by mass.

‘RC components used to be mixed with plaster,’ says Rickli. ‘But today we can say we are plaster-free.’

Plaster is not the only concern. The composition of demolition waste fluctuates depending on the season, construction activity and individual projects. What is predominantly concrete today may be mixed demolition tomorrow, with a high proportion of wood and insulation. A sorting facility that is optimised for only one scenario is inadequate.

Flexible sorting concept

To address this, Sortag relies on an integrated sorting concept from Steinert. This combines magnetic separation and sensor-based sorting within a single process. Six machines cover the entire sorting chain.

Firstly, the material undergoes mechanical pre-sorting and screening according to particle size. Then metal separation begins. A Steinert UME self-cleaning overhead suspension magnet and a Steinert MOR magnetic pulley remove ferrous metals while an eddy current system separates non-ferrous metals.

As well as, removing metals early in the process, downstream equipment is protected from damage and wear.

Next, the UniSort PR EVO 5.0 system delivers near-infrared (NIR) technology. It separates wood, plastics and other non-mineral components from the mineral fraction. A hyperspectral NIR camera improves detection reliability, even for complex materials.

The decisive step, however, comes next. Two Steinert KSS XT | CLI combination sorting machines bring the mineral fraction to its final purity. These systems combine X-ray technologies and camera, laser and induction in a single unit.

Plaster and other impurities are identified and removed with high precision. Moreover, the combination of multiple sensors in one machine allows the facility to remain flexible. It can respond quickly to changing input compositions without requiring hardware changes.

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

Mixed demolition tomorrow

In daily operations, flexibility is essential. ‘We basically have two to three standard programmes with which we can sort throughout the day and produce consistently good quality,’ Rickli explains. ‘In special cases, Steinert’s sorting technology can be used to change waste streams and sort out other products.’

While this may sound straightforward, it has a significant impact in practice. Most construction and demolition sorting facilities are calibrated for a specific material flow. If the composition changes, sorting quality can decline or manual reprocessing may become necessary.

Sortag’s sorting programme can be adapted as needed. This ensures consistent output quality, even under changing conditions. ‘That’s the flexibility we didn’t have before, but we do today.’

Local standards, global relevance

Switzerland has some of the strictest requirements for recycled building materials. Standards such as SIA 2030 and directives from the Federal Office for the Environment define precise limit values. Public clients increasingly demand RC and closely monitor its quality. Meanwhile, landfill space is becoming scarcer and more expensive while natural gravel reserves are steadily declining. Against this backdrop, high-quality recycling is essential.

‘Globally, we see ourselves in a pioneering position because Switzerland has a very high standard for concrete,’ says Rickli. ‘And it simply has to be top quality, otherwise it can no longer be sold.’

The plaster challenge is not unique to Switzerland. Across Europe, the European standard EN 206 limits sulphate content in recycled aggregates to 0.2%. However, studies from Belgium, France and Canada show that many recycled aggregates exceed this value.

Conventional mechanical sorting methods struggle to remove plaster effectively, due to its similar density to concrete and brick. As a result, sensor-based sorting becomes increasingly relevant for operators aiming to produce high-quality mineral fractions for bonded applications. Ultimately, the objective remains unchanged.

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