Optical Sorting Systems Making High-Grade Recycled Concrete Feasible. : FEATURE: Concrete Progress for C&D Waste in Antwerp

When it comes to recycling the huge quantities of concrete that enter the waste stream, contaminants have largely prevented it being recovered for high-grade applications. However, in the port of Antwerp a new project has successfully demonstrated optical sorting systems which make just such high-grade recycled concrete feasible.

By Jef Bergmans, Kris Broos, Peter Nielsen, Philippe Dierckx, Yvan Brijsse and Kurt Jacobs

Construction and Demolition Waste (C&DW) represents one of the EU’s largest waste streams. According to the Directive 2008/98/EC on waste, at least 70% by weight of non-hazardous C&DW must be recuperated by 2020. Eurostat estimates an annual C&DW generation of 970 million tonnes across the EU-27, representing an average value of almost two tonnes per inhabitant, with an average recovery rate of 47%.

A large proportion of the stony fraction of C&DW can be easily re-used or recovered as recycled aggregates within the construction sector, all-be-it almost exclusively in low-grade unbound applications such as (sub)foundations. However, this market is becoming saturated. Therefore, a shift towards more structural concrete applications (requiring a higher quality of the recycled aggregates) is currently being investigated and promoted.

Successful examples of producing recycled concrete using recycled C&DW aggregates have been widely reported, with interest principally focused on the use of concrete aggregates. While the use of Mixed Recycled Aggregates (MRA) has also been considered, the sulfate content is often indicated as a restrictive limitation.

The content of contaminants such as organic matter (wood, plastics, organic foams), gypsum or autoclaved aerated concrete (AAC) in recycled aggregates must be minimised to make these aggregates suitable for high-grade applications. The presence of contaminants can lead to unwanted effects: e.g. cracking, weak points and delay in hardening.

Several studies indicate that the most effective way of minimising the amount of contaminants in C&DW materials is selective demolition, having a positive environmental impact. Moreover, policy programs indicate selective demolition as a key factor for C&DW waste minimisation. OVAM, the public waste agency of Flanders, states that it is their goal to set selective demolition as the standard in the construction sector by 2020. This case study describes the selective demolition of an office building in the Port of Antwerp (PoA).

When source separation is not possible, subsequent separation of undesired compounds is necessary to obtain aggregates suitable for high-grade applications. Traditionally, separation is performed by manual sorting cabins, density separators (e.g. windshifters) and/or magnetic separators. However, more rigorous separation might be needed to meet the required levels of purity for high-grade applications.

Advanced automated sorting techniques by colour or chemical composition have been successfully developed in other industries (e.g. high quality sorting of plastics, glass recycling). The use of these automatic techniques in C&DW recycling could result in a guaranteed supply of pure recycled materials than can be used in high-grade construction applications. Here we assess the performance of Near Infrared (NIR) and Ultraviolet-Visible (UV-VIS) sorting solutions.

The NIR sorting is used to produce MRA with a higher technical and environmental quality because of lower soluble sulfate contents (mostly related to gypsum and AAC particles) and organic matter contents. Afterwards, UV-VIS sorting is performed to obtain a concrete (grey) fraction and a ceramic (red) fraction.

Selective demolition and automatic sorting techniques not only allow the production of high quality aggregates, but will also produces pure fractions of materials that are currently present as impurities in the stony fraction (e.g. AAC). In this case a medium sized industrial building was constructed in the PoA, using recycled concrete aggregates in structural concrete and recycled AAC in new floor screed products

Having selected an office building in the PoA, demolition was performed selectively in order to allow reuse possibilities for a number of building elements and recycled aggregates that are usable in high-grade concrete applications.

The office building was built in a very traditional Belgian structure. Foundations and most other structural elements consisted of concrete. Bricks and gypsum plasterboard were used for the outer and inner walls respectively. Asbestos containing tiles covered the roof.

The elements to be reused (aluminium window frames, radiators, fire protection doors) were dismantled and removed by hand before the start of the demolition works. This dismantling process took about 1 working day.

In a subsequent step the building was decontaminated. The asbestos containing roof tiles (6 tonnes) were removed by hand. The roof tiles were put in a container with a plastic coverage preventing dust and which was subsequently transported to a landfill site. Furthermore, mercury containing fluorescent lamps and electronic devices were removed.

During the subsequent demolition, the inner wall coverage (gypsum plasterboards and XPS insulation boards) was removed first. These materials can greatly decrease the quality of the stony fraction if not collected separately. Afterwards, the building was demolished with a hydraulic crane. Six separate fractions were collected: concrete (foundation), a mixed stony fraction (mainly bricks and mortar from the outer walls), aluminium (light fittings), wood (mainly from the roof structure), gypsum plasterboards, calorific waste (insulation).

The small size of the office building and surroundings did not allow an economically viable onsite crushing and sieving of the stony fraction. Onsite crushing and sieving is usually performed on demolition sites of >3,000 tonnes of stony material and enough available space for the machinery. Crushing and sieving of the stony fraction was performed offsite at a recycling site in the PoA.

The NIR sensor-based sorting targets the selective removal of contaminants (e.g. gypsum, organic material) from the valuable stony particles. Input material is evenly fed onto a conveyor belt, where it is detected by the NIR sensor. The detected impurities are separated from the material flow by jets of compressed air. The particle size of 6mm, guarantees both optimal resolution and efficient removal through air jets. Sorting capacities depend on bulk density and grain size of the input fraction, but reached up to 11 tonnes/h/m in the NIR sorting tests.

Six samples of recycled aggregates from C&DW were collected in 5 different EU countries (Belgium, Germany, Italy, Spain, Sweden). Because selective demolition of the office building did not create a sample of MRA with enough impurities, the Belgian sample (sample 6) was obtained by adding gypsum and AAC particles to the MRA.

A bag of approximately 1 m³ from each sample was sent to the research facilities of TOMRA Sorting GmbH in order to eliminate problem fractions using NIR sensor-based technologies. A representative subsample was taken and used for characterisation. This subsampling was performed in accordance with EN 932-1.

Before and after sorting, the determination of constituents was performed according to EN 933-11. The proportion of each constituent was determined and expressed as a mass percentage.

Most samples after sorting show reductions in the X-fraction higher than 50%. Sample 2 was the only fraction where the reduction was <50%. This was attributed to the presence of high levels of dark blue AAC (most AAC is white). Since the NIR sensor was not programmed to eject this type of AAC, this particular impurity was not removed.

The use of recycled aggregates in high-grade construction applications (e.g. concrete, cement bound products) requires low contents of sulfates. The NIR technology achieves notable reductions in the soluble sulfates by removing large parts of gypsum and AAC.

The obtained MRA can be upgraded further by using a UV-VIS sensor. By using this technology, the MRA can be separated into a grey concrete fraction wit a purity >97%.

After the demolition work was complete a waste collection centre was constructed in the PoA using products with recycled C&DW. The selectively demolished concrete fraction was reprocessed into new concrete products that were used for the production of foundation concrete and polished concrete floors, both inside and outside the building.

The latter application in particular can be considered as very high level, demonstrating the technical possibilities of pure recycled concrete aggregates.

Following selective demolition the concrete aggregates of the office building’s foundation comply with the standard for “high-quality concrete aggregates” of SB 250, the standard specifications for road works in Flanders (Rc > 90; Rcu > 95; Ra < 1; XRg < 0.5; FL < 2).

The produced concretes were ready-mixed and used in foundation concrete (up to 60 m% replacement of the coarse aggregate fraction) and flooring concrete (up to 30 m% replacement of the coarse aggregate fraction). No additional measures (e.g. amount of water or cement) were taken for the use of recycled aggregates.

Initial characterisation tests showed no differences between the different aggregate replacement rates. The well-documented concrete products can be assessed in time. Up till now, two years after construction, no differences with concrete with natural aggregates has been detected.

Selective demolition allows the production of pure fractions from materials that are currently disposed. The development of recycling options for these fractions would lower the amount of material that needs to be landfilled. One of these materials is autoclaved aerated concrete (AAC).

The amount of AAC waste that can be recycled in the production of new AAC is limited because of quality issues. Furthermore, recycling AAC into traditional concrete or as unbound aggregate causes both technical and environmental problems because of the low compressive strength (2-8 MPa) of AAC and its high amount of leachable sulfate: typically > 10,000 mg/kg dm (L/S = 10).

Critical requirements for the immobilisation proved to be sufficiently alkaline conditions and the presence of sufficient Portland clinker aluminate (C3A) to react with the sulfates contained in the AAC. To reach a sufficiently high alkalinity in the developed cement stabilised sand products, the use of CEM I is crucial. The use of blended cements results in a lower leachate pH (<12) and a rise in sulfate leaching. The developed products contained enough reactive aluminate to immobilise the available sulfate.

However, when AAC waste is contaminated with gypsum particles, local hotspots of leachable sulfates can create a depletion in reactive aluminium. This results in a strong increase in sulfate leaching.

We developed recycled products containing crushed AAC from C&DW (210 kg/m³). The crushed AAC (0-8 mm) was mixed with cement, sand and water. During cement hydration a reaction of the AAC leachable sulfate and the aluminate contained in the cement resulted in the formation of (insoluble) ettringite.

The main conditions influencing the formation of ettringite, and hence the leaching of sulfate, were examined. A sufficiently high pH was found to be crucial to meet sulfate leaching standards This high pH was met when using ordinary Portland cement. The presence of additional sulfate as gypsum impurities in the AAC waste proved detrimental towards sulfate leaching.

A floor screed was produced using recycled AAC from a selective demolition and installed in the office building of the waste collection centre. The screed showed a compressive strength of 5.6 MPa and a heat resistance of 2.0 mK/W. The presence of the low-density AAC in the floor screed had a positive influence on its thermal insulation capacities.

This case study illustrates the possible added value of selective demolition and advanced sorting techniques for the creation of high-grade recycling options. Additionally, a recycling pathway for one of the more problematic C&DW types (AAC) was demonstrated.

In this case study, a selective demolition of an office building was performed, allowing reusable items and high-grade aggregate fractions that can be used in structural concrete applications to be obtained. A prior decontamination ensured safe work conditions and recycled products.

Sensor-based sorting technologies proved to be able to produce very pure high-grade aggregates. NIR sorting lowered the amount of unwanted contaminants (X-fraction: gypsum, organic materials, metals) by half, while UV-VIS sorting allowed the production of a grey concrete fraction with a purity of >97%.

The concrete aggregates after selective demolition were used in foundation concrete and polished concrete flooring. Initial characterisation tests showed no differences between the different aggregate replacement rates and concrete with natural aggregates while no additional measures taken for the use of recycled aggregates.

Furthermore, a new recycling route for AAC waste was developed. Crushed recycled AAC was used as a sand replacement in floor screeds. The main problem for the recycling of AAC is the presence of leachable sulfates. These sulfates were immobilised with the formation of ettringite by combination with Portland cement. The use of recycled AAC can have a positive effect on the heat insulating capacities of constructions.

The developed products were all used in the construction of a waste collection centre in the PoA. This case study is a good example of the opportunities for high-grade recycling of pure C&DW fractions and allows for further follow-up on the long term performance of the recycled products.

Jef Bergmans, Kris Broos, Peter Nielsen, Philippe Dierckx, Yvan Brijsse and Kurt Jacobs express their gratitude to the EU FP7 project IRCOW (Innovative Strategies for HighGrade Material Recovery from Construction and Demolition Waste - grant agreement no. 265212) for funding the research.

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