Analytically designed cold-bonding encapsulation of automotive shredder residues for structural lightweight aggregates

This study presents an analytical methodology for producing lightweight aggregates (LWA) from light automotive shredder residue (LASR), a highly heterogeneous non-metallic fraction generated during end-of-life vehicle processing. A quantitative encapsulation model was developed to determine the minimum cement paste volume required to encapsulate LASR particles, based on measurable physical parameters: particle size distribution, particle density, specific surface area, compactness and water absorption. This approach enables a reproducible and physically grounded cold-bonding process, overcoming the empirical formulations commonly reported in previous studies. The resulting LASR-based LWA were characterised in terms of density, porosity, absorption, microstructure, abrasion resistance (LAA), aggregate crushing value (ACV), confined compressive strength (σCR,10%) and leaching behaviour. The aggregates exhibited an oven-dry particle density of approximately 1070 kg/m³ and an apparent density near 1680 kg/m³ , consistent with the density range defined for structural lightweight aggregates in EN 13055. Total accessible porosity (≈27%) and water absorption (≈25%) were within the expected range for cold-bonded LWA. Backscattered-electron imaging showed that the cement paste penetrates the LASR pore network and forms a continuous binder phase, with a mortar-enriched outer zone of about 0.7–1.0 mm consistent with the coating thickness predicted by the model, providing direct microstructural support for the encapsulation design. Mechanical characterisation yielded ACV = 15% and σCR,10% = 0.8 MPa, while LAA averaged 41% (SD ≈ 7%), highlighting the importance of assessing both compressive resistance and impact-abrasion susceptibility during handling and mixing. Leaching tests demonstrated substantial reductions in Cu, Ni, Zn and Pb release, confirming effective contaminant immobilisation by encapsulation. Overall, the proposed analytical methodology provides a reproducible framework for converting LASR into environmentally compatible lightweight aggregates, contributing to circular ELV residue valorisation and sustainable construction materials.