Turning waste into value: Eco-friendly thermal insulating mortars from recycled rigid foams

Plastic waste poses a growing environmental challenge, driving increasing interest in its reuse in sustainable construction materials. This study investigates the use of recycled rigid polyurethane (PUR) and polystyrene (PS) foams as primary lightweight aggregates (>90 vol.%) in premixed thermal insulating mortars for floor and roof applications. The binder system comprises Portland cement and hydrated lime, without the addition of sand, and is optimized with targeted additives to enhance mechanical, physical and thermal performance. The results show that key properties – density, compressive strength and thermal conductivity – are strongly governed by the PUR/PS content and overall mix design. Optimized formulations achieved compressive strengths of 1.0 and 0.85 N/mm², densities of 420 and 340 kg/m³ and thermal conductivities of 0.07 and 0.09 W/mK for PUR- and PS-based mortars, respectively, demonstrating a favourable balance between mechanical integrity and insulation performance. Scale-up was successfully demonstrated through the design and implementation of a prototype production system with a capacity of 1500 L of insulating mortar per h (equivalent to 20 bags of 75 L) [1]. The developed materials show strong potential for practical application, particularly in regions where plastic waste is still predominantly landfilled. Overall, this work supports the development of circular, low-impact construction materials and highlights the potential for further integration of diverse plastic waste streams into highperformance insulating systems.