Development of Lightweight Geopolymer Composites by Combining Various CDW Streams

This study regards the development of lightweight geopolymer composites through the valorization of various construction and demolition wastes. Brick waste was utilized as the sole aluminosilicate precursor for the geopolymerization reactions, expanded polystyrene and polyurethane wastes were used as artificial lightweight aggregates, and short polyethylene fibers developed from CDWs reinforced the geopolymer matrix. The curing conditions of the geopolymer synthesis were optimized to deliver a robust geopolymer matrix (T = 25–80 °C, t = 24–72 h). Both raw materials and products were appropriately characterized with XRD and SEM, while the mechanical performance was tested through compressive strength, flexural strength, Poisson’s ratio and Young’s modulus measurements. Then, a comprehensive durability investigation was performed (sorptivity, wet/dry cycles, freeze/thaw cycles, and exposure to real weather conditions). In contrast to polyurethane waste, the introduction of expanded polystyrene (0.5–3.0% wt.) effectively reduced the final density of the products (from 2.1 to 1.0 g/cm3) by keeping sufficient compressive strength (6.5–22.8 MPa). The PE fibers could enhance the bending behavior of lightweight geopolymers by 24%; however, a geopolymer matrix–fiber debonding was clearly visible through SEM analysis. Finally, the durability performance of CDW-based geopolymers was significantly improved after the incorporation of expanded polystyrene aggregates and polyethylene fibers mainly concerning freeze/thaw testing. The composite containing 1.5% wt. expanded polystyrene and 2.0% v/v PE fibers held the best combination of properties: Compr. Str. 13.1 MPa, Flex. Str. 3.2 MPa, density 1.4 g/cm3, Young’s modulus 1.3 GPa, and sorptivity 0.179 mm/min0.5.