Post-fire thermal and mechanical performance of CDW-based plain and fiber-reinforced geopolymer composites incorporating different recycled aggregates

Geopolymer composites (GPCs) based on construction and demolition waste (CDW) have demonstrated significant potential as a sustainable solution for structural applications. However, despite existing research on the technical properties of CDW-GPCs, their post-fire performance remains largely unexplored. This study investigates the fire resistance of CDW-based GPCs formulated with a binder containing 70 % CDW while exploring the effects of various CDW aggregates, including recycled brick (RBA), ceramic-tile (RTA), concrete (RCA), and conventional silica sand (SSA). The effects of supplementary cementitious materials, such as ground granulated blast furnace slag (GGBS), metakaolin (MK) and class C fly ash (FA-C), alongside polyvinyl alcohol (PVA) fibers, were also considered in fiber-reinforced GPCs (FRGPCs). GPC and FRGPC samples were exposed to temperatures ranging from 23 °C to 800 °C and assessed for residual compressive and flexural strengths, load-deflection behavior, energy absorption, ultrasonic pulse velocity, mass loss, and spalling resistance. Although all mixtures exhibited thermal degradation beyond 400 °C, the use of PVA fibers significantly improved residual strengths and flexural performance. Interestingly, RCA-based GPCs and FRGPCs showed comparable thermal resistance to SSA, outperforming RBA- and RTA-mixtures. Furthermore, the inclusion of GGBS in CDW-binders demonstrated superior fire resistance relative to MK and FA-C formulations, proving a denser microstructure and stable C-A-S-H phase formations after exposure to temperatures up to 800 o C. The retention of key microstructural features confirmed the viability of CDW-based GP composites as fire-resistant construction materials. • No spalling was observed up to 800 °C, PVA and GGBS improved crack control and surface integrity. • GGBS-based composites showed superior thermal stability than their MK, FA-C-based counterparts. • RCA-based composites outperformed SSA, RTA, and RBA in post-fire stability due to improved aggregate-matrix compatibility. • FRGPCs showed improved ductility and stiffness, retaining higher values than GPCs at 800 °C. • GGBS-SSA and RCA-based composites had denser matrices, though RBA-composite showed highest porosity and degradation.