Hygrothermal aging-fire exposure on thermal and mechanical properties of decorative waste aerogel insulation mortar

The rapid growth of decorative waste, driven by building renewal and consumption upgrading, has raised urgent concerns regarding its sustainable utilization. Decorative waste recycled aggregate (DWRA) has been widely applied in construction materials; however, its combination with aerogel for high-performance insulation remains underexplored, particularly regarding thermal and fire performance after hygrothermal aging followed by sudden fire exposure. This sequential exposure was designed to simulate realistic service-life hygrothermal degradation of insulation mortars followed by accidental fire events acting on pre-damaged materials. This study systematically investigated the coupled effects of hygrothermal aging (conditioning at 70 °C and spraying with 15 °C water, 20–80 cycles) and subsequent fire exposure (850 ℃ for 3 h) on decorative waste aerogel insulation mortar (DWAIM) with aerogel replacement ratios of 60 %-100 %. Material degradation was evaluated through measurements of thermal conductivity and interfacial mechanical properties, including tensile bond strength and compressive-shear bond strength, complemented by microstructural characterization. Results indicate that increasing aerogel content leads to reductions in both tensile and compressive-shear bond strengths, with accelerated degradation observed at higher replacement ratios. Micro-defects generated during hygrothermal aging were significantly amplified after fire exposure, producing damage that exceeded the simple superposition of individual deterioration mechanisms. On average, the actual bond strength loss under coupled hygrothermal-fire conditions was 13.94 % greater than the theoretical cumulative loss. Residual mechanical performance consistently followed the order: unaged > hygrothermal aging > fire exposure > coupled effect. From a thermal perspective, thermal conductivity exhibited a stable negative correlation with aerogel content, while its variation amplitude first decreased and then increased. Under coupled conditions, the increase in thermal conductivity reached up to 3.14 times that caused by hygrothermal aging alone and 2.27 times that caused by fire exposure alone. Among all formulations, 80 % aerogel content was identified as optimal. It achieved an initial thermal conductivity of 0.062 W/(m·K), tensile bond strength of 0.965 MPa, and compressive-shear strength of 943.56 kPa. After 80 aging cycles and subsequent fire exposure, it still satisfied Type II insulation standards, demonstrating acceptable retention of thermal insulation performance and mechanical bonding capacity under hygrothermal-fire coupled conditions. Microstructural analysis confirmed its densest structure, minimal cracking, and superior durability. • The tensile bond strength of DWAIM decreases linearly with aerogel content. • DWAIM's compressive-shear bond strength decays exponentially with aerogel content. • Minimal thermal conductivity increase (≤10 %) in aged 80 % aerogel DWAIM. • 0.084 W/(m·K) thermal conductivity in 80 % aerogel DWAIM post-850°C fire • 80 % aerogel content gives best thermal-mechanical balance with stable negative trend