Mechanism of paste-to-aggregate ratio on the thermophysical evolution of aerogel-incorporated recycled mortar

To address the urgent need for ambient thermal insulation and high-temperature fire resistance in buildings, this study engineered a SiO₂ aerogel composite thermal insulation mortar. The paste-to-aggregate (P/A) and water-to-binder (W/B) ratios were investigated to uncover their synergistic regulation on thermal and mechanical properties. Results reveal a distinct primary-secondary mechanism hierarchy: P/A governs the absolute capacity of the load-bearing skeleton and thermal inertia, while W/B provides auxiliary rheological tuning, with an optimal W/B ratio of 0.40 yielding a 103.1% strength leap. Reducing P/A from 1.1 to 0.65 physically severs thermal bridges, successfully dropping thermal conductivity to 0.142 W/(m·K). However, this extreme reduction severely depletes the cementitious matrix, slashing specific heat capacity by 50.0% and causing a 43.5% surge in thermal diffusivity. Under 800°C transient fire exposure, the dominant thermal protection mechanism shifts from ambient physical pore shielding to high-temperature matrix phase-change endothermy. The low P/A system suffers catastrophic structural failure due to a lack of endothermic media and severe thermal strain mismatch. To bridge this trade-off, adjusting the P/A ratio proves highly effective. An optimal mixture (P/A=0.95, W/B=0.40) successfully resolves the contradiction between insulation and fire resistance performanc, maintaining a low ambient thermal conductivity of 0.156 W/(m·K) and adequate compressive strength (1.3 MPa). Furthermore, it provides excellent thermal shock buffering, restricting internal temperature rise to 260°C after 60 min of 800°C fire exposure, thereby fulfilling dual requirements for steady-state insulation and dynamic fire defense.