Effect of nanoparticles on sulfate resistance and thermal insulation performance of glazed hollow bead concrete

In the construction of deep-buried tunnels in high geothermal regions, thermal insulation concrete is required to maintain suitable temperatures within the tunnel. However, during service, this concrete is exposed to sulfate-rich groundwater, which can lead to the deterioration of its performance. This study examines the impact of incorporating nano-SiO 2 (NS) and nano-Al 2 O 3 (NA) into glazed hollow bead concrete (GC) on sulfate resistance and thermal insulation during combined sulfate attack and dry-wet cycling. Microstructural mechanisms were elucidated through SEM, EDS, and XRD analyses. The findings indicate that incorporating nanoparticles markedly improves both sulfate resistance and thermal insulation, with 2 % identified as the optimal dosage for both nano-SiO 2 and nano-Al 2 O 3 .Nano-modified GC demonstrated significantly lower mass loss rates (W n ) and enhanced relative dynamic elastic modulus (E rd ) compared to plain GC. After 120 cycles, GCNS20 and GCNA20 achieved 25.0 % and 21.4 % higher E rd , while W n decreased by 34.1 % and 30.0 %, respectively. After 150 cycles, GCNS20 exhibited a 26.43 % reduction in thermal conductivity and a 34.65 % decrease in thermal diffusivity, while GCNA20 showed reductions of 17.52 % and 23.62 %, respectively. SEM results revealed that NS and NA refined pore structures and suppressed thermal bridging, thereby synergistically enhancing durability and thermal insulation. EDS analysis revealed a decreased Ca/Si ratio and sulfur content in nano-modified GC. XRD results showed that nanoparticles enhanced cement hydration, reduced unreacted C 2 S/C 3 S, and consistently decreased the diffraction peak intensities of gypsum and ettringite, following the order GCNS20 < GCNA20 < GC. Owing to its higher specific surface area and superior reactivity, NS provided more pronounced improvements than NA. Ultimately, a pore-gel synergistic optimization mechanism was identified, enabling simultaneous enhancement of sulfate resistance and thermal insulation performance. • The coupled sulfate attack and wet–dry cycling test for glazed hollow bead concrete is studied. • The effects of nano-SiO 2 and nano-Al 2 O 3 dosage on sulfate resistance and thermal insulation are studied. • The relationship between Ca/Si ratio, sulfate ingress (S), and performance (W n , E rd , thermal properties) is studied. • The microstructural mechanism for reduced gypsum/ettringite formation revealed by SEM/EDS/XRD is studied.