Interface engineering of carbon nanotubes with SiO₂ coating for toughening and densification of alkali-activated composites under high strain rate

Carbon nanotubes (CNTs) and their modified derivatives show considerable potential for enhancing the impact resistance of alkali-activated materials (AAMs) under extreme loading conditions. In this study, three SiO₂-coated CNTs with different coating thicknesses (5Si-C, 10Si-C and 20Si-C) were synthesised, and their effects on the dynamic mechanical behaviour of alkali-activated slag–fly ash (SFA) composites were investigated using split Hopkinson pressure bar (SHPB) tests. The results demonstrate that SiO₂-CNTs provide a more pronounced strengthening and toughening effect than pristine CNTs (p-CNTs) and functionalised CNTs (f-CNTs). In particular, 10Si-C/SFA exhibits increases of 29.65 % and 34.19 % in static compressive and flexural strength, respectively; within the investigated strain-rate range, its dynamic compressive strength is enhanced by 33.97–57.59 %, while the peak and total impact toughness reach up to 6.63 and 3.37 times those of the control, respectively. The post-impact fragment size distributions display typical fractal characteristics, with SiO₂-CNTs—especially 10Si-C—significantly reducing the fragmentation fractal dimension and thus enabling a favourable “high SEA–low fragmentation” response. X-CT analysis shows that 10Si-C/SFA develops the densest and least-connected pore network among all mixtures, while SEM observations further indicate that the reactive SiO₂ coating chemically interacts with the matrix, promoting the formation of C(N)-A-S-H gels and enhancing interfacial bonding. Overall, the SiO₂-CNT interface-engineering strategy achieves synergistic toughening and densification of AAMs under high strain-rate loading, providing important experimental evidence and mechanistic insight for the design and optimisation of impact-resistant, low-carbon cementitious materials.