Comparative study on the influence mechanisms of nano-silica and nano-alumina on the multi-scale behaviour of concrete

Concrete performance critically governs structural durability and safety. The incorporation of nanomaterials in concrete is a promising enhancement strategy, yet the role of particle size, especially across different material types, remains under-explored. In this study, the effects of 20 nm nano-silica (NS) and 20 nm nano-alumina (NA) on concrete properties were systematically investigated through macro-, meso- and micro-scale tests. Concrete specimens were prepared by partially replacing Portland cement with NS and NA at mass fractions of 1–3%. The results showed that, at a fixed particle size, performance first increased with dosage and then declined beyond an optimum dosage. Peak gains were achieved with 2% NS, the compressive strength at curing stages of 3 and 28 days increased by 23.4% and 15.3%, respectively, while the 28-day splitting tensile strength and chloride resistance raised by 17.1% and 29.4%, respectively. For NA, the optimum dosage was approximately 1%, yielding corresponding increases of 13.9%, 8.1%, 12.4% and 7.9%. Both optimal additions refined the pore structure, reduced the total porosity and densified the interfacial transition zone, collectively enhancing overall performance. These findings underscore the dosage-sensitive benefits of NS and NA, and provided comparative insights for nanomaterial selection in concrete modification.