Unlocking the synergy of nano-silica and nano-calcium carbonate for superior frost resistance in recycled aggregate concrete

To examine the effects of compositely incorporating nano-silica (NS) and nano-calcium carbonate (NC) on the freeze-thaw resistance of recycled aggregate concrete (RAC), this study fabricated modified RAC specimens with varying contents of NS (0 %, 1 %, 2 %, 3 %) and NC (0 %, 1 %, 3 %, 5 %), both as single additives and composite modifiers. The macroscopic performance evolution and microstructural damage mechanism of dual nano-modified RAC damaged by frost action were systematically studied by X-ray computed tomography (X-CT), scanning electron microscopy (SEM), and other technologies, combined with statistical analysis methods. The results indicated that with the increase in the number of freeze-thaw cycles, the mass loss rate of all specimens first decreased and then increased, while the relative dynamic elastic modulus (RDEM) continuously decreased. Furthermore, the rate of performance degradation was significantly slower in the composite-modified specimens compared to both single-doped and unmodified samples. Under a constant NS dosage, the mass loss rate and damage degree in the composite-modified specimens first decreased and then increased with higher NC content. Among all combinations, the mix with 3 % NS and 3 % NC (S3C3) exhibited the best performance. Moreover, the synergistic incorporation of NS and NC contributed to both filler and pozzolanic effects, refining the pore structure, enhancing the bonding strength of the interfacial transition zone (ITZ), and inhibiting the propagation of microcracks caused by frost action. A damage evolution model based on the RDEM demonstrated that composite modification could reduce freeze-thaw damage in RAC by more than 40 %. The synergistic effect of 3 % NS and 3 % NC effectively balanced pore structure refinement and hydration product strengthening, significantly improving frost resistance and durability. This study establishes a quantitative relationship between nanomaterial dosage and freeze-thaw damage, providing an experimental basis and theoretical support for the optimal design of recycled concrete structures in cold regions.