Optimum dosage and long-term sulfate resistance of cement-based materials modified by nano-SiO₂ and nano-CaCO₃

To enhance the durability of cementitious composites in sulfate-rich environments, this study investigates the effects of varying dosages of nano-silica (NS) and nano-calcium carbonate (NC) on their macroscopic behavior and microstructural evolution during a 180-day sulfate exposure period. By integrating measurements of mass change, compressive strength, and rupture strength with X-ray diffraction (XRD), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR), the underlying modification mechanisms of both nanomaterials are systematically revealed, and their optimal contents are determined.Experimental results indicate that all specimens exhibit a similar evolution law: mass, compressive strength, and rupture strength first increase and then decrease, whereas porosity first declines and subsequently rises. The optimal dosages corresponding to different evaluation indicators vary. NS, through the dual effects of pozzolanic reaction and physical filling, consumes calcium hydroxide to generate dense C-S-H gel, inhibits the formation of attack products, and delays structural deterioration. Considering both macroscopic mechanical properties and microscopic pore structure performance, the recommended optimal dosage of NS is 3%–5%. NC enhances early-age performance through nucleation effects and micro-aggregate filling, but high dosages exacerbate later-stage expansion and cracking, so the recommended optimal dosage of NC is 1%.At the same dosage level, NS presents a more remarkable overall modification effect than NC, especially in pore structure optimization, micro‑morphology improvement, and long‑term sulfate resistance enhancement.The outcomes of this research offer both a theoretical basis and practical guidance for the nano-modified design and field application of cementitious composites exposed to sulfate-rich conditions.