<p>Sodium carbonate-activated slag mortar (SCASM) exhibits the advantages of low shrinkage, high strength in late age, and low-cost and carbon emissions. However, its microstructure is relatively porous due to the formation of crystalline carbonate phases, which compromise its durability in marine and other aggressive environments. To address this, a highly reactive and dispersive alkali-catalyzed nano-silica sol (NSS) was incorporated to enhance the durability of SCASM. The mechanisms were explored using scanning electron microscopy, X-ray diffraction, and mercury intrusion porosimetry. The results showed that NSS significantly reduced the porosity and pore connectivity of SCASMs, decreasing their surface alkalinity, immersion porosity, capillary water absorption, and susceptibility to freeze-thaw damage. Under sodium sulfate and magnesium sulfate erosion, the strength loss of SCASM decreased markedly with increasing NSS content. Notably, after 180 days in sodium sulfate solution, the incorporation of 8% NSS reduced the compressive and flexural strength loss rates of SCASM by 46.9 and 41.9%, respectively, compared to the control without NSS. Furthermore, NSS significantly enhanced the SCASM’s resistance to chloride ion penetration, and chloride salt dry-wet cycles. The improvements are primarily due to the NSS-induced refinement of pore structure that hindered aggressive ion ingress, combined with the physical adsorption of the aggressive ions by alkali-catalyzed NSS. These collectively suppressed expansive product formation and thus mitigated SCASM degradation in corrosive environments. Additionally, NSS promoted the formation of a more polymerized and stable gel structure, enhancing its resistance to destabilization under aggressive conditions. This study offers an effective and low-cost solution to improve the durability of SCASMs, promoting their practical application in marine and other aggressive environments.</p>