<p>Collapsible loess soils exhibit severe volume reduction upon wetting, leading to major geotechnical failures. Conventional stabilizers often fail to provide sufficient improvement in their mechanical behavior, especially under long-term curing conditions. To address these limitations, this study investigates the impact of different contents of nano calcium carbonate (NCC) on improving the mechanical and long-term properties of this soil type. Both destructive tests, including standard compaction, Atterberg limits, the indirect tensile strength (ITS), the unconfined compressive strength (UCS), consolidated-drained direct shear tests and ultrasonic pulse velocity (UPV) test, were conducted. The experimental findings indicated a significant enhancement in strength parameters following the addition of 0.4% NCC, with UCS and ITS increasing by approximately 90% and 155%, respectively. Moreover, as the NCC content increased, reductions of about 8% and 15% were observed in the liquid limit (LL) and plastic limit (PL) of the soil, respectively. This modification also resulted in a 17% increase in the optimum moisture content (OMC), accompanied by a 6% reduction in the maximum dry density (MDD). The long-term soil behavior in the consolidated-drained direct shear test showed that the cohesion and internal friction angle increased by 81% and 7% with increasing curing times of up to 90 days. The mechanical and shear characteristics of soil decreased at NCC content greater than 0.4% due to the agglomeration of nanoparticles and decreased distribution uniformity. Therefore, the optimum content of NCC was determined to be 0.4% to achieve the best mechanical and geotechnical performance. The UPV also demonstrated a direct relationship with the soil’s cohesion and internal friction angle, indicating this test’s high capability for accurate estimation of soil mechanical characteristics.</p>