<p>The remediation of weak soils contaminated with waste leachate is a key concern in construction projects. Adding additives such as nanomaterials has been regarded as an efficient strategy to alter soil behavior. This study examines the effect of nano calcium carbonate (nano-CaCO₃) on the geotechnical properties of natural and leachate-contaminated clays through Atterberg limits, standard compaction, unconfined compressive strength (UCS), falling head permeability, and consolidation tests. Four nano-CaCO₃ contents (0.2%, 0.4%, 0.8%, and 1.2% by dry weight) were used. Results showed that increasing nano-CaCO₃ reduced the liquid limit (LL) and plasticity index (PI) in both soils. Nano-CaCO₃ also increased the optimum moisture content and decreased the maximum dry unit weight. Permeability decreased with nano-CaCO₃ addition but increased due to contamination. Adding 1.2% nano-CaCO₃ raised the UCS by 21.5% in natural clay and 92.7% in contaminated clay. The secant modulus (E₅₀) increased by 64.3% and 78.2%, and the constrained modulus (E_c) by 131.2% and 124.8%, respectively. SEM analysis confirmed the formation of denser microstructures, improved particle bonding, and reduced voids in stabilized samples, especially in contaminated clay. This study highlights the potential of nano-CaCO₃ as an effective stabilizer for improving both the mechanical behavior and microstructure of natural and contaminated clays.</p>