<p>Leachate contamination significantly alters the geotechnical properties of clay, leading to reduced strength and stability in waste containment and foundation systems. Addressing this challenge, the present study explores the stabilization of natural and leachate-contaminated clay using micro-silica and nano-silica as environmentally friendly additives. The objective was to evaluate their effects on compaction behavior, unconfined compressive strength (UCS), and stiffness, with particular emphasis on comparing the relative effectiveness of micro- and nano-silica and identifying the governing stabilization mechanisms. Experimental tests included Standard Proctor compaction, and UCS performed on samples prepared with varying dosages of micro-silica (3, 6, 9, 12%) and nano-silica (0.3, 0.6, 0.9, 1.2%). All specimens were cured for 28 days before testing. The results revealed that leachate exposure considerably decreased the UCS and stiffness of clay. However, both micro- and nano-silica improved the mechanical response. Notably, nano-silica showed higher efficiency at lower contents, with 0.9% producing the greatest strength gain, whereas micro-silica required around 9% for similar improvement. Compaction outcomes indicated a reduction in maximum dry density (MDD) and an increase in optimum moisture content (OMC), mainly due to particle replacement and higher water affinity of silica. The enhancement in strength and stiffness is attributed to pozzolanic reactions, void filling, and the formation of cementitious products that stabilized the soil fabric. It is concluded that micro- and nano-silica can effectively mitigate the adverse effects of leachate on clay, offering a promising technique for soil stabilization. Future research should address long-term durability, cyclic environmental loading, and the combined use of silica with other additives to optimize field applications.</p>