AbstractDispersive clays are one of the common problematic soils owing to their unstable structure, which causes numerous geotechnical problems in construction projects. One of the main strategies to ameliorate these weak soils is to modify their mechanical properties with additives. Because little research about the influence of nanomaterials on the problematic soils’ characteristics is available, this study aims to evaluate the effect of nanosilica on the mechanical, mineralogical, and microstructural behavior of dispersive clay. A series of experimental tests, including the pinhole, double-hydrometer, and crumb tests, is conducted to determine the dispersivity potential of the soil treated with nanosilica. The Atterberg limits, standard compaction, unconfined compression (UCS), and direct shear tests are also carried out. Additionally, mineralogy and the microstructure of the base soil and treated specimens were analyzed through X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The results indicate that adding the optimum amount of 1% nanosilica to the dispersive clay altered the soil characteristic into nondispersive. The liquid limit (LL), plastic limit (PL), and optimum moisture content (OMC) of the treated soil specimens increased; nevertheless, the plasticity index (PI) and maximum dry density (MDD) decreased with the application of nanosilica. Remarkable improvement in UCS, absorbed energy, secant elastic modulus (E50), and the direct shear strength was achieved in the specimens treated with 1% nanosilica content after 7 and 28 days curing. The formation of calcium–silicate–hydrate (C─ S─ H) gel and substantial reduction of the porous structure were observed through the XRD results and SEM micrograph after 28 days of curing.