Black cotton soil's notable swelling and shrinkage contribute to structural damage. This study examines the impact of nano rice husk ash (nRHA) variants on this soil: One synthesized in 60 h and another through 7 h combined dry-wet milling method. The primary objective is to assess the effects of nRHA treatment on the soil's index properties, engineering characteristics and swelling behavior. Laboratory tests including free swell index, Atterberg's limits, swelling potential, swelling pressure, unconfined compressive strength and consolidation tests were conducted on black cotton soil samples treated with both nRHA variants. Results indicated that the 7-h nRHA treatment led to lower plasticity and reduced swelling compared to the 60-h variant. Specifically, the 7-h treated soil showed decreased swelling pressure, compression index and rate of primary swelling, along with increased pre-consolidation pressure and unconfined compressive strength. The free swell index also decreased by 21% with the 7-h nRHA treatment. The superior performance of the 7-h milled nRHA is likely due to its higher calcium and reactive silica content, enhancing its stabilizing effect. This research highlights the 7-h nRHA as a more effective stabilizer for black cotton soil, offering a promising solution to mitigate its problematic volumetric behavior.