Expansive soils cause significant damage to infrastructure due to their swelling and shrinkage behavior with changing moisture content. This study evaluated the potential of Costus chartaceus mass ash (CCMA), an agricultural waste, in stabilizing expansive lateritic and clay soils from Nigeria for use as pavement subgrade material. Specimens of the problematic soils were mixed with varying proportions (2.5-10%) of CCMA and a fixed 5% cement content. The geotechnical properties of maximum dry density, optimum moisture content, consistency limits, California bearing ratio (CBR), unconfined compressive strength (UCS), and free swell index were determined via standardized tests. Results showed the maximum dry density generally decreased with higher CCMA contents due to ash particles occupying soil voids. However, inclusion of up to 2.5% CCMA increased density through beneficial pozzolanic reactions. Optimum moisture content rose with greater ash amounts owing to its hydrophilic nature. Consistency limits like liquid limit and plasticity index decreased significantly, while plastic limit increased, demonstrating modifications to clay mineralogy restricting water absorption. Both unsoaked and soaked CBR ratios augmented noticeably with rising CCMA up to 7.5%, reflecting strengthened inter-particle bonds. UCS strengths also steadily climbed to a peak at 7.5% CCMA, signifying physicochemical improvements. A judicious combination of 7.5% CCMA and 5% cement achieved optimal performance for controlling volume change and imparting bearing capacities to the problematic soils. This reflects balancing of pozzolanic gain versus dilution effects. The study validates use of CCMA-cement composite for rectifying consistency issues and upgrading strength characteristics of expansive soils. CCMA shows potential as an economical, eco-friendly stabilizer leveraging its high silica and pozzolanic properties compared to other agricultural wastes. Further work could explore CCMA blending with additional activators to maximize stabilization efficacy.