Effect of potassium hydroxide and calcium lignosulfonate on the plasticity, strength, and wetting–drying durability of black cotton soil: engineering performance and microstructural characterization

Expansive black cotton soil (BCS) exhibits high plasticity, swelling potential, low strength and poor durability, which restricts its usage in geotechnical applications. The present study explores the stability of BCS by alkali activation using potassium hydroxide (KOH) as alkali solution and calcium lignosulfonate (CLS) as an ecofriendly soil stabilizer. The mechanistic transformations in the BCS treated with KOH and CLS were investigated by evaluating the Atterberg limits, unconfined compressive strength (UCS), wetting–drying cycles, and microstructural analysis such as XRD. In the present work, 1 N, 2 N and 4 N solutions of KOH alongside 0.5%, 1% and 2% CLS were used. Results indicated that the liquid limit and plasticity index of stabilized specimens decreased, while UCS and durability improved. The BCS with 2 N KOH and 1% CLS exhibited the best performance, enhancing strength and resistance to wetting-drying cycles. KOH treatment increased the alkali activation and may led to the flocculation of clay particles by dissolving aluminosilicate minerals, while CLS may possibly attributed to improved stability through calcium-mediated aggregation and polymeric bonding. The maximum UCS value was recorded as 586 kPa for BCS+2 N KOH + 1% CLS at 90 days curing as compared to untreated soil (181.42 kPa).Microstructural characterization performed on the optimal combination of BCS+2 N KOH + 1% CLS using XRD and FESEM indicated that the BCS stabilization mechanism may be associated with the partial dissolution of clay minerals and retention of quartz, probably leading to the possible formation of potassium/calcium-modified aluminosilicate cementitious products K-A-S-H and C-(K)-A-S-H reaction products, thereby enhancing engineering performance.