Abstract: To investigate in depth the influence of alkaline solution (NaOH) on the physical and mechanical properties of red clay, undisturbed red clay samples were immersed in six NaOH solutions of different concentrations and cured for varying durations. By testing the physical-mechanical indices of alkali-contaminated red clay and conducting indoor simulation experiments, the relationships among physical-mechanical behavior, alkali concentration, and curing time were established. The main findings are as follows: 1) Physical properties of alkali-contaminated red clay are primarily controlled by NaOH concentration. As concentration increases, dry density decreases, specific gravity increases, the liquid limit decreases then increases, and the plastic limit increases then decreases. 2) The mechanical properties exhibit a clear concentration effect. With increasing NaOH concentration, the shear strength first decreases, then increases, and finally decreases again. Before curing, the shear strength is lowest at 3.5%, indicating completion of reactions involving active cementitious substances. Further increases in concentration lead to strength recovery, but at 21%, shear strength decreases again. After 10 days of curing, shear strength increases across all concentrations, with the most pronounced improvement at 14%. 3) An optimal concentration range exists for shear strength of alkali-contaminated red clay: performance is worst at 3.5% and best at 14%. 4) Alkali-contaminated red clay requires a certain curing period to allow newly formed cementitious products to fill pores, thereby increasing density and strength.

Key words: Alkali Contamination, Red Clay, Physical Properties, Mechanical Properties