<p>Alkaline environments significantly affect the engineering properties of red clay. Although the effects of alkali contamination on the microstructure and mechanical characteristics of cohesive soils have been extensively studied, systematic research focusing on red clay remains limited. This study employed NaOH solution to simulate alkali contamination of red clay. By utilizing unconsolidated undrained (UU) triaxial shear tests, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), laser particle size analysis (LPSA), and X-ray diffraction (XRD), this work systematically investigated the influence of different alkali concentrations on the mechanical properties and microstructure of red clay. The results indicate a distinct threshold effect of alkali concentration on red clay behavior, with the existence of a most unfavorable concentration (3.5%) and an optimal concentration (14%). At the 3.5% concentration, destructive dissolution was dominant, in which mineral dissolution led to particle refinement, surface smoothing, and an increase in the proportion of large pores, resulting in a significant reduction in soil strength. At 14% concentration, reconstructive cementation became prevalent. XRD analysis confirmed the formation of new crystalline phases (sodium aluminosilicate), indicating the occurrence of geopolymerization. The newly formed cementitious materials effectively bonded soil particles and filled pores, thereby enhancing shear strength, although their brittle nature resulted in strain softening during shearing. At excessively high concentrations (e.g., 21%), structural degradation and strength reduction were again observed. This research reveals the dynamic evolution of the “dissolution-cementation” competition mechanism in red clay under different concentrations of alkali contamination, providing a theoretical basis for preventing alkali contamination in red clay foundations and for soil reinforcement techniques based on alkali activation principles.</p>