Kaolinitic clay soils are widely encountered in geotechnical engineering projects and often exhibit low mechanical strength, high compressibility, and limited durability under environmental loading conditions. Improving the engineering performance of such soils while reducing cement consumption has therefore become an important research objective. In this study, the effectiveness of alkaline activation in enhancing the mechanical behavior, durability, and microstructural characteristics of chemically stabilized kaolinitic clay was investigated. Soil specimens were prepared using 5%, 10%, and 15% stabilizer contents, while Portland cement was partially replaced with silica fume at replacement levels ranging from 10% to 70%. Alkaline activation was carried out using sodium hydroxide (NaOH) solutions with molarities of 4–10 M and sodium silicate (Na₂SiO₃) dosages of 1–2%.

Based on the preliminary optimization results, the mixture containing 10% total stabilizer with 50% cement replacement by silica fume, activated by 8 M NaOH and 1.5% Na₂SiO₃, exhibited the most favorable performance and was selected for detailed investigation. The mechanical properties of the optimized mixtures were evaluated through unconfined compressive strength (UCS) and indirect tensile strength (ITS) tests after 28 days of curing. Durability performance was assessed under repeated wetting–drying cycles, while microstructural changes were examined using scanning electron microscopy (SEM).

The results demonstrated that alkaline activation significantly enhanced the strength and durability of the stabilized soil. The optimum mixture achieved a UCS value of 3.14 MPa, corresponding to an increase of approximately 31% compared with the non-activated mixture. Improvements in tensile strength and resistance to environmental degradation were also observed. SEM observations revealed the formation of a denser and more integrated matrix resulting from the development of cementitious and geopolymeric reaction products, leading to reduced pore spaces and improved particle bonding.

The findings indicate that alkaline-activated cement–silica fume stabilization can provide an efficient and environmentally sustainable approach for improving the performance of clayey soils in geotechnical applications exposed to adverse environmental conditions.