Mechanical, volumetric, and stress-dependent cyclic behavior of expansive clay stabilized with pumice-based geopolymer

This study investigates the use of a pumice-based geopolymer as a sustainable stabilizer for expansive clay, with emphasis on strength development, volumetric stability, and stress-dependent cyclic behavior. Although geopolymer binders have been widely explored, pumice-based systems for clay soils remain limited, particularly in terms of cyclic stiffness and stress-dependent behavior. An experimental program was designed to evaluate the effects of pumice content (10–30%), alkaline liquid ratio (0.20–0.30), and NaOH molarity (8–12 M). The results demonstrate that geopolymer stabilization significantly enhances soil performance, with unconfined compressive strength increasing by up to 1089% at 7 days and 820.6% at 28 days relative to the corresponding age-matched untreated controls, while swelling potential decreases from 43.75% to approximately 10%. The highest unconfined compressive strength was achieved at a moderate pumice content (~ 20%) and an intermediate alkaline liquid ratio (0.25), whereas the highest resilient modulus was obtained at 10% pumice. Increasing NaOH molarity further enhanced compressive strength. Resilient modulus results reveal stress-dependent behavior, characterized by increasing stiffness with confining pressure and a non-monotonic response to deviatoric stress. A stress–strength-based formulation is proposed, enabling consistent prediction of cyclic stiffness across different mixtures and loading conditions. Sustainability assessment indicates improved strength–carbon efficiency relative to cement-treated soils, highlighting the potential of pumice-based geopolymers as a sustainable solution for ground improvement.