Boundary-condition-dependent mechanical response of clayey sand stabilized with cement and nano-MgO

• Novel use of nano-MgO and cement improves stiffness of clayey sand. • Cement content was the strongest factor in enhancing soil stiffness performance. • Optimum nano-MgO dosage of 1% maximized stiffness; higher dosages caused a decline. • Moderate clay content improved stiffness, whereas higher clay fraction reduced it. • Micromechanical model accurately reproduced stiffness trends with nano-MgO. Although nano-stabilization has been widely explored in geotechnical engineering, most studies have emphasized strength improvement, with limited attention to predicting stiffness–settlement behavior and evaluating the relative contribution of mix components. This research bridges this gap by combining laboratory testing with a micromechanical modeling framework and AI-based sensitivity analysis to interpret stiffness trends in cement/nano-magnesium oxide (nano-MgO) treated clayey sand. A comprehensive experimental program was conducted on Anzali sand with 0–30% clay, treated using 0–9% cement and 0–4% nano-MgO, under both partly confined and fully constrained conditions. The results indicated that stiffness was highly sensitive to clay content, binder dosage, and nanoparticle addition. With 15% clay and 1% nano-MgO, stiffness increased by 38% in partly confined samples, whereas incorporating 9% cement with the same nano content enhanced stiffness by up to 33% under constrained loading. SEM observations of the optimal mix (SC15+9%C+1%NM) revealed a compact microstructure enriched with C–S–H products. The developed micromechanical model, which integrates nanoparticle interphase and agglomeration mechanisms, showed strong agreement with experimental stiffness trends. Moreover, sensitivity analysis using an MLP network confirmed cement as the dominant factor influencing stiffness, while nano-MgO and clay exhibited secondary yet condition-dependent effects. The integration of tests and modeling presents a new route for predicting and optimizing mix design in nano-treated soils. These findings provide practical insight for improving stiffness and controlling settlement of clayey sands under realistic confined conditions, offering direct engineering relevance for foundation systems and deformation-sensitive geotechnical applications.