An orthogonal experimental study on low-activity bottom ash based geopolymers for soft clay stabilization

The excessive accumulation of bottom ash, a low-reactivity by-product of coal combustion, poses severe environmental and land-use challenges worldwide. This study proposes a sustainable approach to reuse bottom ash as a geopolymer precursor for stabilizing soft clay. An orthogonal experimental design was employed to systematically evaluate the effects of BA content (0–20%), alkali content (7–11%), and alkali activator modulus (0.8–1.6) on the Atterberg limits, compaction characteristics, and unconfined compressive strength (UCS) of the treated clay. Test results indicated that as the levels of these factors increased, the liquid limit and plasticity index decreased, while the plastic limit increased. The optimum moisture content decreased, whereas the maximum dry density increased. The UCS demonstrated a consistent upward trend over 7, 14, and 28 days of curing, achieving a maximum 28-day strength of 9.62 MPa. Range analysis and analysis of variance (ANOVA) identified BA content and alkali activator modulus as the dominant factors influencing UCS. A predictive UCS model was established with high fitting accuracy (R² = 0.955–0.968). Microstructural characterization via XRD, FTIR, and SEM revealed that the strength improvement was primarily attributed to the formation of dense aluminosilicate gels, which effectively bonded soil particles. This research highlights the potential of low-activity bottom ash-based geopolymers as eco-efficient binders, contributing to carbon reduction, waste valorization, and sustainable geotechnical infrastructure development.