Geopolymer, Alkali-Activated, and Waste-Derived Binders for Soft-Soil Stabilization: A Critical Review

Geopolymer, alkali-activated, and waste-derived binders offer alternatives to cement for soft-soil stabilization, but their benefits depend on material and site conditions. This review distinguishes four overlapping groups by dominant reaction chemistry: low-calcium geopolymers, high-calcium alkali-activated binders, hybrid systems, and waste-derived cementitious binders. It separates direct soft-soil evidence from supporting studies on other soils and non-soil binders, linking reaction products, soil–binder interactions, and pore evolution to engineering behavior. In one soft soil with 50.4% initial water content and 20% precursor addition by dry soil mass, 28-day unconfined compressive strengths ranged from 0.09 to 8.22 MPa across fly ash–slag formulations cured at 23 °C. Stabilization generally increases strength and stiffness and reduces compressibility, but stronger cementation can increase brittleness and post-peak softening. Hydraulic conductivity depends on pore connectivity, cracking, and stress. Fly ash–slag blends can balance early strength and workability, but optimum proportions remain soil-specific. Of 74 mapped publications, 63 report laboratory unconfined compression and two report cyclic/dynamic response; creep, coupled durability, and long-term field performance remain poorly documented. Activator production and precursor processing can offset environmental gains from waste reuse. Comparisons with cement require equivalent engineering performance, consistent life-cycle boundaries, and leaching assessment. These findings support soil-specific and performance-based binder design.