Integrating phospho-gypsum and alkali-activated binders to develop resilient and sustainable subgrades for future transportation systems

Weak subgrade soils in transportation infrastructure often limit bearing capacity and long-term performance, especially under heavy traffic and harsh environmental conditions. Conventional stabilization methods improve strength but are associated with high carbon emissions. This study presents a sustainable and smart alternative using phosphor-gypsum (PG), an industrial by-product, combined with alkali-activated binders (AAB) made from fly ash (FA) and ground granulated blast furnace slag (GGBFS). The AAB system was activated with a sodium silicate–sodium hydroxide solution to enhance strength development and promote carbon utilization. Laboratory tests, including unconfined compressive strength (UCS) and Brazilian tensile strength (BTS), were performed to assess the mechanical performance of PG–AAB composites with varying FA and GGBFS contents for subgrade applications. The results showed notable gains in strength and durability, supported by microstructural evidence of cementitious phase formation responsible for improved compressive, shear, and tensile behavior after seven days of curing. The integration of industrial waste with AAB enhances subgrade resilience and supports the development of smart, low-carbon, and sustainable transportation systems.