Foamed bitumen stabilisation of recycled steel slag aggregates utilising geopolymer as a secondary binder

Foamed bitumen stabilisation is a sustainable technique for enhancing the mechanical properties of unbound granular materials and rehabilitating aged pavements. This process produces a flexible pavement base and subbase layers with improved mechanical performance and durability. This study investigated foamed bitumen stabilised (FBS) mixtures for pavements in which the aggregate matrix consists entirely of industrial by-products. Three aggregate compositions, including 100% ladle furnace slag (100LFS), 100% electric arc furnace slag (100EAFS) and a combination of 50% EAFS and 50% LFS by mass (50EAFS/50LFS) were used as parent materials for foamed bitumen stabilisation. Alkali-activated fly ash (FA) and slag (S)-based geopolymer was used as a secondary binder to replace conventional carbon-intensive binders such as hydrated lime or cement while enabling rapid strength gain. The mechanical performance of the FBS mixtures was evaluated through unconfined compressive strength (UCS), indirect tensile resilient modulus (ITM r ), repeated load triaxial (RLT) and four-point flexural bending tests, while reinforcement mechanisms were studied using scanning electron microscopy. The results showed that the FBS mixtures incorporating 10%(FA+S) geopolymer achieved higher UCS values than benchmark mixtures with 2% cement, ranged from 4.05 MPa to 20.1 MPa with respected to different aggregate compositions and curing conditions. Two FBS mixtures, 50EAFS/50LFS + 3%B + 10%(FA+S) and 100LFS + 3%B + 10%(FA+S) complied the stiffness requirement for pavement applications, with 50EAFS/50LFS + 3%B + 10%(FA+S) being the optimum mixture and achieved highest ITM r of 4135 MPa and 2685 MPa under unsoaked and soaked condition, respectively. RLT and four-point flexural bending results also showed acceptable performance of the resilient modulus and fatigue resistance of the FBS mixtures incorporating 10%(FA+S) geopolymer. The findings of this study demonstrated the technical feasibility of using steel slag aggregates and geopolymers in foamed bitumen stabilisation for future sustainable road construction, offering improved mechanical properties and durability and a reduced carbon footprint.