Feasibility of solid waste-based geopolymers for anti-scouring protection of offshore scour pits: Macro-performance, micro-characterization, and sustainability

This study proposes a method for anti-scour protection of offshore monopile foundations by using solid waste-based geopolymer materials. Fly ash (FA) and soda residue (SR) were mixed with sodium silicate (SS) solution to prepare fly ash-soda residue-based geopolymer paste (FSGP). Fluidity, setting time, anti-washout, unconfined compressive strength (UCS), and anti-scour properties were tested to assess its feasibility for offshore anti-scour protection. The reaction mechanism was revealed through microscopic methods. Additionally, pH value, chloride ion content tests, carbon emissions, and economic evaluations were conducted to assess its environmental compatibility and sustainability. The results indicated that higher FA content or less solution concentration improved fluidity and prolonged setting time, while increased SR content or solution concentration enhanced dispersion resistance and anti-scour performance. Amorphous C-S-H and C(N)-A-S-H gels formed within the FSGP system, filling pores and creating a dense microstructure that boosted strength. The optimal proportion, FS3C2 (20 %FA + 80 %SR, solution concentration of 2.5 mol/L), exhibited a fluidity of 210 mm, 1-day UCS, and anti-scour UCS of 0.38 MPa and 0.35 MPa, respectively, meeting specification requirements. Cl⁻ content and pH values were within marine environment-safe ranges, and Sustainability assessments indicated its ecological and economic benefits, highlighting its potential for sustainable development. • Solid waste-based geopolymer was prepared for scouring protection of offshore foundations. • FSGP demonstrates favorable fluidity, strength, anti-washout, and anti-scouring properties. • The strength development mechanism of FSGP is revealed. • FSGP is environmentally friendly to marine environments and serves as a filling material for the low-carbon economy.