Sustainable anti-washout geopolymer grout from soil-fly ash-GGBS for scour protection of offshore structures under in-situ conditions

Cement is commonly used to mitigate local scouring around offshore foundations; however, it suffers from poor durability in harsh marine environments and contributes to carbon emissions. Therefore, this study aims to develop sustainable alkali-activated-soil-fly ash-GGBS (AASFG) slurry for scour protection. Slurry was formulated using silty soil with 20%, 30%, and 40% fly ash and GGBS (1:1 ratio), activated with sodium silicate and NaOH at varying modulus ratios (1.0, 1.2, and 1.4) and Na2O contents (6%, 8%, and 10%). Performance was evaluated through fluidity, turbidity, washout resistance, and unconfined compressive strength (UCS). Results showed that slurry behavior is governed by the interplay of modulus, Na2O, and binder content. While the mix S60F20G20M1.2N10 achieved the highest UCS (12.88 MPa in air, 10.29 MPa underwater), it had poor washout resistance. The mix S70F15G15M1.2N8 exhibited balanced performance. Co-existence of ettringite, N-A-S-H, C-A-S-H, and C-S-H gels contributed to strength. AASFG slurry shows promise as an eco-friendly anti-scour.