Geotechnical Behaviour and Seawater Durability of Marine Soft Soil Stabilized Using Nano-Clay Modified Geopolymer with Flax Fiber Reinforcement

• Factorial design optimized geopolymer-stabilized soil with flax and nano-clay. • Additives boosted performance over the unamended geopolymer-stabilized soil control. • Optimal mix (1.5% nano-clay, 1.0% flax fiber) achieved >12 MPa compressive strength. • Flax fiber reinforcement transformed the failure mode from brittle to pseudo-ductile. • Dense geopolymer matrix ensured high seawater durability with Marine soft soils present significant geotechnical challenges due to their low strength, high compressibility, and poor engineering properties. Geopolymer binders offer a sustainable alternative to traditional cementitious materials for soil stabilization. This study experimentally investigates the synergistic effects of binder composition, nano-clay modification, and natural flax fiber reinforcement on the performance of geopolymer-stabilized marine soft soil through a comprehensive factorial design. A 3 × 2 × 3 × 3 full-factorial experiment, encompassing 54 unique mix proportions, was conducted, varying the Fly Ash (FA) to Ground Granulated Blast-furnace Slag (GGBS) ratio (100/0, 70/30, 50/50), NaOH activator molarity (8M, 12M), nano-clay content (0.0%, 1.5%, 3.0% by binder weight), and flax fiber content (0.0%, 0.5%, 1.0% by binder weight), while maintaining a constant 30% soil replacement level, SS/SH, and Alkali Activator ratios. Performance metrics evaluated included Unconfined Compressive Strength (UCS) development (7, 28, 60, 90 days), stress-strain behavior, toughness index (area under the stress-strain curve), and durability (UCS loss after 62 days of simulated seawater immersion, following 28 days of standard curing). Experimental results demonstrated significant positive effects from incorporating GGBS and using higher molarity activator on UCS, toughness, and durability. An optimal nano-clay dosage of 1.5% was identified, exhibiting strong synergistic interactions with both the binder system and flax fiber reinforcement, particularly enhancing toughness. Flax fibers were observed to be crucial for transforming the failure mode from brittle to ductile, dramatically increasing toughness. Optimized mixes, combining high GGBS content, 12M NaOH, 1.5% nano-clay, and 1.0% flax fiber, achieved 28-day UCS values exceeding 12 MPa and toughness values significantly higher than unreinforced mixes, while maintaining excellent durability (strength loss < 10%). Microstructural analysis (SEM, XRD) correlated these macroscopic improvements with observed matrix densification, C-(N)-A-S-H gel formation, pore refinement, and enhanced fiber-matrix interfacial bonding facilitated by the optimized composition. This study provides a valuable experimental framework for designing high-performance, potentially sustainable geopolymer composites for marine soil stabilization.