Ion-guided gelation indices and strength–durability framework of seawater–marine sand geopolymer concrete for sustainable coastal infrastructure

Freshwater scarcity and the depletion of river sand are pressing global challenges for coastal construction. Seawater and marine sand are therefore attracting interest as alternative resources. Geopolymer concrete (GPC), produced from industrial by-products, offers a low-carbon substitute for Portland cement, but its performance under saline activation is sensitive to ionic chemistry. In this study, mixes with 0 %, 35 %, 70 %, and 100 % seawater–marine sand were prepared. Their mechanical properties were tested alongside X-ray diffraction (XRD), which provided the basis for three indices: amorphous gel fraction (λ), stability-to-interference ratio (γ), and the Gel Stability Index (GSI). These descriptors capture the balance between gel continuity and crystalline disruption. At 35 % replacement, a hybrid network of N-A-S-H (sodium-alumino-silicate-hydrate) and C-(A)-S-H (calcium-(alumino)-silicate-hydrate) was retained, giving stable strength gains. Complete seawater use gave the highest compressive strength, largely from Na⁺-induced gelation and delayed zeolitic/C–S–H reinforcement. In contrast, 70 % replacement led to oversaturation, early brucite–halite precipitation, and premature strength loss. The freshwater control showed only modest gains, consistent with limited ionic activation. GSI tracked both compressive and flexural strength, indicating its value as a composite indicator of gel integrity and long-term stability. By linking ion chemistry, gel evolution, and structural performance, this work proposes a microstructural design framework for seawater–marine sand GPC. These results point to durable, low-carbon concretes well suited for coastal infrastructure. Of particular interest, GSI emerges as a mechanistic candidate descriptor for durability-relevant behaviour, including chloride ingress, sulphate/magnesium attack, and sorptivity. Its predictive role, however, awaits systematic validation in future durability studies. • Introduces gelation indices (λ, γ, GSI) for seawater–sand geopolymer concrete. • Identifies salinity thresholds: moderate stabilizes, oversaturation destabilizes. • Positions GSI as strength predictor and durability descriptor under marine exposure. • Establishes structure–property framework linking gels, strength, and phases. • Provides design pathway for low-carbon, resource-efficient marine infrastructure.