Influence of Southeast China’s seawater chemistry on the performance development of cement-sodium silicate grout

Cement-sodium silicate (C-S) double slurry grouts are widely employed for surrounding rock reinforcement and water plugging in subsea tunnel construction. However, their performance is highly sensitive to mix proportions and environmental exposure. The long-term durability of these materials and the tunnels they protect is particularly challenged by persistent seawater immersion. This study systematically investigates the effect of real seawater chemistry from four major sea regions in Southeast China on the durability of C-S grouts prepared with a practical engineering mix ratio. The evolution of mechanical properties, hydration products, and microstructure was analyzed to establish correlations between seawater ionic composition and material degradation. Results indicate that a mix proportion of 0.6 water-to-cement ratio (w/c) with 34 Baumé degree (°Bé) sodium silicate delivered optimal performance across all tested marine environments. Early-age strength and compactness were improved by seawater ion infiltration of 1–10 μm, whereas mid- to long-term degradation varied due to sulfate-induced ettringite formation, linked to differing SO 4 2- concentrations. Optimizing the mix proportion generated a dense matrix that converted expansive ion-reaction products into pore-filling materials, thereby enhancing durability in seawater environments. These findings provide practical guidance for optimizing grout mix design to enhance the long-term durability of subsea tunnels in diverse marine environments.