Long-term performance and RSM-based optimization of fly-ash/bentonite synchronous grout for undersea shield tunnels

High-performance synchronous grouts are crucial for the watertightness and long-term serviceability of undersea shield-driven tunnels, and their mixing design remains largely empirical. This study combined an extensive laboratory campaign with response surface methodology (RSM) to develop a statistically optimized grout. Class F fly ash (0–25 wt%) and sodium bentonite (0–7.5 wt%) were used as additives to Portland cement. Initially, 30 mixes were assessed to determine the dosage of water and superplasticizer. Subsequently, 24 mixes were prepared with varying fly ash and bentonite ratios to assess the fresh properties (flow cone time, mini slump, table spread, and bleeding), compressive strength (28, 90, 180, and 360 days), scour resistance, and marine durability (water absorption, electrical resistivity, sorptivity, mass loss, and chloride penetration after immersion in synthetic seawater). Second-order RSM models (R² ≥ 0.97) accurately captured eight response variables and revealed strong fly ash–bentonite interactions: fly ash improved the long-term strength and chloride resistance, whereas bentonite suppressed bleeding but increased viscosity. Optimization identified 20 % fly ash and 4 % bentonite at a water-to-cement ratio (w/c) of 0.35 as the best compromise between workability and durability. The predicted mix achieved a 28-day strength of 75.95 MPa. The validation tests showed deviations of less than 5 % from the RSM predictions. Compared to the control group, mass loss and chloride ingress at 360 d were reduced by 81.4 % and 47.6 %, respectively. The resulting design charts and regression equations provide tunnel engineers with a data-driven tool for customizing synchronous grouts that meet stringent long-term performance requirements of tunnel boring machines.