Calcium-leaching-resistant shotcrete for subsea tunnels: Effects of alkali-free accelerator, early-strength material and leaching inhibitor

Calcium leaching can reduce the long-term mechanical capacity of shotcrete used as primary support in subsea tunnels. This study investigated the effects of an alkali-free accelerator, an early-strength material (ESM), and a leaching inhibitor on the strength development, pore structure, and calcium leaching resistance of shotcrete. Laboratory-cast mortar specimens and specimens cut from field-sprayed panels were exposed to accelerated leaching in 6 mol/L NH 4 Cl. Compressive strength, water absorption, mass loss, leaching depth, hydration heat, XRD, TG, SEM–EDS, and low-field NMR were used to relate macroscopic deterioration to phase and pore evolution. An accelerator dosage of 6–8% achieved the best balance between early strength and calcium-leaching resistance. Higher accelerator dosages reduced 28-day compressive strength and increased strength loss during the early stage of leaching. Within the Z-series, where the inhibitor dosage was fixed, an ESM/cement ratio of 15–20% provided the best balance among early strength, later-age strength, and leaching resistance under the tested conditions. At fixed background compositions, the 28-day compressive-strength loss rates ranged from 20.21% to 27.58% across the Z-series and from 20.09% to 27.58% across the Y-series, with Z15 and Y2 showing the lowest losses, respectively, indicating non-linear dosage responses of ESM and the inhibitor. EDS mapping showed smaller leaching-induced changes in the area-averaged apparent Ca/Si ratio for the selected functional-material specimens than for the Ref. Accelerated leaching tests on field-sprayed panel specimens were used to compare ESM-enhanced and leaching inhibitor-modified shotcrete, and a mechanical degradation model based on composite mechanics was established to describe compressive-strength deterioration under different leaching degrees (all R 2 ＞0.91). This work provides a scientific basis for durability design and degradation assessment under accelerated leaching conditions of shotcrete in subsea tunnels.