– The efficient utilization of recycled concrete powder (RCP) has attracted much attention. To break through the limitation of single activation technology of RCP, this study investigates the effects of a mechanical-microwave method on the reactivity of RCP.

– The mechanical properties, hydration products, and pore structure of RCP-incorporated mortar were evaluated.

– The results demonstrate that the combined activation reduces the median particle size of RCP and induces a low-frequency shift in the Si-O-T FT-IR characteristic peaks, signifying depolymerization of the silicate network and formation of highly reactive broken bond sites. Concurrently, decreased Si2p and Al2p binding energies in XPS spectra confirm enhanced surface reactivity. The 28-day strength activity index (SAI) of RCP mortar improved from 65.7 (untreated) to 82.2% under optimal activation conditions (90-min ball milling followed by 10-min microwave irradiation), outperforming solely mechanical activation by 3.6–6.1%. Furthermore, combined activation increased chemically bound water content from 22.8 to 33.7%, accompanied by a low-wavenumber shift in FT-IR peaks of hydration products. The total porosity of RCP mortar decreased from 17.2 to 14.6%, indicating a denser pore structure.

– This study explores the feasibility and potential mechanism of mechanical-microwave activation of RCP, aiming to provide valuable insights for the sustainable development of materials. Using activated RCP in cement-based materials reduces the demand for cement and substantially cuts carbon emissions, thereby making a critical contribution to the construction industry’s green and low-carbon transition.

Shield tunneling is widely adopted in major underwater engineering projects, exemplified in Qingdao Jiaozhou Bay Subsea Tunnel, China. As the primary load-bearing component, the mechanical and durability performance of shield segments directly determines the lifecycle safety of subsea tunnels. For the development of “ultra-long distance, ultra-deep burial, and ultra-large cross-section” for subsea tunnels, segment structures face coupled challenges from high hydraulic pressure and aggressive ion erosion, posing severe threats to the long-term service reliability of shield tunnels. The existing research indicates that high hydraulic pressure environments accelerate the corrosion degradation of shield tunnel segments, compromising their load-bearing capacity and durability. Specifically, an elevated hydraulic pressure accelerates chloride ion migration in concrete, alters pore structure characteristics, and reduces microstructural elastic modulus. For underwater projects such as the Qiongzhou Strait Cross-Sea Tunnel, where operational hydrostatic loads can reach up to 2 MPa, the durability deterioration mechanisms, microstructural damage evolution patterns, and chloride ion transport behaviors of shield segment concrete under ultra-high pressure conditions remain unclear.

This study was to conduct an in-depth investigation into the ion transport mechanisms and microstructure deterioration processes of shield tunnel segment concrete subjected to ultra-high hydraulic pressure conditions. In addition, the effects of nano-silica coatings, migratory corrosion inhibitors, and nano-precursor materials on the concrete durability were also investigated, thus providing a theoretical and technical support for ensuring the long-term service safety of shield tunnel segments in subsea environments.

In this study, a typical concrete mix design for shield tunnel segments was used, with enhancements implemented through surface application of nano-silica coatings, migratory corrosion inhibitors, and internal incorporation of nano-precursor materials to improve concrete durability. The experimental methodology encompassed rapid chloride migration (RCM) tests, electrical flux measurements, capillary absorption tests, MIP analysis, and chloride ion transport tests under elevated hydrostatic pressures (i.e., 1-4 MPa). The post-testing evaluations included quantitative free chloride content analysis, high-resolution X-ray computed tomography (X-CT) scanning, and nanoindentation testing to characterize the microstructural degradation.

The free chloride concentration in concrete gradually increases under an elevated hydrostatic pressure. In the control group, the surface chloride concentration is increased by 8.3%, 20.8%, and 29.2% as the pressure is increased from 1 MPa to 4 MPa. However, chloride ingress is significantly mitigated in the specimens treated with corrosion-inhibiting measures. Unloaded concrete exhibits a uniform porosity distribution (1.8%–6.7%) across heights, while hydrostatic pressure induced porosity increases within 10 mm of the surface. A pressure elevation causes a nonlinear decrease in pore connectivity tortuosity by 1.3%, 4.3%, 9.2%, and 14.4%, compared to unloaded conditions, correlating to pressure gradients. Simultaneously, connected pore lengths and equivalent radii increase under a pressure. High hydrostatic pressure degrades the microstructural toughness of mortar and the ITZ, thickens the ITZ, and establishes a coupled deterioration mechanism involving “hydraulic penetration-chemical erosion-mechanical damage.” The application of nano-silica coatings, migratory corrosion inhibitors, and nano-precursor materials significantly reduce the chloride diffusion coefficient, electrical flux, and capillary water absorption of concrete. This enhancement is attributed to the pozzolanic reaction between nano-silica particles and hydration products, generating C-S-H gels that densify the matrix by filling pores. Migratory corrosion inhibitors migrates to the steel-concrete interface via capillary action and vapor-phase diffusion, forming a protective adsorption layer. Meanwhile, nano-precursors generate calcium carboxylate compounds during hydration, occupying submicron pores through hydrophobic and densification effects that substantially improve chloride resistance.

Hydrostatic pressure significantly accelerated chloride ion transport in concrete, and the surface chloride concentrations were increased by 8.3%, 20.8%, and 29.2% as the pressure was elevated from 1 MPa to 4 MPa in the control group. Although chloride concentrations decreased with erosion-inhibiting treatments, their mitigation efficiency diminished under rising hydrostatic pressure. For the post-pressure application, porosity increased within 10 mm of specimen surfaces, correlating to pressure magnitude. Enhanced pore connectivity and reduced tortuosity were obtained, accompanied with increases in the equivalent radii and average lengths of connected pore segments. Elevated hydrostatic pressure degraded the microstructural toughness of mortar matrices and ITZs, while ITZ thickness progressively increased with pressure loading. External application of nano-silica coatings, migratory corrosion inhibitors, and internal incorporation of nano-precursor materials all effectively enhanced the chloride resistance and reduced capillary water absorption of shield tunnel segment concrete, with nano-precursor materials demonstrating the most pronounced effect. These corrosion-inhibiting measures reduced concrete porosity by 30.3% to 64.4%, while decreasing the proportions of severely harmful pores.