Severe electrochemical corrosion and leaching degradation of underground structures are induced by coupled stray current and groundwater in rail transit systems like subways. The microstructural evolution of fly ash geopolymer (FAG) during accelerated leaching was systematically investigated by simulating a coupled direct current (DC) stray current and soft water environment. Pore solution alkalinity and electrolytic cell OH⁻ concentration were utilized as evaluation indicators, combined with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP). Results indicate that substantial OH⁻ leaching occurs under the coupled stray-current and soft-water exposure, leading to reduced pore-solution alkalinity and changes consistent with the degradation of the N–A–S–H binding network. Consequently, porosity, most probable pore diameter, and the quantity of harmful pores are increased. However, no obvious changes in the major diffraction peaks associated with quartz and mullite were detected within the resolution of the qualitative XRD analysis. Furthermore, a linearly decreasing trend over time is observed for the coupled leaching rate. Mix proportion analysis demonstrates that FAG leaching resistance is improved by reducing the water-to-binder ratio; specifically, superior gel phase content and pore structure are maintained at a ratio of 0.30. Additionally, cumulative OH⁻ leaching is effectively reduced by decreasing the sodium silicate modulus, with optimal resistance exhibited between 1.0 and 1.2. Concurrently, pore solution alkalinity before and after leaching is significantly elevated by increasing the alkali dosage. This exerts a pronounced inhibitory effect on OH⁻ leaching, thereby substantially enhancing the overall leaching resistance.

To mitigate the environmental impact associated with the high cement consumption in conventional 3D printable concrete (3DPC) and to enhance the intelligent thermal regulation capacity of modern buildings in complex environments, a novel 3D printable phase-change concrete incorporating industrial solid wastes was developed in this study. Through comprehensive assessments of fluidity, extrudability, and buildability, the critical workability control parameters satisfying the requirements of continuous 3D printing were determined. Subsequently, the response surface methodology based on a Box-Behnken design was employed to analyze the individual and interactive effects of silica fume content, microencapsulated phase-change materials dosage, and water-to-binder (W/B) ratio on the 28-day compressive strength of the 3D printed specimens. The analysis of variance results indicated that the significance of these three factors on compressive strength followed a descending order of W/B ratio > mPCMs dosage > silica fume content. Furthermore, all factors exhibited a highly significant non-linear quadratic effect, characterized by an initial increase followed by a subsequent decrease. A single-response optimization algorithm was utilized to determine the optimal mixture design, targeting the maximization of mechanical properties under a fixed printability constraint: a W/B ratio of 0.265, a silica fume content of 7.22%, and an mPCMs dosage of 2.72%. Experimental validation demonstrated that the measured 28-day compressive strength of the optimal mixture was 46.54 MPa, yielding a relative error of only 6.26% compared to the predicted value. This confirmed the high accuracy and reliability of the established quadratic regression prediction model. This study provides a reliable material foundation and scientific basis for the development of highly efficient 3D printed concrete components with integrated thermal management functionalities.