Since the late nineteenth century, railway construction has been a key driver of national development in China. The Yuehan Railway is one of the earliest modern railways in the country, playing an important role in advancing transportation infrastructure and regional economic growth. Over the past century, the concrete structures of the Yuehan Railway have been exposed to complex environmental actions, including high humidity, seasonal fluctuations in the Yangtze River water level, chemical ions, and sediment-induced abrasion. These factors have collectively accelerated surface deterioration and microstructural degradation. Recent decreases in river water levels have revealed the concrete remains of Xujia Peng Station, providing a rare opportunity to examine century-old concrete. This study conducts field sampling and laboratory analyses to characterize the composition and microstructure evolution of the relic concrete. Techniques including X-ray diffraction (XRD), thermogravimetric analysis (TG), scanning electron microscopy (SEM), backscattered electron imaging (BSE), and X-ray computed tomography (X-CT) are employed. The findings offer scientific support for the preservation of historic constructions and provide reference insights for durability design in cement materials.

Concrete samples were collected during the dry season from the exposed remains of the Xujia Peng Station section of the Yuehan Railway. Representative fragments were obtained from pile caps and bridge piers via manual chiseling, while intact cores were extracted from sleeper components. After drying and surface preparation, samples were processed for mechanical testing and microstructural analysis. Compressive strength and chloride ion penetration resistance were measured on φ 100 mm × h 100 mm and φ 100 mm × h 50 mm cylindrical specimens, respectively. Phase assemblages were quantified using (XRD) with Rietveld refinement, complemented by thermogravimetric analysis. Microstructural morphology, ITZ distribution, and elemental composition were examined via (SEM), backscattered electron imaging with image analysis (BSE/IA), and energy dispersive spectroscopy (EDS). Three-dimensional pore architecture was evaluated using high-resolution X-CT, enabling quantitative pore size distribution.

The sleeper concrete exhibited exceptional long-term stability, with an average compressive strength of 110.68 MPa and an electrical charge passed of 868 C, corresponding to a Q-IV chloride resistance grade. XRD and TG/DTG analyses revealed nearly complete hydration of clinker phases after nearly a century of service. C₃A and C₄AF were fully consumed, and only trace amounts of C₃S and C₂S remained. Due to prolonged exposure to air, pile cap and pier samples showed severe carbonation, with CaCO₃ mass fractions exceeding 20% and minimal portlandite. By contrast, sleeper samples exhibited limited carbonation. Quantitative BSE/IA demonstrated clear ITZ gradients in all samples. The ITZ thicknesses for pile cap, sleeper, and pier concretes were approximately 50, 30 μm, and 40 μm, respectively. ITZ regions consistently exhibited elevated porosity and reduced hydration product content near aggregate surfaces. The gradient of hydration product volume fraction stabilized beyond the ITZ boundary, indicating a distinct microstructural transition from porous interface zones to dense bulk matrix. SEM/EDS analyses revealed dense C-S-H gel, AFt/AFm products, localized carbonate phases, and well-preserved matrix integrity in the sleeper samples. While microcracks existed near aggregate boundaries, the overall matrix remained chemically stable. X-CT imaging showed a predominantly disconnected pore system consisting of fine capillary pores and a small number of large voids. The critical pore volume of the sleeper concrete was 0.1 mm³, above which macropores dominated total porosity but did not form continuous pathways.

This study provides a comprehensive microstructural and performance assessment of century-old Yuehan Railway concrete. Sleeper concrete retained high compressive strength (110.68 MPa) and low chloride permeability (868 C), demonstrating excellent durability after nearly 100 years of service. Pile cap and pier concretes underwent significant carbonation, while sleeper concrete exhibited limited carbonation due to environmental shielding. ITZ thicknesses were approximately 50 μm (pile cap), 30 μm (sleeper), and 40 μm (pier), with higher porosity and microstructural heterogeneity compared to bulk matrix. X-CT analysis identified a critical pore volume of 0.1 mm³, indicating that large isolated pores govern overall porosity without forming permeable networks. Overall, the Yuehan Railway concrete demonstrates outstanding long-term mechanical and durability properties, offering valuable insights for cementitious material design.