With the global push towards energy transition, understanding the mechanical behavior of rocks under high-temperature conditions is becoming increasingly significant, such as nuclear waste disposal. Previous studies have demonstrated that the high-temperature heating has a great impact on the physical and mechanical properties of sandstone, exhibiting both thermal strengthening and deterioration phenomena. However, research on the characteristic stress and microcrack evolution of sandstone under thermal strengthening and deterioration conditions remains limited. The objective of this study is to examine the characteristics of stress, microcrack evolution, and failure mechanisms in sandstone under a range of high temperatures. Sandstone samples were heated respectively to 200°C, 400°C, 600°C and 800°C. The deformation and failure processes were monitored using acoustic emission (AE) and digital image correlation (DIC) techniques. The evolution of characteristic stress and microcrack development under both thermal strengthening and deterioration conditions was analyzed. The results show that the crack closure stress and its threshold ratio are proportional to temperature. When sandstone exhibits strength deterioration, the crack initiation stress and its threshold ratio decrease with increasing temperature. Conversely, when strength enhancement is observed, the crack initiation stress increases, while the threshold ratio initially rises and then declines. Moreover, the effect of thermal treatment on axial deformation is more pronounced than on radial deformation, with the elastic constant and Poisson’s ratio gradually decreasing as temperature rises. Microcrack evolution was predominantly tensile across temperatures, but the macroscopic failure modes varied: monocline shear failure at 25°C, tensile failure at 200°C, 400°C, and 600°C, and a mixed shear-tensile failure at 800°C, where shear cracks became most prominent. This study enhances the understanding of sandstone’s mechanical behavior under high-temperature conditions and provides valuable insights for engineering applications involving thermal treatment.

This study aims to enhance the damage controllability and overall seismic resilience of assembled underground structures under earthquake actions. To achieve this, three types of prefabricated roof–sidewall composite joints are proposed based on the design concepts of cogging for force transfer and local strengthening. These include the high-strength bolt–cogging–grouting sleeve joint (HCG), the prestressed steel strand–cogging–grouting sleeve joint (PCG), and the UHPC–cogging–grouting sleeve joint (UCG). Following the principle of positioning joints in regions of low structural stress, four 1/4-scale reinforced concrete (RC) specimens were designed and fabricated, including one cast-in-place (CIP) reference specimen and three precast RC specimens. Quasi-static tests were carried out to systematically evaluate the seismic behavior and internal force distribution of each specimen. Numerical validation was also performed using ABAQUS. The results show that both UHPC and a reasonable application of prestressing can effectively inhibit crack initiation and damage propagation at the joint seams. When the composite joints are positioned outside the plastic hinge region, they provide a reliable load transfer path for the reinforcement. The HCG and UCG joints significantly enhance the load-bearing capacity and energy dissipation capacity of the specimens. Their ductility and energy dissipation both achieve a seismic performance equivalent to that of the CIP specimen. Furthermore, damage in these specimens is predominantly confined to the designated plastic hinge region of the roof. This effectively mitigates shear damage in the roof–sidewall connection zone (RSC). Although the PCG joint improves the initial stiffness of the specimen, its energy dissipation capacity and ductility are reduced. It also causes damage to be transferred to the RSC. This leads to increased shear deformation and premature shear failure in this zone. Consequently, both UHPC and a reasonable application of prestressing can be used for the prefabrication of underground structures. Positioning the joints outside the roof plastic hinge zone can effectively achieve the seismic design goal of “strong joint, weak component”.

This study systematically examines the mechanical performance and microstructural evolution of concrete incorporating alkali-mechanically activated lithium slag (LS). Unconfined compressive strength (UCS) tests were conducted at 7 and 28 days of curing to assess the influence of LS content on early and later-age strength development. Multiscale characterization techniques, including scanning electron microscopy (SEM), X-ray Fluorescence (XRF), X-ray Diffraction (XRD) nuclear magnetic resonance (NMR), and Fourier-transform infrared spectroscopy (FTIR), were used to reveal the dual mechanism of chemical activation and physical pore refinement. The results show a non-monotonic trend in compressive strength with increasing LS content, with an optimum at 20% replacement, where the UCS reached 103% and 105% of that of the control mix at 7 and 28 days, respectively. NMR analyses reveal that activated LS contributes to refinement of the pore structure, resulting in reduced pore size and enhanced matrix densification. SEM, energy dispersive spectroscopy (EDS), and thermogravimetric (TG) analyses indicate that the improved strength is primarily attributed to the pozzolanic reaction between activated LS and cement hydration products, forming additional calcium silicate hydrate (C-S-H) and other binding phases. These findings highlight the dual role of alkali-mechanically activated LS in enhancing concrete performance through coupled microstructural refinement and chemical reactivity, offering a sustainable approach for valorizing lithium slag in cementitious materials.