Using industrial by-product lithium slag (LS) as a raw material for ultra-high performance concrete (UHPC) is an important way to achieve low-carbon prefabricated structures. However, existing studies lack a constitutive model for LS-UHPC and its application in prefabricated beam connection nodes. To fill this gap, this paper first established a tensile-compressive constitutive model for LS-UHPC through mechanical tests; then it was embedded into the finite element model to simulate the bending performance of the connection nodes of the post-cast LS-UHPC prefabricated beams and verified by the test results. Finally, parameter analysis is carried out. The results show that moderately increasing the diameter of longitudinal reinforcement can significantly improve the flexural bearing capacity of the connection node, but when the diameter exceeds 18 mm and HRB500 high-strength steel bars are used, the node exhibits over-reinforced failure characteristics; increasing the strength grade of ordinary concrete has a limited effect on the improvement of flexural bearing capacity (<5%). This study clarified the mechanical constitutive relationship of LS-UHPC, revealed the failure mechanism and bearing capacity evolution law of its prefabricated connection nodes under parameter changes, and provided a theoretical basis and design suggestions for the application of low-carbon lithium slag UHPC in prefabricated assembly structures.

With the continuous expansion of the lithium industry, lithium slag (LS) has been generated in large quantities, and its potential reuse in cement-based materials has become increasingly important. In this work, LS was introduced into ultra-high-performance concrete (UHPC) as a partial substitute for cement to explore its applicability in low-cement UHPC systems. The fresh properties of UHPC, including flowability and setting behavior, were measured, and its mechanical performance was evaluated through compressive and flexural strength tests. In addition, early-age autogenous shrinkage was monitored to clarify the effect of LS on dimensional stability. To further reveal the mechanisms associated with the macroscopic performance changes, isothermal conduction calorimetry, backscattered electron microscopy (BSE), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and mercury intrusion porosimetry (MIP) were adopted. These techniques were used to characterize the hydration behavior, phase composition, hydration product evolution, and pore-structure characteristics of UHPC containing different LS contents. Results demonstrate that LS incorporation effectively reduces autogenous shrinkage and accelerates setting. An optimal LS content enhances long-term strength development; however, LS incorporation compromises early-age compressive strength and flowability. Calorimetric, thermogravimetric, and BSE analyses collectively reveal that LS retards early hydration heat release and delays initial strength gain, attributable to its dilution effect, but exhibits latent pozzolanic reactivity, consuming Ca(OH)₂ and promoting secondary C–S–H formation, thereby increasing the 28-day degree of cement hydration. Pore-structure analysis further confirms that an appropriate LS content significantly reduces total porosity, average pore diameter, and the volume fraction of pores with PD > 50 nm, leading to a more refined and compact microstructure. Integrated macroscopic and microscopic evidence identifies 20 wt.% LS as the optimal replacement level: relative to the reference mixture LS0, the 28-day compressive and flexural strengths of LS20 are increased by 10.86% and 27.93%, while flowability decreases by 13.97%, initial setting time shortens by 10.54%, and autogenous shrinkage is reduced by 57.41%. The results provide a scientific basis for the resource utilization of lithium slag in UHPC and contribute to the development of cement-reduced UHPC mixtures with improved mechanical and microstructural characteristics.

Prefabricated post-pouring UHPC (ultra-high performance concrete) beam–column joints have the advantages of strong integrity, excellent seismic performance and good durability, but there are also problems of easy cracking of old and new concrete interfaces and concrete near the interface. Therefore, a new type of post-cast UHPC joint is designed in this paper. The joint is connected by post-cast UHPC at the beam and column sections far from the core area, and the keyway is set in the connection section to solve the defect that the old and new interfaces easily crack. The refined finite element model of the joint was established by using the finite element software ABAQUS (2023). Through the simulation of 10 working conditions, the typical failure modes and seismic performance of the joint were discussed in depth, and the influence of key parameters such as the lap length of steel bars, the strength of steel bars and the strength of post-pouring UHPC was analyzed. The results show that the joint cracks first appear at the junction of the beam–column core area and develop along the cut-off interface between ordinary concrete and UHPC. No macroscopic cracks were observed in the post-pouring UHPC connection section, and the structure was finally destroyed due to the crushing of ordinary concrete. Increasing the lap length of the steel bar can improve the peak bearing capacity and stiffness of the joint, but it will accelerate the stiffness degradation. Increasing the strength grade of steel bars can significantly improve the bearing capacity and stiffness of the joints, but the energy dissipation capacity is slightly reduced. In addition, the improvement effect of UHPC strength is closely related to the strength of steel bars: when an HRB500 steel bar is used, high-strength UHPC can show better bearing capacity, stiffness and energy dissipation performance, while the improvement effect is not significant when an HRB400 steel bar is used. The research results can effectively inhibit the development of interface cracks and provide a theoretical reference for the subsequent full-scale test and the seismic design of precast joints with post-cast UHPC connections.