Abstract: Against the backdrop of the “dual-carbon” goals, calcium looping (CaL) technology is regarded as an important route for large-scale CO₂ capture owing to the high theoretical adsorption capacity and low raw-material cost of CaO-based sorbents; however, CaO-based sorbents readily suffer from rapid performance decay caused by high-temperature sintering during cycling, which restricts their industrial application. Steel slag, a calcium-bearing solid waste produced in large quantities but underutilized in China, serves as a low-cost precursor for CaO-based sorbents by virtue of its high calcium content, and converting it into carbon capture materials offers the dual benefits of solid-waste valorization and low-cost CO₂ emission reduction. Using converter steel slag as the raw material, leaching was carried out with the small-molecule acids formic, acetic, oxalic, and citric acid to prepare the calcium-based sorbents FA-CCM, AA-CCM, OA-CCM, and CA-CCM, and the mechanism underlying the enhancement of CO₂ capture performance was explored by combining phase-structure characterization, cyclic adsorption–desorption tests, and kinetic analysis based on the shrinking core model. The results show that the type of acid determines the leaching form of calcium and the composition of the materials. The calcium contents of FA-CCM, AA-CCM, and CA-CCM were 80.3%, 78.2%, and 61.3%, respectively, whereas the material derived from oxalic acid contained only 1.70% calcium owing to the formation of poorly soluble calcium oxalate. The CO₂ capture performance of FA-CCM and AA-CCM was markedly superior to that of OA-CCM and CA-CCM. The CO₂ uptake of FA-CCM increased from 2.79 mmol/g in the 1st cycle to 3.08 mmol/g in the 5th cycle and remained as high as 2.82 mmol/g in the 15th cycle, whereas that of AA-CCM decreased continuously from 3.89 mmol/g in the 1st cycle to 1.78 mmol/g in the 15th cycle. Based on systematic characterization and literature review, it is inferred that calcium formate releases small-molecule gases (CO, H₂, CO₂, and HCHO) during calcination, creating open channels that allow CO₂ to penetrate deep into the particle interior. In the initial cycles, the hard skeleton undergoes continuous reconstruction, constantly exposing new active surfaces; as the cycle number increases, the proportion of the soft skeleton gradually rises, thereby achieving self-reactivation. In contrast, the acetone released from the ketonic decarboxylation of calcium acetate has a relatively larger molecular size and a more obstructed escape path, forming a dense structure that encloses the hard skeleton within the particles, thereby hindering the exposure of active surfaces, while the limited soft skeleton suffers from continuous loss during high-temperature desorption due to CaO grain growth and pore collapse, leading to rapid sintering deactivation over the course of cycling. The kinetic shrinking core model analysis further reveals that the reaction-controlled rate constant of FA-CCM increased from 4.39×10⁻⁴ to 5.98×10⁻⁴ and then leveled off, with a low decay rate of the diffusion rate constant, whereas the reaction-controlled rate constant of AA-CCM decreased from 5.67×10⁻⁴ to 3.52×10⁻⁴ and its diffusion rate constant also declined continuously, with a decay rate far exceeding that of FA-CCM. This study demonstrates that formic-acid activation can synergistically improve the pore structure, carbonation kinetics, and cyclic stability of the material, providing a basis for the resource utilization of steel slag to prepare low-cost CO₂ capture materials.