Recycled fine powder is an important pathway for the resource utilization of construction and demolition waste; however, its low reactivity and the unstable influence on the performance of cement-based materials after incorporation remain critical barriers to large-scale application. Carbonation treatment not only enables CO2 sequestration and provides environmental benefits but also improves the physicochemical properties of recycled fine powder. In this study, recycled fine powder was treated by a dry carbonation method and used to partially replace cement at substitution levels of 10%, 20%, and 30% to prepare mortar specimens. The effects of carbonated recycled fine powder on the macroscopic mechanical properties, early hydration heat evolution, and pore structure development of mortar were systematically investigated, with the aim of providing data support and theoretical basis for the optimized utilization of recycled fine powder in green and low-carbon construction materials. The results demonstrated that carbonation treatment generated CaCO3, which served as nucleation sites for secondary hydration of cement, thereby accelerating hydration, increasing heat release, refining pore structure, and enhancing mechanical performance. Compared with mortars containing uncarbonated recycled fine powder the 3-day compressive strength increased by 15.2%, 14.3%, and 8.0%, while the 28-day compressive strength showed little difference, indicating that carbonation primarily enhanced early-age strength. For flexural strength, the 28-day values increased by 9.7%, 4.0%, and 0.7%, consistent with the 3-day results. Pore structure analysis revealed that total porosity decreased by 0.43%, 0.45%, and 0.51%, while the cumulative heat release within 72 h increased by 0.8%, 9.4%, and 7.9%. These findings elucidate the mechanism by which carbonation treatment enhances early-age strength of mortar through promoting hydration reactions and improving pore structure.