<p>Alkali-activated slag mortars (AASM) face durability challenges, particularly the loss of mechanical strength under prolonged carbonation. This study investigated the mechanical, chemical, and pore structural evolution of AASM subjected to accelerated carbonation. Mortars were prepared by alkali-activating a mixture of blast furnace slag and steel slag, followed by exposure to pure CO₂ for different durations. Under 10 M NaOH activation, compressive strength initially increased, reaching 27.8 MPa after 6 hours of carbonation, but declined by 21 pct after 12 hours. TG/DSC analysis showed the maximum CaCO₃ content of 3.43 pct at 12 hours. The early strength gain was attributed to the formation of C–(A–)S–H gels and matrix densification, as confirmed by XRD, FT-IR, and BSE. However, the subsequent strength loss was not directly correlated with total porosity. Instead, MIP and micro-CT analyses revealed that pore size distribution—particularly the development of capillary pores (10 to 10,000 nm)—was critical in maintaining load-bearing capacity. These findings clarify the dual role of carbonation, showing how short-term exposure enhances strength while extended carbonation undermines it, and highlight pore size distribution control as a key strategy for improving AASM durability in sustainable construction.</p> Graphical Abstract <p></p>