Carbonation of Ca-rich solid waste is a transformative method to manage CO2 emissions and eliminate waste and by-products, such as steel slag (SS). This technology is considered a feasible approach to limit volumetric instability,due to the hydration of free CaO and MgO, and bring economic and environmental value to SS aggregates. Alkali-activated materials (AAM) are a potential alternative to Portland cement (PC)due to their low environmental impact potential, utilization of waste materials, and superior properties compared to PC. The employment of SS in AAM has beenrecently explored, as a promising route for reuse. However, the literature on the subject is still scarce and inconclusive, with conflicting results. Therefore, this study evaluated the effect pre-treatment carbonation conditions (temperature, moisture content, and CO2 pressure) of SS, and the subsequent performance of carbonated SS (C-SS) as aggregates in AAM. The mechanical, physical and microstructural properties of alkali-activated mortars containing 100% C-SS aggregates from different conditions were assessed. The results indicated that the carbonation process of SS aggregates is mainly influenced by its moist content, followed by CO2 pressure. In general, C-SS aggregates presenta rougher surface after the pre-treatment, characterized by a higher specific surface area that improves the bonding between the matrix and aggregate, with a consequent improvement in the mechanical properties of alkali-activated mortars.