<p>This study reveals the value-added use of carbon dioxide (CO₂) in the manufacturing of sustainable binders. The efficiency of accelerated carbonation of slag cement is evaluated through the continuous immersion of particle surfaces in a saturated CO₂ environment and compared with that of alternative alkali activation processes and preheating thermal treatments. Accelerated carbonation, a sustainable measurement, was intended to increase the early carbonation reaction rate. The alternative comparisons included alkali-activated mixtures comprising six slag cement blends produced from SiO₂–CaO–MgO–Al₂O₃ reagent-grade chemicals and preheated water-quenched waste slag obtained from a local steel plant, which was characterised by low reactivity. Physical tests and chemical analyses were conducted on slag cements and their mixtures to track the phase transformation, microstructural changes, and carbonation state associated with changes in strength. Precuring with carbon dioxide transforms low-value inert slag into a more reactive phase, forming further calcite and other hydrates (e.g., silica gel), which are denser and more stable phases. The strength results revealed that the application of accelerated carbonation or the use of calcium-based activators efficiently increased the strength activity index of the mortars produced with low-reactivity slags. Although the achieved strengths are lower than those of mixtures made with cement or with high-reactivity commercial slags, the classification of this binder system as Grade 80 is supported.</p>