The impact of NaSCN and Na₂S₂O₃ on the reactivity, microstructure and morphology of C₃S and model cement (with a clinker containing 85% C₃S and 15% C₃A) pastes was systematically investigated. Results concluded that both alkali salts mainly act enhancing the reactivity of the C₃S phase while not significant influence on the reactivity of C₃A was measured. While both admixtures rose the reactivity of C₃S over the studied 7 days of hydration, they only increased the reactivity of model cement pastes up to 14–20 h. NaSCN and Na₂S₂O₃ did not modify the C–S–H stoichiometry but they influenced its morphology. In particular, thicker convergent C–S–H needles were formed in pastes containing Na₂S₂O₃ compared to non-admixed systems, while a higher number of thinner C–S–H needles were formed in presence of NaSCN. Furthermore, greater portlandite clusters and intermixing of AFm and C–S–H were observed in admixed C₃S and model cement pastes, respectively, compared to plain systems.