Reactivity of C3S and model cement in presence of Na2S2O3 and NaSCN

Abstract 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.