This study presents an advanced methodology to monitor the reactivity of the individual components in a model blended cement, and to reveal the working mechanism of an accelerating admixture such as Na 2 S 2 O 3. This is achieved by solid-state nuclear magnetic resonance (NMR) measurements in a model blend using selective isotope labelling of C 3 S or of an aluminosilicate-glass (taken as a model supplementary cementitious material). Results were complemented with TGA, XRD and thermodynamic modelling. NMR measurements with selective 29 Si isotopic labelling captured the spatial and compositional heterogeneity of the C-A-S-H phase, with different Al/Si ratio and Ca/Si. This is a consequence of the different reactivity of the C 3 S and of the glass phases which induces local and temporal fluctuations in Ca and Al availability. The addition of Na 2 S 2 O 3 enhanced the initial reactivity of both components of the blend, thus reducing the chemical heterogeneities of the C-A-S-H. In parallel, Na 2 S 2 O 3 addition enhanced the precipitation of S(II)-AFm. This reduced the concentration of aluminates in solution, as manifested by a lowered average Al/Si ratio in C-A-S-H, which lowered the passivation of C 3 S and glass surfaces, directly benefiting their reactivity. Finally, the impact of Na 2 S 2 O 3 on the reactivity was validated on blends of Portland cements with slag.