Accelerators (or activators) are commonly used as additives in cement to advance setting and/or hardening. Activation is often associated with the favoured hydration of particular cementitious phases, which can be induced by complexation mechanisms. However, the dispersion state of pastes with admixtures may also influence hydration rates. The aim of this paper is to highlight the correlation between mesostructural organisation and chemical reactivity of cement pastes with an additive. The affinity of triethanolamine (TEA), an activator molecule, with cement grains was assessed by measurement of total organic carbon. Moreover, the influence of the amine on the dispersion state, sedimentation kinetics and rheological behaviour was studied. Finally, X-ray diffraction, aluminium nuclear magnetic resonance spectroscopy and isothermal calorimetry were used in order to study the chemical reactivity of cement in the presence of the admixture. The combination of these granular and chemical characterisations helped to understand the mode of action of TEA.