Ordinary Portland cement (OPC) production contributes significantly to greenhouse gas emissions. Substituting a part of clinker by supplementary cementitious materials as slag allows reducing this impact. However, these materials have a slower hydration kinetics, which makes a mechanical, thermal or chemical activation necessary. Triethanolamine (TEA) is an amine commonly used as an accelerator, as it allows activating chemically the hydration. The mechanisms involved in this activation still need to be further investigated. Therefore, this study focuses on the interaction of TEA with slag blended cement. The slag blended cement paste was firstly studied without accelerator to highlight the impact of this mineral addition on the OPC reactivity. The effect of TEA on chemical reactivity was investigated by isothermal calorimetric measurements and scanning electron microscopy. The effects on the structural organization of the paste were highlighted by comparing particles size distributions and physicochemical stabilities of slightly diluted pastes. By combining the analysis of paste dispersion state and chemical reactivity, this study allowed identifying different modes of action of triethanolamine in slag blended cement in comparison with an OPC and a limestone blended cement used as references.