Small oscillatory rheology (i.e., SAOS) has been used to describe the structural build-up of cement pastes (i.e., reactivity) and has recently proven a promising method to measure the nature of interactions between cementitious particles. Indeed, we show that the evolution of the storage modulus (G’) with time, imposing a deformation within the paste linear viscoelastic regime (LVE), tracks the development of interaction forces inside pastes and quantifies their cohesion. This gives information about the physico-chemical mechanisms happening at the microscopic scale, otherwise difficult to access experimentally through direct measurements. This study applies this approach to the characterization of the residual reactivity of a milled high-performance concrete (mRHPC) targeted to be used as a supplementary cementitious material. Here, the use of SAOS is combined with X-ray powder diffraction (XRPD) and calorimetry. The characterization of the hardened properties of OPC-mRHPC binders was performed at different mRHPC replacement rates. The good mechanical performance of these samples confirms the residual reactivity measured by SAOS, linked to the presence of unreacted C3S and C2S (measured by XRPD). Our findings confirm rheology as a powerful tool to measure interaction in cementitious pastes and open a path to quantify the residual reactivity of recycled binders in order to optimize their use in new recipes, thus reducing OPC content.