Abstract : We report a comprehensive and non-destructive study of the hydration kinetics and of the microstructure evolution of plaster pastes, by low magnetic field proton Nuclear Magnetic Resonance (NMR) Relaxometry. This technique enables to follow continuously and non-destructively the degree of hydration and the evolution of the microstructure during the setting and hardening of plaster pastes. The bi-exponential transverse magnetization decay reveals the existence of two water populations in slow exchange. Using the NMR relaxometry measurements, two modes of organization of the microstructure of this material are identified, as a function of the initial water to plaster ratio (w/p), for 0.4 <= w/p <= 0.6 and 0.7 <= w/p <= 1. An original model of exchange between water populations in a porous medium was established. A new multi-scale and multi-technique approach is proposed to relate continuously the microstructure and the macroscopic mechanical properties of the evolving porous medium (measured by ultrasound). The experimental approach and the modelling presented here can be applied to other porous media with evolving microstructures.