Hydration of MgO-Based Cement: Water Dynamics by 1H Fast Field-Cycling NMR Relaxometry

H-1 fast field-cycling (FFC) NMR relaxometry was applied for the first time to monitor the state of water during the hydration reaction of MgO and silica that leads to the formation of magnesium silicate hydrate (M-S-H), the binder phase of innovative cements with promising applications in the containment of radioactive waste. To this aim, water proton longitudinal relaxation rates (R-1 = 1/T-1) were measured in the Larmor frequency range between 10 kHz and 30 MHz at different hydration times ranging from 0.5 h to similar to 4 months. The obtained R-1 versus frequency (NMRD) curves were analyzed considering fast exchange of water molecules between a hydration layer, where dynamics is affected by interactions with the surface of solids present in the reacting mixture, and a bulk phase. For the hydration layer, water molecules undergoing fast local molecular dynamics on the surface gave a constant contribution to R-1 throughout the investigated frequency range. On the contrary, water molecules undergoing slow dynamics on the surface gave a dispersion of R-1 and their motions were modeled as reorientations mediated by translational displacements in the length scale of a particle and of a cluster of particles, where particles are silica nanoparticles and/or M-S-H globules that form during hydration. The model parameters reflected the different typical steps of cement hydration, showing smooth trends in the induction and diffusion steps and sudden changes during the nucleation and growth period in which water is consumed and M-S-H forms.