Exploiting in-situ solid-state NMR spectroscopy to probe the early stages of hydration of calcium aluminate cement.

We report a high-field in-situ solid-state NMR study of the hydration of CaAlO (the most important hydraulic phase in calcium aluminate cement), based on time-resolved measurements of solid-state Al NMR spectra during the early stages of the reaction. A variant of the CLASSIC NMR methodology, involving alternate recording of direct-excitation and MQMAS Al NMR spectra, was used to monitor the Al species present in both the solid and liquid phases as a function of time. Our results provide quantitative information on the changes in the relative amounts of Al sites with tetrahedral coordination (the anhydrous reactant phase) and octahedral coordination (the hydrated product phases) as a function of time, and reveal significantly different kinetic and mechanistic behaviour of the hydration reaction at the different temperatures (20 °C and 60 °C) studied.