Magnetic resonance studies of hydration kinetics and microstructural evolution in plaster pastes

This paper details a comparison of the evolution in the microstructure of the alpha and beta forms of gypsum plaster that occur during hydration. The comparison has been performed using a combination of rapid Nuclear Magnetic Resonance (NMR) relaxation measurements and Scanning Electron Microscopy images, acquired as a function of hydration time. The alpha plaster hydrates to an interconnected network of uniform gypsum crystals providing a homogeneous structure, whereas the beta plaster exhibits growth of crystals with irregular shape leading to a more open pore network and a heterogeneous product. An additional NMR T (2) relaxation time component is observed in the beta plaster compared to the alpha plaster, suggesting the presence of large pores in the beta plaster. This conclusion is confirmed by pore volume distributions determined from X-ray micro-computerised tomography (mu-CT) images of the set plasters. To the best of our knowledge, this is the first study of both forms of plaster utilising this combination of experimental techniques. The hydration kinetics have also been compared using one-dimensional NMR profiles, from which effective rate constants are determined. Consistent with previous results, the hydration reactions of the alpha and beta forms of plaster are seen to occur at very different rates: the alpha plaster has a short initiation period and a slow hydration reaction. In contrast, the beta plaster has a long initiation period, although the hydration reaction proceeds more rapidly thereafter. This work demonstrates the applicability of several NMR techniques to monitor, in situ, the hydration kinetics and microstructural evolution in plaster pastes, which will be crucial to the further understanding of mechanical properties (e.g. moisture transport) in these systems.