Simultaneous monitoring of hydration kinetics, microstructural evolution, and surface interactions in hydrating gypsum plaster in the presence of additives

In this article, we use rapid two-dimensional nuclear magnetic resonance T (1)-T (2) relaxation time correlations to monitor hydration kinetics, microstructural evolution, and surface interactions simultaneously in gypsum plaster pastes of industrial relevance. In particular, we explore the influence of ground gypsum mineral (GGM) as an accelerator and citric acid as a retarding agent in the hydration of the beta form of plaster. Quantitative relative water fractions are determined and the modified hydration processes are observed as expected. These measurements also reveal the evolution of a secondary water population in the presence of the retarding agent when sedimentation of the hemihydrate occurs prior to crystallisation. In these samples, the ratio of relaxation times T (1)/T (2) provides an indication of both the number and the strength of interactions between water molecules and solid gypsum particles. It is observed that the GGM decreases the T (1)/T (2) ratio, whilst the citric acid increases the T (1)/T (2) ratio, relative to the T (1)/T (2) ratio in the unmodified plaster paste. We interpret this as an increase in the water-hemihydrate interaction in the presence of GGM, and a decrease in the presence of citric acid, consistent with the current understanding of the behaviour of these additives as an accelerator and retardant, respectively. The T (1) relaxation time measurements provide an indication of the pore structure evolution: larger pores are present in the accelerated product, and smaller pores in the retarded product, compared to the unmodified plaster. These conclusions are supported by helium porosimetry and scanning electron microscopy. The rapid T (1)-T (2) measurement described here will be applicable to other reactions that evolve on a time scale of several minutes.