Gypsum has been traditionally used as a building material, but can also be a model for the development of injectable biomaterials for bone substitution. Set plaster, or gypsum (CaSO₄.2H₂O), is prepared by mixing dry hemihydrate powder (CaSO₄.0.5H₂O) with water. Its properties as a solid binder are largely influenced by the setting reaction occurring when CaSO₄.0.5H₂O dissolve and crystals of gypsum precipitate. It is well known that the resulting solid is formed by an interlocked network of gypsum needles and platelets, but the exact setting process is not well understood yet. The objective of our study is therefore to monitor the evolution of crystals and understand the entire setting process, using operando observations, using Liquid-phase scanning electron microscopy.

The most convenient set-up for such monitoring has found to be sealed Quantomix cells. They are indeed water- and airtight, and therefore plaster evolves under the same conditions as in real use, especially in terms of water-to-plaster ratio. The influence of several experimental parameters (addition of a retardant to monitor the whole setting process, image acquisition conditions and irradiation damage, presence of a membrane) will be discussed.

We will describe image processing and analysis procedures we developed to distinguish the hemihydrate particles dissolving in the mixture from the gypsum crystals growing over time, and therefore plot the fraction of gypsum vs time. To go into more details in the analysis of gypsum growth, we will propose an indexation of the gypsum crystal facets based on geometric information and the knowledge of the equilibrium morphologies. The growth rates of the different planes will be discussed and compared with values found in the literature.