Because of their setting ability, hydraulic binders are used in different applications (e.g. construction materials, bone substitutes, ...). Processing is initiated by mixing one or several fine powders with an aqueous solution. The dissolution of the initial reactive powders results in the formation of a viscous and moldable paste, which properties evolve with time to form a porous monolithic ceramic through the nucleation and precipitation of more stable phases.

The need to control the rate of reaction and final properties requires the precise understanding of the setting process in a multiphysic and multiscale approach. This issue has already been addressed in studies that were somehow limited regarding one or several aspects:

• only one aspect of the setting process was studied (e.g. chemical reaction, evolution of the mechanical properties or of the microstructure), hindering any global multiphysic approach.
• only one length scale was investigated when phenomena occur at multiple scales.
• the setting process was stopped at discrete terms, prior to characterization, preventing the follow-up of one specific sample.

In this study, gypsum plaster (CaSO4·2H2O) is studied in standard conditions (e.g. liquid to solid mass ratio), to develop in-situ and ex-situ multiphysic and multiscale characterization techniques to monitor the evolution of:

• the phase composition (rate of dissolution and precipitation) using XRD and FTIR,
• the microstructure using SEM and μ-CT,
• the mechanical properties using DMA, rheology and compression tests.

This panel of techniques enabled to monitor and to correlate the different physical transitions occurring during the setting process and to draw a global picture of the on-going phenomena.

Gypsum plaster is obtained from the dissolution of bassanite (CaSO4·0.5H2O) releasing the ionic species needed for the precipitation. The physico-chemical characterizations have shown that both the dissolution and precipitation reactions are concomitant. Also, as the reaction extent increase, the development of the microstructure was observed concomitantly to the rise of the mechanical properties. It was also shown that the mechanical properties evolved significantly in terms of fluidity already at the beginning of the reaction, while the reaction extent remained low. Furthermore, the mechanical properties in terms of stress at fracture reached a plateau before reaction completeness. It is argued that the gypsum crystals precipitate and grow to a point where the microstructure formed of entangled crystals is fixed. Then, gypsum crystallization goes on from the residual solution within the microstructure and do not reinforce much the strength at fracture.

The authors would like to thank the French ANR for its financial support on SUN7 project (ANR-19-CE08-0008).