Thanks to their setting ability, hydraulic binders are used in different applications, such as mineral construction materials, cement for biomedical applications, 3d-printing of architectured ceramics… This setting process is continuous and initiated by mixing fine powders with an aqueous solution. The dissolution of the initial powders results in the formation of a viscous paste, whose composition, microstructure and mechanical properties evolve with time to form a porous monolithic ceramic through the nucleation and precipitation of more stable phases.
In this study, we aim at monitoring in situ two hydraulic binders during their setting, using a multiphysic and multiscale approach to understand how mechanical properties (e.g., complex shear modulus G* = G’ + iG’’) evolve along with the reaction extent and structural build-up.

The setting reactions of β-gypsum plaster and of an apatite calcium phosphate cement (CPC) were studied in standard conditions (e.g., liquid to solid mass ratio) at a time-resolved beamline at ESRF (ID02). X-ray scattering data were recorded simultaneously at small (SAXS) and wide (WAXS) momentum transfer. The analysis of the high Q region enabled to accurately quantify the evolution of crystalline phases correlated with the reaction extent. On the other hand, the low Q region enabled to monitor changes occurring in the pastes for characteristic distances of 1 to100 nm. Experiments were conducted both at rest and under mechanical solicitation applying small amplitude oscillatory strain (ω = 10 rad/s, γ = 3×10-3) using a Rheo-SAXS Couette geometry setup, specifically designed for the experiment.

In the case of gypsum plaster, the evolution of the storage (G’) and loss (G’’) moduli exhibited two regimes. At the beginning of the experiment, G’’ > G’: the material is liquid-like. Then, both moduli increase along with the reaction extent, the evolution of G’ being faster than the one of G’’. While the setting reaction extent, as measured by SAXS-WAXS, is still rather low, G’’ ≈ G’.  G’’ << G’ for further consolidation simultaneously with the setting of the paste and strong variations of the high-Q signal. While qualitatively similar results were obtained in the case of CPC, the low-Q signal emphasized a remarkable slope discontinuity (“bump”), possibly arising from the precipitation of nano-crystalline apatite.
The complementarity of in situ X-ray scattering at small and wide angles, with and without external solicitations, brought an unprecedented understanding of setting of hydraulic binders.

We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities, the French ANR for funding of the SUN7 project (ANR-19-CE08-0008), and the COFECUB for funding the cERCA Project (Ph936/19).