The Scientific Challenge: Decoding the Complexity of THCM Couplings

Concrete production accounts for approximately 8% of global CO2 emissions. While the accelerated carbonation of recycled aggregates is a promising avenue for carbon sequestration, the physics of the process remains a “black box.” Under industrial conditions (80°C, pure CO2 flow), thermo-hydro-chemical-mechanical (THCM) interactions create a complex feedback loop in which the chemical reaction alters the porous structure, which in turn controls fluid transport.

The Experimental Approach: Bimodal Synergy at NeXT-Grenoble

The study is based on a new carbonation cell operando (see figure) and takes advantage of the unique configuration of the NeXT (Neutron and X-ray Tomograph) instrument at the ILL, developed in collaboration with the University of Grenoble Alpes, which enables simultaneous and co-localized data acquisition. This 5D tomography approach operando is essential for quantifying phenomena that occur on the same spatiotemporal scale.

Phase Identification Using Dual Contrast

While X-rays map changes in mineral density (carbonation front; see figure at bottom left) and the kinetics of cracking, neutrons isolate the response of the liquid phase (figure at bottom right). This combination makes it possible to unambiguously distinguish between the consumption of bound water, the accumulation of free water, and changes in porosity—measurements that are impossible with conventional measurement systems.

Operando Correlation Between Transport and Damage

The strength of the instrumentation at NeXT lies in its ability to correlate, at any given moment, the formation of microcracks (visible with X-rays) with moisture fluxes (visible with neutrons). This coupling reveals how mechanical damage acts as either an accelerator or an inhibitor of transport, depending on local saturation.

Key Findings and Contributions of the Program

Water Transport Induced by Chemical Processes

The experimental results indicate that carbonation cannot be described as a simple gas diffusion phenomenon. It is accompanied by a significant release of water that was initially chemically bound, as shown in the box titled “ Water Content Profile " in the figure. Neutron imaging reveals that this water migrates toward the center of the sample. At the same time, reaction products (particularly CaCO₃) precipitate within the nanoporosity, leading to the formation of a hydraulic barrier that reduces the effective permeability and limits CO₂ penetration (see box “ Advances in Carbonation ").

Crack Propagation Dynamics

Bimodal imaging has made it possible to observe the nucleation of microcracks induced by calcite crystallization stresses. These cracks initially act as preferential transport pathways for the gas before undergoing partial sealing, a phenomenon known as “structural breathing.” This phenomenon was quantified through time-series monitoring with scans taken every 25 minutes, as shown in the box “ Crack Network Development ".

Limitations of Classical Diffusion Models

La résolution spatiale atteinte (< 25 µm), combinée à la sensibilité aux phases fluides, démontre que les modèles purement diffusifs sont inadaptés. Ces approches négligent les mécanismes couplés de transport, l’évolution de la perméabilité ainsi que les rétroactions mécaniques, pourtant déterminants dans le contrôle de la cinétique globale du processus de carbonatation.