Revealing air-liquid dynamics during long-term saturation of cementitious materials via X-ray CT

Water strongly influences the durability of cementitious materials in civil infrastructure. While physically plausible models exist to describe the long-term water absorption behavior of concrete, their underlying mechanisms remain unverified under relevant conditions. Here, using X-ray computed tomography, we track the gas-liquid-solid configuration within thousands of overcapillary-sized voids during 9 months of water absorption. Results reveal that long-term water uptake is linked to progressive macro-void saturation, evidenced by the gradual shrinkage of trapped gas bubbles. Their behavior is governed by buoyancy, surface wettability, and capillary forces acting at their curved interfaces and driving their dissolution. Contrary to prevailing models, void size has limited influence; instead, gas bubble dissolution is controlled by diffusion of dissolved gases and thus transport distance. Detailed analysis of the evolving air-water-void interface configuration enables critical evaluation of existing concepts and refines the mechanistic understanding of water in mortar and concrete – a topic of increasing importance with eco-friendly binders.