X-ray dark-field contrast imaging of water transport during hydration and drying of early-age cement-based materials

In this study, we investigated by X-ray dark-field contrast imaging the internal displacements of water in early-age cement-based materials due to their spatial microstructural heterogeneities. We performed time-lapse multi-contrast X-ray radiography measurements with a laboratory-scale Talbot-Lau X-ray interferometer during drying and hardening of a model system. Such system consisted of two mortar layers with distinct pore size distribution and local porosities. With these measurements we propose a new approach to imaging water transport in such materials at early hardening ages. The results show that such approach provides higher sensitivity to local water content changes and higher temporal and spatial resolutions as compared to standard X-ray attenuation contrast imaging. In this work, we assessed both qualitatively and quantitatively the roles of key drivers of such water displacements in the evolving microstructure: capillary force gradients created by the spatial heterogeneity in the pore size distribution and by evaporative drying. This was accomplished by correlating the dark-field contrast imaging results with information about the system's pore space features, obtained by attenuation contrast X-ray micro-tomography and respective 3D image analysis. Such correlative analysis provides new evidence of the existence of strong couplings between pore-scale water displacements and the microstructure formation in cement-based materials at early ages.