Multi-contrast X-ray imaging of water transport in cement-based materials
Water spatial-temporal (4D) distribution in cement-based materials critically drives and/or affects several processes which in turn influence their final engineering performance and durability properties. An incomplete list thereof includes the microstructure development during cement hydration, drying shrinkage, with its consequent cracking, and a manifold of chemomechanical degradation processes due to water-based solutions coming from the outer environment. In these processes, the water (or solution) transport is strongly entangled with the material microstructure and, at early-ages, with its evolution. Any attempt at improving the durability properties of such materials or at trying to influence by design their engineering and functional macroscopic properties can significantly benefit from an in-depth knowledge of such entanglement.
A concrete technology-relevant and practical application example addressed in this PhD thesis concerns trying to reduce early age shrinkage-related cracking in concrete initially cast with low water-to-cement ratio (by mass), so called “high performance concrete”, by adopting water internal curing approaches, based on a gradual and continuous supply of water from artificial “water reservoirs” initially dispersed in the mixed slurry. In this case, the design and adoption of different materials for producing such reservoirs relies upon being able to assess how (fast) is the water released and how (far) it travels through the evolving microstructure of the surrounding cementitious matrix.
Key components of the “tool-bag” to study the water transport-microstructure entanglement are non-destructive imaging methods allowing visualizing both water and the microstructural features mostly affecting it and being affected by it, mainly the pore space features. A constraint for such methods is their ability to perform such visualization without the need of substituting water with another liquid for which the imaging achieves higher “contrast”, because water transport in cement-based materials is chemically reactive, especially at early ages.
This PhD work aimed at enabling the advancement of the entanglement understanding by porting to and optimizing for the concrete materials science and engineering fields multicontrast (phase-sensitive) X-ray imaging methods which have been developed in the last decade. Among all types of multi-contrast X-ray imaging techniques, the one based on X-ray grating interferometry was chosen to achieve that goal. In comparison with other existing techniques, available only at large scale research facilities or via very expensive instruments, the chosen multi-contrast X-ray imaging methods have been proven, within this PhD work, feasible to be implemented at the laboratory-scale with further development of pre-existing and widespread standard X-ray imaging instruments. The obtained results have confirmed the possibility of achieving an intermediate temporal resolution and very high image contrast to changes in water saturation degree and, simultaneously, to the microstructure development, in the absence of any contrast agent, thus without perturbing cement hydration.
The first part of this PhD work contains proof-of-concept studies on the entanglement with multi-contrast X-ray imaging achieved with different types of instruments. Two approaches are proposed within this part. On the one hand, X-ray phase-contrast imaging has been shown to achieve high sensitivity to water saturation degree changes and to the spatial heterogeneity of the pore space, with sufficiently high spatial resolution to be able to investigate a significant fraction of the pore space (down to a minimum length scale of a few m and potentially a few hundreds of nm) and considerable temporal resolution (a few minutes for tomography) for catching the dynamics of the transport, when implemented at synchrotron radiation facilities. On the other hand, X-ray dark-field contrast imaging has been shown, extensively and systematically for the first time in this work, to be capable of visualizing changes in water saturation degree, coupled with microstructural changes, based on laboratory-scale imaging instruments equipped with moderate spatial and temporal resolutions (fast time-lapse radiography or tomography).
The second part of the PhD work, which ran in parallel to the investigations of the first part, consisted in the development of an X-ray grating interferometer at Empa. The instrument, in the form of a Talbot-Lau interferometer, was implemented based on a pre-existing X-ray microtomography system and it was specifically optimized for studying water transport processes in cement-based materials.
Following its implementation, it was exploited to investigate, by time-lapse X-ray dark-field contrast tomography, internal curing of cement pastes by different types of curing particles. The reported results show its capability of visualizing in 3D the release of water from the particles with high contrast and fast tomographic acquisition. The results of this study suggests the potential usefulness of such type of X-ray tomography in investigating other and faster water (or liquid, in general) transport processes in porous building materials with certain pore space features (mainly certain pore size ranges), by increasing the temporal resolution of the instrument with the use of dynamic tomography acquisition protocols and iterative tomographic reconstruction approaches.