X-ray Computed Tomography for Characterization of Expanded Polystyrene (EPS) Foam.

Expanded polystyrene (EPS) foam is widely used in building and construction applications for thermal and acoustic insulation. This material is nearly transparent for X-rays, making it difficult to characterize its pore structure in 3D with X-ray tomography. Because of this difficulty, the pore network is often not investigated and is, thus, poorly known. Since this network controls different physical properties, such as the sound absorption, it is crucial to understand its overall structure. In this manuscript, we show how to reveal the pore network of EPS foams through the combination of high resolution X-ray tomography (micro-CT) and saturation techniques. The foams were saturated with CsCl-brine, which acts as a contrasting agent in X-ray micro-CT imaging. This allowed us to separate the beads, making up the foam, from the pore network. Based on the 3D micro-CT results, we were able to assess a representative elementary volume for the polystyrene, which allows for calculating the acoustical parameters from the Johnson-Champoux-Allard (JCA) model, the pore and bead size distribution. The 3D data was also used as input to simulate sound absorption curves. The parametric study showed that an increase in the bead size influenced the sound absorption of the material. We showed that, by doubling the diameter of beads, the absorption coefficient was doubled in certain ranges of frequency.

Motion compensated micro-CT reconstruction for in-situ analysis of dynamic processes.

This work presents a framework to exploit the synergy between Digital Volume Correlation (DVC) and iterative CT reconstruction to enhance the quality of high-resolution dynamic X-ray CT (4D-µCT) and obtain quantitative results from the acquired dataset in the form of 3D strain maps which can be directly correlated to the material properties. Furthermore, we show that the developed framework is capable of strongly reducing motion artifacts even in a dataset containing a single 360° rotation.

In Situ Triaxial Testing To Determine Fracture Permeability and Aperture Distribution for CO2 Sequestration in Svalbard, Norway.

On Svalbard, Arctic Norway, an unconventional siliciclastic reservoir, relying on (micro)fractures for enhanced fluid flow in a low-permeable system, is investigated as a potential CO2 sequestration site. The fractures' properties at depth are, however, poorly understood. High resolution X-ray computed tomography (micro-CT) imaging allows one to visualize such geomaterials at reservoir conditions. We investigated reservoir samples from the De Geerdalen Formation on Svalbard to understand the influence of fracture closure on the reservoir fluid flow behavior. Small rock plugs were brought to reservoir conditions, while permeability was measured through them during micro-CT imaging. Local fracture apertures were quantified down to a few micrometers wide. The permeability measurements were complemented with fracture permeability simulations based on the obtained micro-CT images. The relationship between fracture permeability and the imposed confining pressure was determined and linked to the fracture apertures. The investigated fractures closed due to the increased confining pressure, with apertures reducing to approximately 40% of their original size as the confining pressure increased from 1 to 10 MPa. This coincides with a permeability drop of more than 90%. Despite their closure, fluid flow is still controlled by the fractures at pressure conditions similar to those at the proposed storage depth of 800-1000 m.

Laminar gypsum crust on lede stone: Microspatial characterization and laboratory acid weathering.

Gypsum crusts are typical decay forms on limestone in polluted urban environments. Their origin and relation to the stone facies have been thoroughly investigated in the past three decades. Here, we present the combined use of novel techniques for a microspatial structural, chemical and mechanical characterization of a laminar black gypsum crust on a sandy limestone. These techniques include i.a. X-ray computed microtomography, X-ray Fluorescence micromapping, permeability mapping and the scratch test. They reveal the typical architecture of a laminar gypsum crust, with an outer opaque layer, a subsurface gypsum crystallization layer and a deeper cracked zone passing irregularly into the sound stone. Gypsum crystallization is mostly restricted to an irregular outer zone with an average thickness of 500µm, while cracks are found deeper within the rock. These cracks decrease the rock strength to more than 27.5mm below the surface. Because of their surface parallel orientation and thickness of >10-100µm, they create the potential for surface scaling. This is shown by a laboratory acid test where the crack network extensively developed due to further exposure to an SO2 environment. The use of novel techniques opens potential for the study of different decay forms and can be used for stone diagnosis with regards to conservation studies.

A pore-scale study of fracture dynamics in rock using X-ray micro-CT under ambient freeze-thaw cycling.

Freeze-thaw cycling stresses many environments which include porous media such as soil, rock and concrete. Climate change can expose new regions and subject others to a changing freeze-thaw frequency. Therefore, understanding and predicting the effect of freeze-thaw cycles is important in environmental science, the built environment and cultural heritage preservation. In this paper, we explore the possibilities of state-of-the-art micro-CT in studying the pore scale dynamics related to freezing and thawing. The experiments show the development of a fracture network in a porous limestone when cooling to -9.7 °C, at which an exothermal temperature peak is a proxy for ice crystallization. The dynamics of the fracture network are visualized with a time frame of 80 s. Theoretical assumptions predict that crystallization in these experiments occurs in pores of 6-20.1 nm under transient conditions. Here, the crystallization-induced stress exceeds rock strength when the local crystal fraction in the pores is 4.3%. The location of fractures is strongly related to preferential water uptake paths and rock texture, which are visually identified. Laboratory, continuous X-ray micro-CT scanning opens new perspectives for the pore-scale study of ice crystallization in porous media as well as for environmental processes related to freeze-thaw fracturing.