Fluid flow and mass transport in geological materials are crucial in diverse Earth science applications. To fully understand the behavior of geological materials in this context, the pore scale ...properties of these materials have to be investigated and related to effective material properties. Imaging techniques are becoming ever more valuable tools to characterize the microstructure (especially in three dimensions), while numerical models to calculate transport properties based on experimental images of the microstructure are quickly maturing. The results of image-based modeling studies depend crucially on both the employed model and the quality of the pore space image on which the model runs. Given the technicality and the cross-disciplinary nature of this matter, this review aims to provide a practical and accessible introduction to both the experimental and numerical state-of-the-art, intended for students and researchers with backgrounds in experimental geo-sciences or computational sciences alike.
Self-healing strategies are regarded as a promising solution to reduce the high maintenance and repair cost of concrete infrastructures. In the present work, a bacterial-based self-healing by use of ...hydrogel encapsulated bacterial spores (bio-hydrogels) was investigated. The crack closure behavior of the specimens with/without bio-hydrogels was studied quantitatively by light microscopy. To have a view of the self-healing inside the specimens, a high resolution X-ray computed microtomography (X-ray μCT) was used. The total amount and the distribution of the healing products in the whole matrix were investigated. This study indicates that the specimens incorporated with bio-hydrogels had distinct improved healing efficiency compared to the reference ones with pure hydrogel only. The healing ratios in the specimens with bio-hydrogels were in the range from 70% to 100% for the cracks smaller than 0.3mm, which is more than 50% higher than for the ones with pure hydrogel; and the maximum crack bridging was about 0.5mm (in 7d), while pure hydrogels only allowed healing of cracks of about 0.18mm. The total volume ratio of the healing product in the specimens with bio-hydrogels amounted to 2.2%, which was about 60% higher than for the ones with pure hydrogel (1.37%).
Three-dimensional concrete printing (3DCP) has progressed rapidly in recent years. With the aim to realize both buildings and civil works without using any molding, not only has the need for reliable ...mechanical properties of printed concrete grown, but also the need for more durable and environmentally friendly materials. As a consequence of super positioning cementitious layers, voids are created which can negatively affect durability. This paper presents the results of an experimental study on the relationship between 3DCP process parameters and the formed microstructure. The effect of two different process parameters (printing speed and inter-layer time) on the microstructure was established for fresh and hardened states, and the results were correlated with mechanical performance. In the case of a higher printing speed, a lower surface roughness was created due to the higher kinetic energy of the sand particles and the higher force applied. Microstructural investigations revealed that the amount of unhydrated cement particles was higher in the case of a lower inter-layer interval (i.e., 10 min). This phenomenon could be related to the higher water demand of the printed layer in order to rebuild the early Calcium-Silicate-Hydrate (CSH) bridges and the lower amount of water available for further hydration. The number of pores and the pore distribution were also more pronounced in the case of lower time intervals. Increasing the inter-layer time interval or the printing speed both lowered the mechanical performance of the printed specimens. This study emphasizes that individual process parameters will affect not only the structural behavior of the material, but they will also affect the durability and consequently the resistance against aggressive chemical substances.
Solute transport is important in a variety of applications regarding flow in porous media, such as contaminant groundwater remediation. Most recent experimental studies on this process focus on ...field‐scale or centimeter‐scale data. However, solute spreading and mixing are strongly influenced by pore‐scale heterogeneity. To study this, we developed a novel methodology to quantify transient solute concentration fields at the pore scale using fast laboratory‐based microcomputed tomography. Tracer injection experiments in samples with different degrees of pore‐scale heterogeneity (porous sintered glass and Bentheimer sandstone) were imaged in 3D by continuous scanning at a time resolution of 15 s and a spatial resolution of 13.4 μm. While our calibration experiments indicated a high uncertainty (1σ) on the concentration in single voxels due to imaging noise (± 27% of the total concentration range), we show that coarse gridding these values per individual pore significantly lowers the uncertainty (± 1.2%). The resulting pore‐based tracer concentrations were used to characterize the transport by calculating the solute's arrival time and transient (filling) time in each pore. The average velocities estimated from the arrival times correspond well to the interstitial velocities calculated from the flow rate. This suggests that the temporal resolution of the experiment was sufficient. Finally, the pore‐based transient filling times, the global concentration moment and the global scalar dissipation rate calculated from our experiments, indicated more dispersion in the sandstone sample than in the more homogeneous sintered glass. The developed method can thus provide more insight in the influence of pore‐scale heterogeneity on solute transport.
Plain Language Summary
In groundwater reservoirs, contaminants are often dissolved in the water that flows through the porous sediment or rock layers. Different transport processes like advection, diffusion, and mechanical dispersion influence the concentration distribution of these contaminants. A thorough understanding of the transport processes is thus key for, for example, groundwater remediation and waste management. This can be studied by simulations and experimental work. Most of the recent experimental studies gather field‐scale or centimeter‐scale data. However, transport processes are significantly impacted by the microscopic structure of the pores in the sediment or rock. In this methodological study, we thus focus on direct observations of pore‐scale solute transport in porous materials with a different amount of pore‐scale heterogeneity (sintered glass and sandstone samples) by fast laboratory‐based microcomputed tomography. Tracer concentrations within individual pores are quantified with a standard deviation of 1.2% during tracer injection experiments. Based on these concentration fields, an arrival time and transient time for every pore are defined. These parameters provide insight in pore‐scale mechanical dispersion processes. This methodology can contribute to understand the influence of pore‐scale heterogeneity on solute transport.
Key Points
Fast micro‐CT (15‐s temporal resolution) was used to quantify transient pore‐scale concentration fields in tracer injection experiments
Enhanced image analysis reduced the average uncertainty of the micro‐CT based concentration in each individual pore to 1.2%
Solute spreading and mixing were analyzed by defining pore‐scale transport properties like arrival and transient (filling duration) time
•Freeze-thaw weathering of Savonnières limestone was investigated.•Macroscopic behaviour was characterized by strain and temperature measurements.•Microscopic observations of crystal and water ...localities were done with µCT.•The presence of oolithic ink-bottle pores decreases the frost susceptibility.
Many natural building stones are altered when subjected to freeze-thaw (FT) cycles. On the one hand, the sensitivity of a material to FT damage has been quantified in the past by the existence of a material-specific critical water saturation. On the other hand, it was noticed that natural stones with a high volume of ink-bottle pores, normally hold a relatively large FT resistance. The relationship between the pore structure, the saturation and the FT resistance is however not well understood. In this paper, the influence of water content and the porosity on FT behaviour is investigated macroscopically with temperature and strain measurements, established techniques in FT related research, and X-ray computed micro-tomography (µCT). Strain measurements performed on Savonnières limestone samples clearly show an increasing expansion with increasing water content, with the existence of a critical water saturation level between 70 and 80%. Differential X-ray imaging on differently saturated limestone samples in an unfrozen and frozen state then aided in explaining the origin of this critical saturation degree. By draining water from surrounding micro-pores through cryo-suction, ink-bottle ooid voids served as expansion reservoirs. When the majority of the ooid voids is water saturated prior to freezing, these voids lose their expansion reservoir ability and damaging pressures arise when water freezes inside undrained micro-pores. These findings not only help to understand the FT resistance of this limestone, but also give insight in the general FT behaviour of materials with bi- or multimodal pore-size distributions.
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.
X-ray computed micro-tomography typically involves a trade-off between sample size and resolution, complicating the study at a micrometer scale of representative volumes of materials with broad ...feature size distributions (e.g. natural stones). X-ray dark-field tomography exploits scattering to probe sub-resolution features, promising to overcome this trade-off. In this work, we present a quantification method for sub-resolution feature sizes using dark-field tomograms obtained by tuning the autocorrelation length of a Talbot grating interferometer. Alumina particles with different nominal pore sizes (50 nm and 150 nm) were mixed and imaged at the TOMCAT beamline of the SLS synchrotron (PSI) at eighteen correlation lengths, covering the pore size range. The different particles cannot be distinguished by traditional absorption µCT due to their very similar density and the pores being unresolved at typical image resolutions. Nevertheless, by exploiting the scattering behavior of the samples, the proposed analysis method allowed to quantify the nominal pore sizes of individual particles. The robustness of this quantification was proven by reproducing the experiment with solid samples of alumina, and alumina particles that were kept separated. Our findings demonstrate the possibility to calibrate dark-field image analysis to quantify sub-resolution feature sizes, allowing multi-scale analyses of heterogeneous materials without subsampling.
•State of the art in fast laboratory-based X-ray micro-computed tomography is outlined.•Real-time, pore-scale visualization of drainage in Bentheimer with lab-based scanner.•Solute transport is ...imaged at the pore scale in a limestone at 12 s per scan.
Over the past decade, the wide-spread implementation of laboratory-based X-ray micro-computed tomography (micro-CT) scanners has revolutionized both the experimental and numerical research on pore-scale transport in geological materials. The availability of these scanners has opened up the possibility to image a rock's pore space in 3D almost routinely to many researchers. While challenges do persist in this field, we treat the next frontier in laboratory-based micro-CT scanning: in-situ, time-resolved imaging of dynamic processes. Extremely fast (even sub-second) micro-CT imaging has become possible at synchrotron facilities over the last few years, however, the restricted accessibility of synchrotrons limits the amount of experiments which can be performed. The much smaller X-ray flux in laboratory-based systems bounds the time resolution which can be attained at these facilities. Nevertheless, progress is being made to improve the quality of measurements performed on the sub-minute time scale. We illustrate this by presenting cutting-edge pore scale experiments visualizing two-phase flow and solute transport in real-time with a lab-based environmental micro-CT set-up. To outline the current state of this young field and its relevance to pore-scale transport research, we critically examine its current bottlenecks and their possible solutions, both on the hardware and the software level. Further developments in laboratory-based, time-resolved imaging could prove greatly beneficial to our understanding of transport behavior in geological materials and to the improvement of pore-scale modeling by providing valuable validation.
Degradability of organic matter (OM) in soil depends on its spatial location in the soil matrix. A recent breakthrough in 3D-localization of OM combined dual-energy X-ray CT-scanning with OsO
...staining of OM. The necessity for synchrotron-based µCT and the use of highly toxic OsO
severely limit applications in soil biological experiments. Here, we evaluated the potential of alternative staining agents (silver nitrate, phosphomolybdenic acid (PMA), lead nitrate, lead acetate) to selectively enhance X-ray attenuation and contrast of OM in CT volumes of soils containing specific mineral soil particle fractions, obtained via lab-based X-ray µCT. In comparison with OsO
, administration of Ag
and Pb
resulted in insufficient contrast enhancement of OM versus fine silt (< 20 µm) or clay (< 2 µm) mineral particles. The perfusion procedure used in this work induced changes in soil structure. In contrast, PMA staining resulted in a selective increase of OM's attenuation contrast, which was comparable to OsO
. However, OM discrimination from other soil phases remained a challenge. Further development of segmentation algorithms accounting for grey value patterns and shape of stained particulate OM may enable its automated identification. If successful in undisturbed soils, PMA staining may form an alternative to OsO
in non-synchrotron based POM detection.
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