Jan Dewanckele

Three-dimensional analysis of high-resolution X-ray computed tomography data with Morpho+

Three-dimensional (3D) analysis is an essential tool to obtain quantitative results from 3D datasets. Considerable progress has been made in 3D imaging techniques, resulting in a growing need for more flexible, complete analysis packages containing advanced algorithms. At the Centre for X-ray Tomography of the Ghent University (UGCT), research is being done on the improvement of both hardware and software for high-resolution X-ray computed tomography (CT). UGCT collaborates with research groups from different disciplines, each having specific needs. To meet these requirements the analysis software package, Morpho+, was developed in-house. Morpho+ contains an extensive set of high-performance 3D operations to obtain object segmentation, separation, and parameterization (orientation, maximum opening, equivalent diameter, sphericity, connectivity, etc.), or to extract a 3D geometrical representation (surface mesh or skeleton) for further modeling. These algorithms have a relatively short processing time when analyzing large datasets. Additionally, Morpho+ is equipped with an interactive and intuitive user interface in which the results are visualized. The package allows scientists from various fields to obtain the necessary quantitative results when applying high-resolution X-ray CT as a research tool to the nondestructive investigation of the microstructure of materials.

4D imaging and quantification of pore structure modifications inside natural building stones by means of high resolution X-ray CT.

Weathering processes have been studied in detail for many natural building stones. The most commonly used analytical techniques in these studies are thin-section petrography, SEM, XRD and XRF. Most of these techniques are valuable for chemical and mineralogical analysis of the weathering patterns. However, to obtain crucial quantitative information on structural evolutions like porosity changes and growth of weathering crusts in function of time, non-destructive techniques become necessary. In this study, a Belgian historical calcareous sandstone, the Lede stone, was exposed to gaseous SO(2) under wet surface conditions according to the European Standard NBN EN 13919 (2003). Before, during and after the strong acid test, high resolution X-ray tomography has been performed to visualize gypsum crust formation to yield a better insight into the effects of gaseous SO(2) on the pore modification in 3D. The tomographic scans were taken at the Centre for X-ray Tomography at Ghent University (UGCT). With the aid of image analysis, partial porosity changes were calculated in different stadia of the process. Increasing porosity has been observed visually and quantitatively below the new superficial formed layer of gypsum crystals. In some cases micro-cracks and dissolution zones were detected on the grain boundaries of quartz. By using Morpho+, an in-house developed image analysis program, radial porosity, partial porosity, ratio of open and closed porosity and equivalent diameter of individual pore structures have been calculated. The results obtained in this study are promising for a better understanding of gypsum weathering mechanisms, porosity changes and patterns on natural building stones in four dimensions.

Characterizing saline uptake and salt distributions in porous limestone with neutron radiography and X-ray micro-tomography

Samples of Savonnières limestone subjected to repeated wetting–drying cycles were investigated by both neutron radiography and X-ray micro-tomography to collect information on saline uptake and salt precipitation. Capillary uptake of water, 1.4 molal sodium sulphate and 5.8 molal sodium chloride solution was visualized with neutron radiography. The liquid penetration coefficients and diffusivities were determined and are markedly lower for the salt solutions than for water, due to the higher surface tension and viscosity of salt solutions. Halite distributions were derived from neutron radiographs. Porosity analysis of X-ray tomographic datasets allowed quantifying thenardite distributions and porosity decrease due to salt crystallization.

3D characterization of sandstone by means of X-ray computed tomography

Due to technological development, stateof-the-art high-resolution X-ray computed tomography (CT) systems can be developed, enabling the internal visualization of geomaterials in three dimensions. However, in order to obtain structural information one also needs proper three-dimensional (3D) analysis software. In this paper, the potential for petrographic purposes of high-resolution X-ray CT in combination with the 3D analysis software Morpho+ is explored for a Belgian sandstone sample. The advantage of the CT technique is the fact that it is an ideal tool to characterize the internal structure of a rock in three dimensions in a nondestructive way while a limitation of this CT technique is that only small samples can be combined with a high spatial resolution and therefore often many samples will need to be scanned in order to obtain representative volumes. The relationship between sample size and obtained spatial resolution are discussed as well as the infl uence on the spatial resolution exerted by some important technical aspects like the used X-ray source and detector. This paper focuses in detail on the structures that can be determined by means of micro- and high-resolution X-ray CT in combination with 3D algorithms.

High-Resolution X-Ray CT for 3D Petrography of Ferruginous Sandstone for an Investigation of Building Stone Decay

Diestian ferruginous sandstone has been used as the dominant building stone for monuments in the Hageland, a natural landscape in east‐central Belgium. Like all rocks, this stone type is sensitive to weathering. Case hardening was observed in combination with blackening of the exterior parts of the dressed stones. To determine the 3D petrography and to identify the structural differences between the exterior and interior parts, X‐ray computed tomography was used in combination with more traditional research techniques like optical microscopy and scanning electron microscopy. The 3D characterization of the ferruginous sandstone was performed with a high‐resolution X‐ray CT scanner (www.ugct.ugent.be) in combination with the flexible 3D analysis software Morpho+, which provides the necessary petrophysical parameters of the scanned samples in 3D. Besides providing the required 3D parameters like porosity, pore‐size distribution, grain size, grain orientation, and surface analysis, the results of the 3D analysis can also be visualized, which enables to understand and interpret the analysis results in a straightforward way. The complementarities between high‐quality X‐ray CT images and flexible 3D software and its relation with the more traditional microscopical research techniques are opening up new gateways in the study of weathering processes of natural building stones. Microsc. Res. Tech., 2011.

Holistic approach of pre-existing flaws on the decay of two limestones

This study aims to understand the influence of the microfacies and the determination of pre-existing flaws on the weathering behavior of two types of limestones. Therefore, both Lede and Noyant limestones were independently weathered by strong acid tests and freeze-thaw cycles. In order to characterize the weathering patterns inside the stones, a combination of high resolution X-ray CT, SEM-EDS and thin section microscopy was used. The advantage of high resolution X-ray CT is its non-destructive character and the obtained 3D structural information. By using this technique, a time-lapse sequence of the weathering patterns was obtained for both gypsum crust formation as well as crack formation due to freezing and thawing. This way, a clear link could be made with the initial non-weathered state. Thin section microscopy and SEM-EDS provided additional chemical information. The focus of this study lies in the processes that occur in the bioclast fragments in the stone and the influence of the surrounding cement or matrix. The results show that weathering patterns vary for both limestones although the causes of weathering were similar. In case of the Noyant stone, the weathering by crystallizing gypsum was mainly restricted to the microporous matrix of the stone, while in case of the Lede stone, several foraminifera and shell fragments were preferentially recrystallized. In general, the underlying microstructure determines the weathering pattern of the stone.

Three-dimensional phase separation and identification in granite

Typical granite is mainly composed of quartz, plagioclase, and alkali feldspars, together with some smaller fractions of ferromagnesian phases and trace phases. The occurrence and abundance of these minerals can be determined by destructive methods, but the three-dimensional (3D) visualization of these mineral structures is very difficult. For geological samples, X-ray microcomputed tomography (mu CT) is a very powerful tool to visualize 3D structures. However, mu CT imaging is based on the X-ray linear attenuation coefficient, which depends on atomic composition, density, and X-ray energy, sometimes making it hard to identify different phases inside the sample. This problem can be overcome by combining mu CT with other techniques that provide chemical information. The combination of mu CT with micro-X-ray fluorescence (mu XRF) and X-ray diffraction (XRD) allows segmentation and identification of the different minerals at the surface of the sample. Combining this information with the 3D density model obtained through mu CT scanning of the granite allows 3D phase identification. In this paper the results of the analysis of a representative Precambrian granite by these three complementary techniques are discussed and combined with the results of more traditional techniques like thin-section petrography. Although the granite sample is shown to be very heterogeneous, correct phase identification in 3D is obtained.

Replacement stones for Lede stone in Belgian historical monuments

Abstract The Lede stone (Lutetian, Eocene) is an important historic building stone used in the NW of Belgium. In Ghent, it is dominant in the post-Romanesque built cultural heritage. Its use was restricted several times by socio-economic constraints. Since quarrying and production started to cease from the seventeenth century, periodic revivals favoured the use of Lede stone for new buildings and restoration projects. Sulphation is the main threat for the Lede stone as black crusts are the most common degradation phenomena on this arenaceous limestone. Around the turn of the nineteenth century, the Belgian Gobertange stone was the most widely used replacement material. Throughout the twentieth century, the use of replacement material shifted towards French limestones. However, their colour, texture and petrophysical properties differ from the Lede stone, for which a natural yellow–brown patina is very characteristic. In order to solve this mainly aesthetic issue, several new stone types are used as replacement stone in the twenty-first century, while many others have been suggested. It remains, however, difficult to find a replacement stone that matches the visual and petrophysical properties of the Lede stone. One remaining Lede stone quarry pit has increased its activity since 2011, offering the opportunity to use new Lede stone as replacement stone.

Multi-disciplinary characterization and monitoring of sandstone (Kandla Grey) under different external conditions

Nowadays there is an increase of imported natural building stones, often used as replacement of local, more traditional building stones. The durability of these traditional stones is generally well known; however, when new imported materials are used, it is essential to study their behaviour under the current and local climatological conditions to predict their weathering resistance. In addition to water exposure, these building materials have to be resistant to significant mechanical stress created by the imbibition of de-icing salt solutions, frequently used during winter in Western Europe, with temperature periodically changing from freeze to thaw conditions. Porous network modifications related to the materials’ chemical composition are very complex when different forces are acting on the stone itself. Therefore it is crucial to determine the internal structure of the building stone under changing external conditions with and without the presence of de-icing salts, to understand the influence of these additional salts. In this paper, particular attention was paid to the multi-characterization of compact Kandla Grey laminated sandstone, a building stone frequently imported from India to Belgium recently. Traditional as well as highly advanced research techniques were used for the characterization and monitoring of changes under different external conditions. This study demonstrates that the structural characteristics of the laminations have an effect on the frost resistance of the stone and its response to salt weathering. From the experiments carried out, it can be concluded that Kandla Grey can be sensitive to frost and salt weathering under the current climatic conditions in Western Europe.