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Pore formation during dehydration of polycrystalline gypsum observed and quantified in a time-series synchrotron radiation based X-ray micro-tomography experiment

Fusseis, F., Schrank, Christoph, Liu, J., Karrech, A., Llana-Funez, S., Xiao, X., & Regenauer-Lieb, K. (2012) Pore formation during dehydration of polycrystalline gypsum observed and quantified in a time-series synchrotron radiation based X-ray micro-tomography experiment. Solid Earth, 3(1), pp. 71-86.

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Abstract

We conducted an in-situ X-ray micro-computed tomography heating experiment at the Advanced Photon Source (USA) to dehydrate an unconfined 2.3 mm diameter cylinder of Volterra Gypsum. We used a purpose-built X-ray transparent furnace to heat the sample to 388 K for a total of 310 min to acquire a three-dimensional time-series tomography dataset comprising nine time steps. The voxel size of 2.2 μm3 proved sufficient to pinpoint reaction initiation and the organization of drainage architecture in space and time.

We observed that dehydration commences across a narrow front, which propagates from the margins to the centre of the sample in more than four hours. The advance of this front can be fitted with a square-root function, implying that the initiation of the reaction in the sample can be described as a diffusion process.

Novel parallelized computer codes allow quantifying the geometry of the porosity and the drainage architecture from the very large tomographic datasets (20483 voxels) in unprecedented detail. We determined position, volume, shape and orientation of each resolvable pore and tracked these properties over the duration of the experiment. We found that the pore-size distribution follows a power law. Pores tend to be anisotropic but rarely crack-shaped and have a preferred orientation, likely controlled by a pre-existing fabric in the sample. With on-going dehydration, pores coalesce into a single interconnected pore cluster that is connected to the surface of the sample cylinder and provides an effective drainage pathway.

Our observations can be summarized in a model in which gypsum is stabilized by thermal expansion stresses and locally increased pore fluid pressures until the dehydration front approaches to within about 100 μm. Then, the internal stresses are released and dehydration happens efficiently, resulting in new pore space. Pressure release, the production of pores and the advance of the front are coupled in a feedback loop.

Impact and interest:

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6 citations in Web of Science®

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ID Code: 55182
Item Type: Journal Article
Keywords: Gypsum, Dehydration, Synchrotron micro-tomography, Rock mechanics
DOI: 10.5194/se-3-71-2012
ISSN: 1869-9529
Subjects: Australian and New Zealand Standard Research Classification > EARTH SCIENCES (040000) > GEOPHYSICS (040400) > Geophysics not elsewhere classified (040499)
Divisions: Current > Schools > School of Earth, Environmental & Biological Sciences
Past > Institutes > Institute for Sustainable Resources
Current > QUT Faculties and Divisions > Science & Engineering Faculty
Copyright Owner: Copyright 2012 The Authors
Deposited On: 29 Nov 2012 09:37
Last Modified: 30 Nov 2012 07:54

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