Pore formation during dehydration of a polycrystalline gypsum sample observed and quantified in a time-series synchrotron X-ray micro-tomography experiment 1Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Germany
08 Mar 2012
2School of Earth, Environmental and Biological Sciences, Queensland University of Technology, Brisbane, Australia
3Western Australian Geothermal Centre of Excellence, Perth, Australia
4School of Earth and Environment, University of Western Australia, Crawley, Australia
5CSIRO Earth Science and Resource Engineering, Kensington, Australia
6School of Mechanical Engineering, University of Western Australia, Crawley, Australia
7Departamento de Geología, Universidad de Oviedo, Oviedo, Spain
8Advanced Photon Source, Argonne, USA
Received: 20 Sep 2011 – Published in Solid Earth Discuss.: 21 Oct 2011 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.
Revised: 08 Feb 2012 – Accepted: 10 Feb 2012 – Published: 08 Mar 2012
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.
Citation: Fusseis, F., Schrank, C., Liu, J., Karrech, A., Llana-Fúnez, S., Xiao, X., and Regenauer-Lieb, K.: Pore formation during dehydration of a polycrystalline gypsum sample observed and quantified in a time-series synchrotron X-ray micro-tomography experiment, Solid Earth, 3, 71-86, doi:10.5194/se-3-71-2012, 2012.