Abstract
A series of porous clay samples prepared at different pretreatment temperatures have been tested in a diffusion chamber. Diffusivity and permeability were examined in a temperature range from ambient to 900 °C. Gaseous mixtures of O2, CO2, and N2 have been applied, as these species are the relevant gases in the context of clay brick firing and similar thermochemical processes. Diffusive transport characteristics have been determined by means of the mean transport-pore model, and permeability has been evaluated by Darcy’s law. CO2 diffusivity increased strongly with temperature, whereas O2 diffusion was limited to a certain level. It is proposed that one should consider CO2 surface diffusion in order to explain this phenomenon. The diffusion model was expanded and surface diffusion was included in the model equation. The results of the model fit reflected the important role of incorporated carbonates of the clay foundation in gas-phase (molecular or Knudsen) diffusivity. CO2 surface diffusion was observed to exhibit similar coefficients for two different investigated clays, and is therefore indicated as a property of natural clays. Permeability showed a progressive rise with temperature, in line with related literature.
Highlights
Diffusion in porous media is a widely investigated field in materials science and reaction kinetics research
Samples were provided by a clay brick manufactory and, the results were assumed to exhibit a high suitability for practical application
While gas-phase diffusion is a strictly mechanistic process, surface diffusion can be seen as a chemical complement of the overall phenomenon
Summary
Diffusion in porous media is a widely investigated field in materials science and reaction kinetics research. Natural clay is generally a mixture of several minerals. It is characterized by the constitution of the dense phase, and by a certain range of particle size of the bulk phase. Clays are often attributed to be low cost, mainly inorganic materials, which are available in many regions of the earth in almost unlimited amounts and exhibit manifold desirable physical and/or chemical properties. The transport characteristics of clay samples have been examined in a broad range of temperatures by applying the mean transport-pore model (MTPM) and. Samples were further characterized by mercury intrusion porosimetry of the clay mixtures and evaluation of particle size distribution and chemical composition of the raw clays. Samples were provided by a clay brick manufactory and, the results were assumed to exhibit a high suitability for practical application
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