Diffusion and electromigration in clay bricks influenced by differences in the pore system resulting from firing

摘要

Ion transport in porous materials has been subject of study for several decades. However, the interaction between the pores and the overall pore system make it complicated to obtain a clear picture and predict diffusion and electromigration (transport induced by an applied electric field). Specific bricks were examined with respect to shape, size and interconnection of the pores. The pores were studied at a microscopic level, the interconnected pore system at a macroscopic level and the results obtained were compared with the measurement of the corresponding ion transport of Cl~ and Na+ through the pore system to contribute to an overall understanding of ion transport in porous materials. The pore system in bricks are influenced by the firing degree, clay mixture composition and ion content. The present paper focuses on the pore system and effects from clay mixture composition and ion content were neglected by using the same brick type from the same brickwork (delivered at the same pallet). The used bricks were fired in a circular kiln were uneven heating during firing occurs. Significant color differences were visible between the bricks in the pallet and for the investigation purpose they were subdivided into three groups: bright, medium and dark colored bricks. The increasing color intensity is most probably caused by increasing firing temperatures. These three groups of bricks were investigated for saturation coefficient, open porosity, dry density and water absorption coefficient which revealed significant differences were encountered. The pore system was studied by using thin sections which demonstrated a change from relatively many fine pores to fewer wide pores with increasing brick firing temperatures. An additional significant difference in the pore system of each brick was found to be related to the distance to the surface. The influence of the pore system on ion transport through the water saturated pore system of the bricks was supported by measurements for calculation of the electrical resistance and an increasing resistance was found for increasing brick firing temperatures. The effective diffusion coefficient was empirically determined for chloride and sodium through the application of an electric DC field across the bricks. The lowest effective diffusion coefficient was found for the dark colored brick, increasing for the medium and bright colored respectively. This finding suggests that in clay bricks the presence of many fine pores improves ion transport compared to fewer wider pores.