Diffusion potentials in non-saturated, porous mineral materials – error and information source

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In porous materials such as concretes or soils, the pore solution is likely to exhibit spatial differences in chemical composition in cause of environmental exposure. Thus, diffusion potentials are present and may easily reach several tens of millivolts or exceed 100 mV. A major influence has been identified in gradients of pH due to the higher mobility of anions in comparison to the accompanying cations.Diffusion potentials in porous mineral materials have a significant effect on measurements in research and practice. Consequently, it may lead to considerable misinterpretation of the results and eventually affects the assessment. The present state of the art allows prediction of diffusion potentials in saturated, porous mineral materials. We will continue with further study considering diffusion potentials in non-saturated materials with perm-selective behavior.The overall objective of this project is to significantly improve the current understanding of diffusion potentials in porous mineral materials, particularly in cement-based materials and in sandy soils. The aim is to enhance the accuracy of predictions of diffusion potentials based on material properties and exposure conditions. The situations of main interest are conditions arising from chloride exposure as well as pH differences due to carbonation or leaching. So far, saturated materials were considered. In practice, concrete is normally non-saturated, if structures are not submerged in water. Therefore, this condition state is of higher practical relevance than the saturated state. Consequently, the aim is the development of a tool to predict diffusion potentials in case of non-saturated concrete.The combination of experimental measurements (TUM) with numerical simulations (ETHZ) is the basis for developing the predictive tool for diffusion potentials. Measurement of diffusion potentials is carried out. Characterization (porosity, moisture content) of materials under study will be done by mercury intrusion porosimetry, nuclear magnetic resonance relaxometry, electrochemical impedance spectroscopy. Determination of ion concentration gradients will be done by methods such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Numerical simulations are based on Poisson-Nernst-Planck equations.Being able to predict diffusion potentials is crucial to apply adequate corrections in case of diffusion potential is an error source. Furthermore, an improved understanding of the perm-selective properties of the material and their influence on diffusion potentials may also allow using measurements of diffusion potentials as a means to obtain information about the material under study. Several engineering tasks and current research will benefit from developing the scientific basis for the prediction of diffusion potentials. This includes potentiometric measurements, corrosion monitoring, and cathodic protection.