NTERACTION BETWEEN SURFACE CHARGE PHENOMENA AND MULTI-SPECIES DIFFUSION IN CEMENT BASED MATERIALS

Measurements strongly indicate that the ‘inner’ surface of the microscopic structure of cement based materials has a fixed negative charge. This charge contributes to the formation of so-called electrical double layers. In the case of cement based materials the ionic species located in such layers are typically potassium -, sodium - and calcium ions. Due to the high specific surface area of hydrated cement, a large amount of ions can be located in theses double layers even if the surface charge is relatively low. The attraction force, caused by the fixed surface charge on ions located close to surfaces, is one possible explanation for the observed low global diffusion rates in the pore system of positively charged ions compared to the negatively charged ones. Here it is of interest to simulate the multi ionic diffusion behavior when assigning positively charged ions a comparably lower diffusion constant and also including a negatively charged ‘ion’ with an extremely low diffusion constant so as to represent a fixed negative surface charge. The theoretical results from such simulations, using a tailor made finite element technique, indicates a strong influence of surface charges on global diffusion of different ionic species in the pore system of cement based materials.