Nanoscale chloride diffusion in alkali-activated steel slag and ultrafine blast furnace slag considering the electrical double layer effect

Utilizations of industrial byproducts and wastes as much as possible, together with desirable durability, are essential to sustainable development of building materials. In this work, the chloride diffusion behavior of alkali-activated steel slag (SS) and ultrafine blast furnace slag (UFS) is studied, towards deeper insights into the effect of electrical double layer on the resistance to chloride penetration. The microstructure improvement of alkali-activated SS with increasing addition of UFS was examined by means of X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, mercury intrusion porosimetry and nitrogen sorption. The zeta potentials above the pore surface of various alkali-activated SS-UFS systems were measured and compared. The electrostatic resistance of the electrical double layer to chloride diffusion was analyzed and discussed. Results indicate that UFS dosages above 40% substantially refines the pore structure of alkali-activated SS and then the mechanism governing chloride penetration shifts from capillary transport to gel transport. The exposure of higher chloride sodium concentration results in the pore surface of alkali-activated SS-UFS systems to be less negatively charged and the zeta potential is reversed to be positive value after continuous ion exchange between sodium and calcium. The presence of UFS can remarkably increase the proportion of physically bound chloride that attains approximately 80% of the total binding capacity. The impediment of electrical double layer to chloride diffusivity becomes increasingly pronounced with pore structure refinement, especially for mixtures with diffusion coefficients below 5 × 10−12 m2/s obtained based on Fick's law of diffusion.