Influence of surface electrical properties of C–S–H on chloride binding in slag–blended cementitious materials

Abstract

Knowledge on interface characteristics of C–S–H is important to understand the adsorption and transport behavior of ionic species into cementitious materials. In this study, the surface electrical properties of C–S–H and their influence on ionic adsorption have been investigated to be able to predict chloride binding on slag containing materials. The experimental data showed that the surface charge or electrokinetic potential of slag cement paste (SCP) are intermediate between Portland cement paste and C–S–H with Ca/Si of 1.0. The surface site density of the C–S–H and the associated equilibrium constant value for dissociation are derived from potentiometric titration experiment and a surface complexation model. Further, the surface complexation modelling parameter for chloride adsorption on C–S–H with Ca/Si of 1.0 were estimated by fitting the experimental data to simulation results. The estimated parameters for C–S–H together with surface complexation modelling parameters for Portland cement, which was derived and verified in a previous study, were used to predict the chloride binding on SCP. A batch experiment was performed to determine the total binding of chloride on SCP. It is considered that total binding of chloride is the amount of adsorbed chloride on the C–S–H surface as well as the chemical binding of chloride as Friedel’s salt. The simulation results on chloride binding show good agreement with experimental data, follows a Freundlich isotherm. When Portland cement is partially replaced by slag, it modifies the surface electrical properties of C–S–H in addition to mineralogy and pore structure and leads to change in the chloride binding behavior.