Chloride binding isotherms for Alkali-Activated Slag systems — Pitfalls in the equilibrium method and data interpretation

Determining chloride binding isotherms through equilibrium experiments is common practice in studies on cementitious materials. However, inaccuracies and inconsistencies in experimental procedures and evaluation methods result in non-comparable isotherms. This study critically evaluates the equilibrium method and its interpretation for determining chloride binding isotherms. By comparing the equilibrium method for Ordinary Portland cement (OPC) paste with two alkali-activated slag (AAS) pastes, inaccuracies that originate from different experimental evaluation protocols are evaluated. Microstructural evidence from TGA, XRD and SEM–EDS in combination with thermodynamic modelling is used to further investigate processes affecting the measurement of the chloride binding capacity of alternative binders.
It was found that the simplification of neglecting the initial water content of the paste causes a diluting effect that lowers the measured equilibrium chloride concentration and leads to an overestimation of the bound chloride content. It is also demonstrated that, especially for AAS pastes, water-binding reactions during equilibration can cause a reduction in liquid volume, which increases the measured equilibrium chloride concentration. ... mehrThis volume effect leads to an underestimation of the chloride binding capacity of AAS paste. A key novelty of this study is the quantitative correction of this error by measuring the chemically bound water content before and after chloride exposure. Overall, this study highlights critical methodological pitfalls and provide clear guidelines for determination of chloride binding isotherms using the equilibrium method.