Influence of activator composition on the chloride binding capacity of alkali-activated slag

In this work, the mechanisms of chloride binding in alkali-activated slag (AAS) pastes are studied, towards a better understanding of the role of activator composition (i.e., NaOH, Na2CO3, Na2SO4, KOH, and K2CO3 solutions) on its chloride binding capacity. The impact of chloride on the mineralogical and compositional alteration of AAS is investigated by means of X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS), and thermodynamic modeling. The results show that hardened AAS paste has an approximately 70%-150% stronger chloride binding capacity than ordinary portland cement (OPC) pastes in a 1.0 M NaCl environment, depending on the activator type. The AFm-type and hydrotalcite-type phases in hardened AAS pastes contribute to about 40%-70% of the total chloride binding capacity. The type of preoccupied anions in the AFm-type phases (e.g., OH-, CO32-, SO42-) affects the polymorphs of Friedel's salts formed in AAS. The sulfate-activated slag has a slightly higher chloride binding capacity than hydroxide- and carbonate-counterparts likely due to the transformation of existing ettringite to Friedel's salts.