Leaching of monolithic and granular alkali activated slag-fly ash materials, as a function of the mixture design

Abstract

This study explores the leaching of oxyanionic metalloid species (As, Mo, Se, V and Cr) from alkali activated slag-fly ash materials (AAM), dependent on various mixture parameters i.e., activator molarity, slag-fly ash precursor/binder compositions, liquid to binder ratio, curing time and strength. The analyses focusses on the leaching of potentially hazardous elements in a monolithic and granular material state. For monolithic state AAMs (concrete) overall leaching is within comparable range with traditional Portland cement and in both systems their leaching is far below the regulatory leaching limit values even though AAM strongly differs in mixture composition. For granular state AAMs (aggregate) the parameters, activator alkalinity and the slag-fly ash precursor/binder composition, significantly influence the leaching. The release of As and V strongly increases with a higher activator molarity as an effect of changes in the system alkalinity and related material pH. The release of As, Mo, Se and V strongly increase with a higher fly ash content within the precursor/binder composition. Overall, the leaching of aggregate state AAMs meets the Dutch leaching limits for open application of granular building materials, when the fly ash content within the binder composition ≤ ≈30 wt%. Typically, the pH dependent leaching data show oxyanionic metalloid species have a relatively high leaching potential, being less effectively bound as a result of the amorphous AAM microstructure. However, the leachable concentrations of a AAM system are within the bandwidth with that of blended (slag and or fly ash) Portland cement system.