Steel slag (S) is a by-product of the steel industry with an annual global production of 280 MMT. A significant amount of the S is landfilled, causing economic loss and waste of valuable land space. S is deemed to have great potential as an aggregate because of its mechanical properties; however, the high swelling potential and alkalinity caused by large amounts of free calcium, hence the risk for metal leaching, limit its utilization; thus, a pretreatment process is needed. Amending the S with alum-based water treatment residual (WTR), which may form insoluble complexes with the free calcium, is expected to yield successful results, yet the leaching behavior and controlling mechanisms need to be determined. This study aims to evaluate the pH-dependent leaching characteristics of the pure and WTR-treated S as well as determine the dominant oxidation states of the trace metals and the controlling mechanisms. Acid neutralization capacity and pHstat leaching tests were performed on S–WTR mixtures with WTR contents varying from 0 to 100 % at a pH range of 2 to 14. The tests showed that leaching of all metals is significantly affected by pH and WTR content. Aluminum and copper exhibit an amphoteric behavior in which minimum leaching is observed at the neutral pH values and the metal concentrations are elevated at extreme acidic and alkaline pHs. On the other hand, cadmium, magnesium, calcium, and barium show a cationic leaching pattern. Furthermore, geochemical analyses were undertaken via Visual MINTEQ to determine predominant oxidation states of the metals with a redox potential and the speciation within the aqueous solutions, which showed that most of the releases from the elements were solubility controlled.