Iron and steel slags from legacy and modern operations in the Chicago-Gary area of Illinois and Indiana, USA, are predominantly composed of Ca (10 - 44 wt. % CaO), Fe, (0.3 - 28 wt. % FeO), and Si (10 - 44 wt. % SiO2), with generally lesser amounts of Al (< 1 15 wt. % Al2O3), Mg (2 11 wt. % MgO), and Mn (0.3 9 wt. % MnO). Mineralogy is dominated by CaMgAl silicates, FeCa oxides, Ca-carbonates, and high temperature SiO2 phases. Chromium and Mn concentrations in most samples may be environmentally significant based on comparison with generic soil contaminant guidelines. However, simulated weathering tests suggest these elements are present in generally insoluble phases making use in water treatment applications possible; generation of high pH and alkaline solutions may be an issue. As for water treatment applications, batch and flow-through experiments document effective removal of phosphate from synthetic solutions for nearly all slag samples. Air-cooled fine fractions (< 10 mm) of modern slag were most effective; other types, including modern granulated, modern air-cooled coarse fractions (> 10 mm), and legacy slag removed phosphate, but to a lesser degree. An additional water treatment application is the use of slag to neutralize acidic waters. Most slag samples are extremely alkaline and have high net neutralization potentials (NNP) (400 830 kg CaCO3/t), with the highest approximately equivalent to 80% the neutralization potential of calcite. Overall, phosphate removal capacity and NNP correlate positively with total Ca content and the dissolution of Ca minerals facilitates secondary Ca phosphate formation and consumes acid during hydrolysis. Utilizing locally available slag to treat waste or agricultural waters in this region may be a higher value alternative than use in construction, potentially offsetting restoration costs to degraded legacy areas and decreasing steel manufacturers current waste footprint.