Ettringite and monosulfate formation to reduce alkalinity in reactions of alum-based water treatment residual with steel slag

•Steel slag can create highly alkaline leachates detrimental to aquatic life.•Degree of alkalinity mitigation was proportional to the reductions in Ca release.•Water treatment residual addition to steel slag mitigated alkalinity.•Ettringite and monosulfate formation can explain the reduced Ca(II) and OH− release. Aged steel slag has a potential use as a highly durable aggregate in roadway construction; however, its high capacity for creating alkaline leachates (pH > 12.4) poses a severe environmental risk. In batch and column leach tests, 10% alum-based water treatment residual (WTR) addition to aged steel slag resulted in a 67% decrease in acid neutralizing capacity of steel slag and leachate alkalinity, but this alkalinity mitigation effect was accompanied with markedly increases in dissolved Al concentrations in the leachates (