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

Abstract

Aged steel slag has a potential use as a highly durable aggregate in roadway construction; however, its high capacity for creating alkaline leachates (pH\u202f>\u202f12.4) 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 (<4.6\u202fmM) compared to steel slag-only samples. Measurements of dissolved ions, saturation index evaluations, and results of geochemical modeling analysis indicated that ettringite and monosulfate formations were favored and that it is probably the responsible mechanism for the observed mitigation of alkalinity and Ca release under alkaline conditions.