Resistance of alkali-activated slag mixed with wastewater towards biogenic sulfuric acid attack

The role of Ni and Cr wastewater on the mitigation of the deterioration rate of alkali-activated slag (AAS), caused by biogenic sulfuric acid produced by a sulfur oxidizing fungus (SOF: Aspergillums Niger), is evaluated. The cationic leaching rate, the fungal biomass and the pH of a medium are the compositional parameters which determine the ability of a prepared AAS to resist SOF. After the first 30 days of immersion the fungal strain demonstrated low reactivity, resulting in low Ca, Na, Ni and Cr leaching rates. Increasing the curing time to 180 days caused an enhancement in fungal growth and biogenic sulfuric acid production rates, which were associated with an increase in cationic leachability and AAS corrodibility (through the calcification of calcium silicate hydrate and the formation of gypsum). Nevertheless, the sample containing Ni and Cr wastewater showed the lowest leachability, gypsum content and compressive strength regression, indicating the high efficiency of these heavy metals in mitigating the harmful effects of biogenic sulfuric acid, via the retardation of fungal growth and reactivity. Regardless of variations in heavy metals’ leachability over time, all SOF-media recorded Ni and Cr concentrations lower than the permissible limit in typical leaching procedures, as well as a soluble threshold limit concentration.