Sustainable geotechnical applications of mixtures of incineration bottom ash and mine tailings

The increasing generation of waste has become a global problem. Policies to implement management strategies focused on waste minimization and valorisation have been a tool to avoid landfilling. In this context, incineration appears as a treatment option for volume reduction and energy production from Municipal Solid Waste, which also generates residues composed mainly by Bottom Ash. Mine tailings are also a concern in this scenario as one of the most heavily produced waste streams, which are disposed of in relatively dangerous constructions, mine tailings dams. In the attempt of expanding the life span of these wastes, this project aimed to characterize Bottom Ash and mixtures of Bottom Ash and Mine Tailings assessing their mechanical behaviour as well as the possible environmental impacts due to geotechnical application in road construction. Chemical characterization of both materials was mostly made through leachate and total content evaluation. Bottom Ash exhibited low heavy metal concentration complying with the limit values for non-hazardous waste landfills, regulated by Council Decision No. 2003/33/EC. Two different mine tailings were analysed, namely from Panasqueira and Torre de Moncorvo mines. These mine tailings presented a higher heavy metal content and acidic pH, but only Panasqueira Mine Tailings showed high heavy metal concentrations in leachate. The mixture of 90:10 of Bottom Ash and Torre de Moncorvo Mine Tailings did not reveal a relevant pH alteration compared to Bottom Ash alone, while the mixture with Panasqueira Mine tailings resulted in a slight pH decrease. The mobility of heavy metals from both mixtures with mine tailings was reduced as a result of pH increasing when compared to the leachate of the individual mine tailings. Ecotoxicological assessment of leachates from Bottom Ash alone and from both mixtures with mine tailings displayed positive results suggesting that the materials tested are not ecotoxic. Compressibility and shear strength of the material were analysed for compacted specimens. Results from the one-dimensional consolidation tests showed that the stiffness is within the range of granular soils and there were no significant benefits from adding mine tailings in the mixtures. All materials presented small settlements and are insensitive to the presence of water. Shear resistance was also within the values for dense granular soils, presenting great resistance. The addition of mine tailings slightly decreased the resistance of the material but not enough to exclude the hypothesis of using the mixture if the environmental benefits prevail. In general, Bottom Ash and both mixtures presented good mechanical behaviour for application in road construction