Effect of crushed concrete waste on leaching behaviour of alum shale: A layered column study

Leaching from alum shale waste rock, particularly acid rock drainage (ARD), poses a significant environmental risk due to oxidation of sulphide minerals, which generates acidic leachates enriched in heavy metals, sulphate, and uranium. Simultaneously, crushed concrete represents a large waste stream with strong alkaline properties and high potential for acid neutralisation. In this study, we quantify the effect of crushed concrete waste on leaching behaviour of alum shale using layered column experiments with crushed concrete and two types of alum shale waste rock, to which de-ionised water was added. To account for the possibility that the alum shale would not become acid-generating during the experiment, ARD (pH 2.7) was added directly to additional columns containing crushed concrete. Weekly wetting and drying cycles were repeated over approximately 3 months. Results show that all alum shale columns containing concrete maintained highly alkaline leachate (pH ∼ 12.0-12.7), effectively neutralising acidity and reducing leaching of U, Al, Fe, SO₄^2-, and trace metals such as Mn, Zn, Ni, and Cu. Notably, U concentrations decreased by > 99 % in all concretetreatments compared with controls, despite pH and carbonate conditions that would normally promote U mobilisation. The U immobilisation is likely due to sorption to cementitious phases or co-precipitation with Ca-bearing minerals. However, leaching of Cr(VI) increased in the presence of SO₄^2-, suggesting release from ettringite via anion exchange. The results indicate that crushed concrete waste has potential as a low-cost, passive treatment option to mitigate leaching from alum shale, although increased Cr(VI) mobilisation must be considered.