Effect of acid mine drainage (AMD) on the alteration of hydrated Portland cement and calcareous sandstone

Abstract

AMD is the acidic discharge generated by the oxidation of sulfide minerals from active or disused mine workings and contains high concentrations of toxic metals and sulfuric acid. Host rocks and concrete-based infrastructures in permanent contact with AMD are highly vulnerable to the geochemical processes caused by acidity (e.g. concrete corrosion). Experiments using columns filled with calcareous sandstone and hydrated Portland cement were performed to quantify these geochemical processes. AMD mono-ionic and poly-ionic solutions containing sulfate, Fe(II), Al, Ca, Mg and different divalent cations (Cd, Zn, Cu and Ni) and As(V) at pH 2 were injected in the columns.The temporal variation of the chemical composition of the output solutions was monitored. The retrieved solid samples were analyzed by X-Ray Diffraction (XRD), thin section petrography and Scanning Electron Microscopy (SEM). A quantitative interpretation of the reactions was carried out by means of 2D reactive transport modeling.Results indicated that the main reactions were the dissolution of portlandite (hydrated Portland cement) and calcite (rock). These two reactions, together with the pH variation, played a pivotal role (1) in the precipitation of gypsum, aragonite, brucite and Fe- and Al- poorly crystalline phases (e.g., basaluminite, schwertmannite and ferrihydrite) and (2) in the precipitation of Cd, Ni and Zn hydroxides and Zn carbonates and adsorption or co-precipitation of Cu and As on schwertmannite and ferrihydrite.