Pilot-scale field experiments were conducted as a strategy to treat and inhibit acid mine waters by addition of an alkaline cover of fly ash waste from coal combustion on a pyrite-bearing mining waste. When both waste materials are deposited together, the metal mobility decreases due to the precipitation of newly-formed phases, which leads to the development of a double physical barrier that inhibits the oxidation process, microencapsulation of the pyrite grains, and hardpan formation at the contact. These mechanisms may inhibit the diffusion of oxidizing agents to the surface of sulfide grains, and hence, provide a long-term solution for acid mine drainage. © 2010 Taylor & Francis Group, London.