The efficiency of three slurries was tested for treating of acidity caused by pyrite weathering. Slurries of BauxsolTM, powdered limestone, and brucite [Mg(OH)2] were sprayed onto 10-ton masses of pyritic aggregate, and the acidity and sulfate concentrations of outflows were monitored for 21.5 months. After addition in identical procedures, neutralization of the outflow to a pH of above 6 was achieved by Bauxsol for about 1 day, by limestone for 4 days, and by Mg(OH)2 for the 652 days of the testing. Limestone control of pH was limited by armoring of the CaCO3 by gypsum [CaSO4·2H2O] and by restricting of water flow to channels so that most of this slurry remained unreacted. Channels were not evident in the Mg(OH)2-treated aggregate. With the limestone and Mg(OH)2 slurries, sulfate concentrations were controlled by crystallization of gypsum, or at higher concentration of sulfate by hexahydrite [MgSO4⋅6H2O]. The limestone slurry limited sulfate concentration to approximately 25 percent less than that occurring with the Mg(OH)2 slurry. An ideal slurry might include Mg(OH)2 for neutralization, limestone for limiting sulfate concentration, and other components to alter rheological behavior. Thixotropic behavior that favors initial dispersion of the slurry solids yet reduces loss in outwash and also limits channelization of water flow is best.