Application of Portland cement to control acid mine drainage generation from waste rocks

Cement has been little investigated as a control for acid mine drainage, despite having advantages over cheaper alternatives. Leaching column experiments (set up in triplicate) investigated the effect of applying Portland cement slurries with three different water/cement (w/c) ratios (0.8, 1.0, 1.2) to pyrite-bearing waste rock from Brukunga mine in southeastern Australia. After application of the cement, leachate from the cemented columns showed a reduction in acidity compared to the control columns of ∼85% and 100% for w/c ratios of 0.8 and both 1.0 and 1.2 respectively. This was due to direct neutralisation of acidity by cement dissolution and encapsulation of the pyrite-bearing rock fragments by the cement (as shown by a reduction in oxygen consumption rates of up to 70%). In columns with the higher w/c ratios (1.0, 1.2), the cement slurry penetrated to the base of the columns and greatly decreased the permeability. The greater coverage of the waste rock by the cement and increased contact time between the leachate and cement meant that no acid drainage leached from these columns (pH 7–8, Fe, Al, Mn, Zn and Cu concentrations rarely above detection limits). In columns with a w/c ratio of 0.8, the cement slurry was mostly retained in the upper 30–50% of the waste rock, leaving a large fraction of the waste rock below without any cement coverage. Leachate drained relatively quickly from these columns, probably through a small number of pathways through the cement plug and carried relatively small loads of acidity and metals from uncemented waste rock beneath the cement. Thus cement may be a viable method for controlling acid mine drainage generation from waste rock dumps, and could be applied with both lower and higher w/c ratios could be used to achieve an optimal balance between deep penetration and surficial retention.