Applications of Portland cement blended with fly ash and acid mine drainage treatment sludge to control acid mine drainage generation from waste rocks

Portland cement is an effective but expensive option for source control of acid mine drainage (AMD); the cost could be reduced by blending the cement with cheap waste materials such as fly ash from coal combustion and sludge produced from neutralization of acid mine drainage with lime. To test how these two additives affect cement performance in reducing AMD generation, blended cement slurries were applied to sulfidic waste rocks from Brukunga, South Australia, in leaching columns monitored for around 1 year. Acidity, metal and sulfate loads decreased greatly (by 80–95%) for all cement applications but the effect varied, depending mostly on the depth of penetration of the cement in the column (a function of slurry viscosity). The fly ash-blended cements penetrated further into the columns than the more viscous surface-capping sludge-blended cements, causing a greater decrease of leachate acidity loads and sulfide oxidation due to better coverage of the waste rocks and more acidity neutralization by cement dissolution. The AMD sludge showed no evidence of releasing its adsorbed heavy metals, but the fly ash released some Si, indicating that it is not chemically stable in the cement. There was little secondary mineral growth in the blended cements; the ettringite in the AMD sludge-blended cement formed from bassanite present in the sludge rather than exposure to acid mine drainage. Overall analysis of the long-term effectiveness of the blended cement applications shows that cement placed as a surface cap on top of the waste rock provides more value, because the slower cement dissolution rates ensure continued effectiveness for many years. Scaling up the results from the laboratory study to the field scale at the Brukunga mine site suggests that benefit/cost ratios in the order of 20–30 might be achieved compared to the present use of lime neutralization for AMD treatment.