Alkali-Activated Paste Backfills for Underground Mine Cavities: Insights into Strength, Shrinkage, Flow, Setting, and Environmental Sustainability Perspectives

This article presents a comparative study on the utilization of three different mine tailings viz., red mud (RM), iron tailings (FeT), and zinc tailings (ZT) as paste backfill for underground mine cavities through alkali activation. A series of comprehensive laboratory experiments are conducted to examine the unconfined compressive strength (UCS), drying shrinkage (Vs), flowability, and setting characteristics of various alkali-activated paste backfills (AAPBs). While doing so, the effect of varying water to solid ratios, curing period, and the alkali activator concentration are investigated. To further improve the strength, shrinkage, and setting performance of AAPBs, granulated blast furnace slag (GBFS) is used as a precursor to accelerate the geopolymerization reaction. In addition, cemented paste backfills (CPBs) are also prepared as a reference mix using portland pozzolana cement (grade 43). Furthermore, the geoenvironmental implications of synthesized AAPBs are assessed through an environmental sustainability analysis, which includes evaluating heavy metals leachate by the toxicity characteristic leaching procedure, measuring CO2 emissions, and analyzing energy consumption. The experimental study demonstrates that the 28-days UCS in AAPBs derived from RM, FeT, and ZT can reach up to 4.26 MPa, 2.80 MPa, and 3.54 MPa, respectively, as compared to 0.78 MPa, 1.25 MPa, and 1.78 MPa in CPB equivalents. Furthermore, it is found that the addition of GBFS not only enhances the UCS but also reduces the Vs and setting time of AAPBs which is beneficial in mitigating the risk of ground movement and surface subsidence. Among all paste backfills, ZT-derived AAPBs requires the least activator solution to achieve the optimal flow characteristics (required for pipe transport) which makes them less expensive than FeT- and RM-based AAPBss. The findings of the environmental sustainability analysis reveal that the concentrations of heavy metals leached from various AAPBs are well within the threshold limits set by the USEPA. Furthermore, it is found that the production of AAPBs reduces the CO2 emissions by 46%–82% and energy consumption by 18%–78% when compared to traditional CPBs. Overall, the current experimental study successfully demonstrates the potential of AAPBs as a substitute to conventional CPBs for mine backfilling operations which could help in reducing the environmental impact associated with CPBs.