Mechanical properties of cemented tailings backfill with chloride-free antifreeze

In cold regions with sub-zero surface temperatures, the addition of chloride-free antifreeze (Ca(NO₂)₂, Ca(NO₃)₂, and CO(NH₂)₂) is an inexpensive method to prevent pipeline freezing during cemented tailings backfill (CTB) transport. However, the curing temperature of CTB after reaching the mine cavity tends to be above-zero. The mechanical properties of CTB with a chloride-free antifreeze in above-zero environments have not been investigated. Therefore, this study aimed at thoroughly exploring the effect of chloride-free antifreeze on the mechanical properties of CTB in above-zero environments. CTB samples with chloride-free antifreeze (Ca(NO₂)₂, Ca(NO₃)₂, and CO(NH₂)₂) and different concentrations (0, 5, 15, and 35 g/L) were prepared and cured in different above-zero environments (2, 20, and 35 °C). The unconfined compressive strength tests were performed after 1, 3, 7, and 28 days. In addition, a series of microstructural analyses and monitoring experiments were conducted. The results indicated that the addition of a 15-g/L chloride-free antifreeze decreased the strength of CTB curing at 20 °C after 1, 3, and 7 days and increased the strength after 28 days. Moreover, the CTB strength evolution with the curing time depends on the chloride-free antifreeze concentration and above-zero curing temperature. According to the TG/DTG analyses results, calcium ions had a promoting effect on the carbonation of calcium hydroxide. The findings of this study can provide a guideline for the application of chloride-free antifreeze on the mine backfill in cold regions.