Cemented Paste Backfill Hydraulic Conductivity Evolution from 30 Minutes to 1 Week

The first one to four days constitutes a critical period in filling mining voids with cemented paste backfill (CPB, a mixture of mine tailings, binder, and water) and determining the saturated hydraulic conductivity (k_sat) during this period is important to rational engineering design. However, most published studies started testing 24 h after specimen preparation, and those that started earlier did so by preconsolidating the specimens, which results in void ratios lower than those occurring in the field. This study uses a new test method that retains the backfill’s representative bulk properties and starts testing about one-half hour after specimen preparation. During backfilling, CPB undergoes three distinct k_sat stages: a relatively constant high-value k_sat stage associated with the portland cement’s (PC’s) dissolution phase; a rapidly declining k_sat stage associated with PC’s acceleration phase; and then a slowly declining k_sat stage associated with PC’s steady-stage and subsequent deceleration phase. The last two stages can be represented by modified Kozeny-Carman equations, in which the hydration effects are constant but different in each stage, and the k_sat changes can be related to void ratio changes in each stage. In contrast, the distinct stages are not adequately represented by traditional PC “maturity” models, and, more importantly, the k_sat values determined here are at least an order of magnitude higher than previously published results on similar materials.