Rheology and setting time of saline cemented paste backfill

As many mines around the world are situated in freshwater-scarce regions and corporations are challenged to meet increasing sustainability and environmental responsibility standards, many mining operations are seeking to use locally available saline groundwater or ocean water as mixing water in cemented backfill. However, the implications of this decision on the rheological properties (which control the flow ability or transportability) and setting time of cemented backfill must be better comprehended to allow for a confident selection of this practical solution. Hence, the effects of mixing water salinity and binder type on the rheological properties and setting time of cement paste backfill (CPB) were investigated. Viscosity, yield stress, and setting time testing were conducted on CPB made with Portland cement type I (PC) and varying slag replacement quantities, silica tailings (ST), and mixing water with NaCl concentrations from 0 g/L to 300 g/L. Supplemental testing and analyses included pH determination, TG/DTG and XRD microstructure analyses, zeta potential measurements, and electrical conductivity monitoring. Results indicate that low NaCl concentrations (10 g/L − 35 g/L) in PC-CPB increased the yield stress above the control while higher concentrations (100 g/L − 300 g/L) decreased it. An increase in NaCl concentration led to slower increase in viscosity over the first two hours of curing. Low NaCl concentrations reduced the time to initial and final setting while high concentrations increased it. These behaviors were mainly due to the effects of the Na and Cl ions on the dissolution of the cement components, and on the changes in zeta potential due to chloride adsorption. Increases in slag replacement quantities on saline CPB had minimal effect on the yield stress during the first two hours after mixing but decreased the viscosity. All tested slag contents increased the time to initial setting, though a replacement content of 75% did not increase the initial setting time as much as lower quantities. A slag replacement content of 25% increased the final setting time, though higher contents decreased it. This behavior was due to the slower reaction rate of slag, combined with slag’s consumption of NaCl and CH and greater chloride binding capabilities. The results of this study will aid mines in designing more cost-effective and efficient backfill programs.