Study on performance of Cement Paste Backfill (CPB) with different binder under various situation

Abstract

The mining process involves the removal and recovery of economically valuable minerals from the earth’s crust. The resulting excavations are commonly revived by a process referred to as backfill. The fill performs as both a support system and a working platform; its different roles determine the requirements of its mechanical properties. Cemented Paste Backfill (CPB) is a high-density slurry composed of dewatered tailings, a cementitious binder and processed mine water, which is thickened to obtain a non-settling character for facile pumping into mined cavities resulted from underground mine operations. Over the past few decades, CPB technology has increasingly been used to revive mined cavities in underground mine operations, owing to its low operating cost. It minimises the adversities associated with tailings disposal, and it has higher mechanical performance compared with other backfilling methods. The present study intends to examine various factors influencing CPB mechanical and rheological properties to get a better understanding of backfill design. The experimental program consisted of the combined capacity of ordinary Portland cement (PC), a newly developed slag-blended cement (Minecem, MC), the binder, and fly ash (FA), the additive, as a sustainable solution towards improving the mechanical performance of a copper-gold underground mine CPB system. A series of unconfined compression tests were carried out on various binder and binder + additive mix designs to evaluate the effect of binder and additive contents and curing time on strength, toughness and stiffness of the CPB system. For some PC + FA mix designs, the failure patterns of the tested samples were also investigated using the three-dimensional DIC technique. Furthermore, the rheological properties of CPB slurry are studied with different concentrations Minecem (MC), Portland cement (PC) and fly ash (FA), under various water and temperature condition using the same tailings material as well. A series of rheometer tests (for yield stress) were carried out on different MC + FA and PC + FA designs for evaluating the effects of binder and additive contents, as well as water content, water type and temperature on the rheological properties of CPB slurry. After placing the CPB material in the stope, and during the setting and hardening processes, the weight of CPB stope applies an axial load over the CPB paste, from the upper layer to the bottom layer of the stope, may result in a consolidation of early-aged backfill material in the lower layers which is called self-consolidation. This self-applied load leads to the strength of CPB specimens obtained by coring from in situ backfill stope are 50% to 200% higher than CPB specimens prepared by using conventional plastic molds over the same curing period. The present study intends to investigate the impact of axial applied stress (As) during curing, which represents the various self-consolidation condition, on the mechanical properties of CPB material. Also, the complicated underground situation may lead to delayed drainage or re-saturation by external water after backfilling. The unsaturated condition will happen due to the irrigation and drain down of backfill material over time. A series of triaxial compression test for CPB consist of MC only were carried out on various saturation condition and confining pressure to evaluate the effect of saturation condition on the mechanical properties of the CPB system.