Abstract
After placing the Cement Paste Backfill (CPB) slurry in mined cavities underground, during the setting and hardening processes, the weight and hydrostatic pressure of the upper-layer CPB slurry applies an axial load over the bottom-layer CPB materials, which is called the self-consolidation of CPB slurry. Due to this phenomenon, the mechanical properties of in situ CPB could be considerably different from laboratory results. Hence, it is crucial to understand the effect of self-consolidation behaviour on the mechanical properties of backfill material. This paper presents an experimental study on the impact of axial applied stress (As) during curing, which represents the various self-consolidation conditions and curing times on the mechanical properties of CPB material prepared using the tailings of a copper mine in South Australia and a newly released commercially manufactured cement called Minecem (MC). A curing under pressure apparatus (CPA) is designed to cure CPB samples under axial applied stress. The equipment can apply and measure axial load during curing and measure the passive lateral stress due to axial load which represents the horizontal stresses at a certain depth of CPB stope on the retaining structure. The prepared samples with axially applied pressure during curing were tested under uniaxial and triaxial compressive loading conditions. Microstructural tests by scanning electron microscopy (SEM) were also used to study the fabric evolution in response to various applied stresses during curing. Overall, the increase in As during curing leads to higher resultant CPB peak strength and stiffness under uniaxial and triaxial compression tests. For instance, a sample cured under 3.6 MPa axial load for 28 days demonstrates a uniaxial compressive strength (UCS) value of five times more than a sample cured under atmospheric curing conditions. Passive lateral stress was measured during the curing period and was representative of underground barricade stress. Furthermore, during curing, the axial applied stress changed the initial CPB pore structure after placement. With the increase in applied stress, the stress compressed CPB samples at the macroscale, leading to much smaller pores or cracks prior to the hydration process. At an early stage, the increase in UCS due to axial applied stress mainly arises from a dense microstructure caused by the compression of tailings and cement particles. With the increase in curing time, the observation also shows that a CPB matrix with fewer pore spaces may improve the hydration progress; hence, the influence of axial applied stress becomes more pronounced in long-term UCS.
Highlights
The mining industry generates a large quantity of tailings every year during the extraction of valuable minerals from ore [1,2]
Uniaxial compressive strength (UCS) is one of the major indexes used to evaluate the mechanical performance of Cement Paste Backfill (CPB)
With the same 7-day curing time—AaT7, where a = {0, 1.2, 2.4, 3.6}—while the applied stress increased, the uniaxial compressive strength (UCS) resulted in 476.5 KPa, 659.1 kPa, 751.3 kPa, and 1097.5 kPa, respectively
Summary
The mining industry generates a large quantity of tailings every year during the extraction of valuable minerals from ore [1,2]. Compared with other backfill methods, i.e., rock fills and hydraulic fills, the paste backfill has higher mechanical performance, lower operating costs, and the benefit of disposing tailings underground [13,14,15,16] For these reasons, CPB has been increasingly applied worldwide over the past few decades to fill underground cavities. The maximum applied pressure during curing was limited to 400 kPa. A case study on in situ measurements of cemented paste backfill shows that for a height difference of 4 m, the peak vertical stress measured could be as high as 200 kPa [32]. It is essential to understand how the passive lateral stress occurred while applying axial stress during the curing of CPB samples This present study investigates the influence of various axially applied pressures using a newly developed slag-blended cement for three different curing times in confined and unconfined loading conditions. The chemical composition mainly consists of silicon dioxide (SiO2) and iron oxide (Fe2O3), with mass fractions of 38.27% and 37.70%, respectively
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