Strength Development and Strain Localization Behavior of Cemented Paste Backfills Using Portland Cement and Fly Ash.

Zhao Zhao,Deng Deng,Taheri Taheri,Soltani Soltani,Karakus Karakus

doi:10.3390/ma12203282

Abstract

This study examines the combined performance of Portland cement (PC), the binder, and fly ash (FA), the additive, towards improving the mechanical performance of the South Australian copper-gold underground mine cemented paste backfill (CPB) system. A series of unconfined compressive strength (UCS) tests were carried out on various mix designs to evaluate the effects of binder and/or additive contents, as well as curing time, on the CPB’s strength, stiffness and toughness. Moreover, the failure patterns of the tested samples were investigated by means of the three-dimensional digital image correlation (DIC) technique. Making use of several virtual extensometers, the state of axial and lateral strain localization was also investigated in the pre- and post-peak regimes. The greater the PC content and/or the longer the curing period, the higher the developed strength, stiffness and toughness. The use of FA alongside PC led to further strength and stiffness improvements by way of inducing secondary pozzolanic reactions. Common strength criteria for CPBs were considered to assess the applicability of the tested mix designs; with regards to stope stability, 4% PC + 3% FA was found to satisfy the minimum 700 kPa threshold, and thus was deemed as the optimum choice. As opposed to external measurement devices, the DIC technique was found to provide strain measurements free from bedding errors. The developed field of axial and lateral strains indicated that strain localization initiates in the pre-peak regime at around 80% of the UCS. The greater the PC (or PC + FA) content, and more importantly the longer the curing period, the closer the axial stress level required to initiate localization to the UCS, thus emulating the failure mechanism of quasi-brittle materials such as rock and concrete. Finally, with an increase in curing time, the difference between strain values at the localized and non-localized zones became less significant in the pre-peak regime and more pronounced in the post-peak regime.

Highlights

Mine tailings are among the largest and most problematic sources of solid waste, owing to their extensive production, durability over time, and potential health hazards—approximately 14 billion tons of tailings were produced globally by the mining industry in 2010 [1]
The liquid limit (LL)—as determined for 20-mm cone penetration depth using the 80 g–30◦ fall-cone device—and standard thread-rolling plastic limit (PL) were measured as LL = 19.2% and PL = 13.1%, respectively; a plasticity index (PI = LL − PL) of 6.1% was produced, such that the fines fraction of the tailings was classified as clay–silt with low plasticity (CL–ML) in accordance with the Unified Soil Classification
The axial strain at failure—an indication of the material’s ductility—demonstrated a trend similar to that observed for strength and stiffness; in an adverse manner

Summary

Introduction

Mine tailings are among the largest and most problematic sources of solid waste, owing to their extensive production, durability over time, and potential health hazards—approximately 14 billion tons of tailings were produced globally by the mining industry in 2010 [1]. A CLSM can be defined as a high-density slurry composed of soil (mainly sands and/or silts), a cementitious binder (mainly Portland cement), and water; depending on the intended application, the slurry may be thickened to obtain a desired rheological behavior to accommodate facile pumping and effective field implementation. In this context, recent studies have reported innovative solutions to utilize mine tailings as an “additive” in the production of cement clinker, concrete and ceramic products; such approaches, which are becoming routine in practice, have promoted the sustainability of the mining industry [3,4,5]. Mine tailings prepared as CLSMs are self-compacting and flowable in character, and can be employed as a sustainable replacement for conventional structural fillings (e.g., backfilling of mined voids, bridge abutments, pipeline beddings and subbases in pavements) [6,7]

Methods

Results

Conclusion