Self-Healing Ability and Behaviour of Cemented Paste Backfill

Abstract

Cemented paste backfill (CPB), as an innovative cementitious material, has been extensively employed to cost-effectively manage mine wastes, ensure workplace safety, and improve mine productivity in the mining industry from a sustainable perspective. CPB is an engineered mixture typically consisting of dewatered tailings (70-85 wt.%), hydraulic binder (3-7 wt.%), and water to achieve a homogeneous paste. It is usually prepared in a plant located at the surface of a mine site and transported to refill the underground mined-out stopes and voids.

CPBs are designed to satisfy an adequate load-bearing capacity for safe mining operations. The primary geotechnical performance criterion of CPB is mechanical stability, which ensures resistance against deformation and prevents failure, thereby stabilizing surrounding rock masses. In parallel, the low permeability of CPB, as an essential environmental design criterion, plays a pivotal role in ensuring structural stability and long-term durability by minimizing the migration of aggressive chemicals or contaminants that could otherwise weaken the CPB structure and pollute groundwater systems. Upon placement, both mechanical and permeability properties are governed by complex multiphysical processes, including thermal (T), hydraulic (H), mechanical (M), and chemical (C) processes. However, cracks may initiate in the CPB matrix as a result of various factors, such as shrinkage, sulphate attack, initial structural defects, excessive overburden pressure, stresses induced by surrounding rocks and ground movement, rock bursts, or combined effects of these conditions, during the curing stage under the interaction of the multiphysical processes. The progressive generation and propagation of cracks can severely deteriorate the integrity of the CPB matrix, impairing its mechanical stability, environmental performance, and serviceability. Moreover, CPB structures often extend tens to hundreds of meters underground in at least one dimension, which makes manual maintenance and repair of cracks in CPB structures infeasible in practical manners. Given that, self-healing in CPB has been proposed as a promising strategy to mitigate crack-induced deterioration. Yet, existing studies are scarce, focusing primarily on autonomous self-healing with externally added agents, while the intrinsic autogenous self-healing behaviour of CPB remains unexplored. Furthermore, the effects of different factors (e.g., multiphysical THMC factors) on the autogenous self-healing capacity and performance of CPBs have not been comprehensively evaluated, presenting a critical research gap.

This Ph.D. study addresses this gap through a series of systematic experiments investigating the autogenous self-healing behaviour of CPB under a wide range of factors/conditions, including age of cracking, pre-cracking level, crack width, self-healing period, thermal (e.g., healing/curing temperature), hydraulic (e.g., drainage condition), mechanical (e.g., different crack-inducing stresses), chemical (e.g., sulphate content), as well as addition of mineral additives (e.g., blast furnace slag and fly ash). Self-healing efficiency was evaluated based on crack closure observations, recovery of mechanical properties (e.g., uniaxial compressive strength, deviator stress, indirect tensile strength), recovery of permeability (e.g., hydraulic conductivity), changes in physical properties (e.g., porosity, void ratio), and characterization of self-healing products. Results demonstrate that CPB exhibits a promising autogenous self-healing capability, which is mainly attributed to the precipitation of self-healing products, primarily consisting of C-S-H, CaCO3, Ca(OH)2, ettringite, and/or gypsum (under sulphate exposure). The relative proportions of these products vary considerably under different self-healing conditions. Both the age of cracking and the self-healing period significantly influence the self-healing efficiency of CPBs. The initiated cracks within the CPB matrix can ameliorate the hydration reactions, favouring the self-healing performance. Elevated curing temperatures (e.g., 35 °C and 50 °C) significantly accelerate the self-healing process via enhanced binder hydration, whereas low temperatures (e.g., 2 °C) exhibit negligible self-healing performance. Internal sulphate exposure exerts either positive or negative effects depending on sulphate concentration and self-healing duration. Improved drainage enhances self-healing performance through the combined effects of increased hydration and microstructural refinement. In the same way, shear cracks generated under confinement and tensile cracks with small apertures show favourable healing performance due to advantageous crack geometry within the matrix. Moreover, the impacts of incorporating mineral additives (e.g., blast furnace slag and fly ash) on self-healing performance are reflected in their contributions to binder hydration mechanisms and associated microstructural modifications. To validate and extend these findings, natural mine tailings with diverse mineralogical compositions were also tested under site-specific CPB formulations.

The findings of this research provide fundamental insights into the autogenous self-healing mechanisms of CPB, with significant implications for improving structural design, mechanical stability, permeability, durability, and environmental performance under field-relevant conditions. This work also demonstrates a comprehensive scientific basis for linking laboratory observations to engineering practice and for advancing the long-term sustainability of CPB systems in underground mining.