Effects of curing time, cement content, and saturation state on mode-I fracture toughness of cemented paste backfill

Highlights

• Mode-I fracture toughness shows a strongly time-dependent and nonlinear evolution.

• A larger chemical shrinkage improves the mode-I fracture toughness.

• A vacuum-based rapid re-saturation approach was proposed.

• Re-saturated CPB has a lower mode-I fracture toughness.

Abstract

Due to the wide existence of tensile stresses in cemented paste backfill (CPB) mode-I fracture toughness (K_IC) plays a crucial role in the mechanical stability of CPB under complex field loading conditions. However, the studies on K_IC of CPB materials and associated influential factors are very limited. Hence, the main objective of this study is to experimentally investigate the effects of curing time (3, 7, 28, and 90 days), cement content (2%, 4.5%, and 7%) and saturation state (unsaturated and saturated states) on the evolution of K_IC through single-edge-notch bending tests on CPB specimens. The obtained results show that K_IC displays a strongly time-dependent and nonlinear evolution with curing time, especially during the early age. Moreover, the development of K_IC is sensitive to changes in cement content, which can be attributed to the precipitation of cement hydration products as well as the self-densification process in CPB. In addition, through the vacuum-based rapid re-saturation approach proposed in this study, it has been concluded that the re-saturated CPB has a lower K_IC compared with the CPB without re-saturation treatment and thus confirm the dependence of K_IC on the saturation state.

Introduction

Cemented paste backfill (CPB, a mixture of waste tailings, hydraulic binder, and mixing water) is used more commonly around the world in underground mines over the last decade [1], [2], [3]. After placed into underground excavations (called stopes), CPB is required to provide secondary ground support to prevent roof subsidence [4] and ensure the safety of underground mining workers [5]. Therefore, the mechanical stability of CPB has been considered as a key design criterion of CPB structures. Correspondingly, the material strength-based design approach (MS-DA) has been widely adopted for the design of CPB structure. However, the traditional MS-DA assumes the intact construction materials exist before the application of field static and dynamic loadings [6], which may result in the unsafe design. Specifically, stopes resulted from the production blasting possess rough rock walls [7]. The rough rock walls own sharp convex and concave wedges along their surfaces at both microscale and macroscale. Therefore, after placement into a stope, the surface topography of soft CPB materials is governed by the rough rock walls and floors and thus may result in randomly distributed micro- and macro-notches along CPB surface. Consequently, when the mining activities proceed to the next level beneath the filled stope through underhand cut and fill method, CPB mass with pre-notched surface are exposed and the tensile stress develops at the bottom of CPB sill beams above the current mining level. Consequently, tensile cracking is the most critical failure type of CPB sill beam [8], [9]. As a result, the stress concentration takes place near the notch tips. When the nominal stress acting on CPB is less than its design strength, the stress intensity factor may reach its critical value (i.e., critical stress intensity factor) and thus causes the crack initiation, propagation and coalescence inside CPB mass. Therefore, the failure of CPB mass at macroscale can take place when the nominal stress is less than the design strength, which indicates the unsafe design through the conventional MS-DA.

Therefore, to yield a safe design for CPB structure, the fracture mechanics-based method is needed. Correspondingly, as the key fracture mechanics property, the fracture toughness is required to be determined. Due to the adopted mining methods and complex field loading conditions, tensile stress widely exists in CPB mass. For example, the recovery of adjacent ore pillars causes the exposure of CPB wall faces and thus dramatically reduces the minor principal stress in the vicinity of exposed CPB surfaces. Based on the studies on geomaterials [10], [11], it has been confirmed that, under the polyaxial stress state with a low minor principal stress close to zero, the tensile cracks (i.e., surface parallel cracks) are generated in the orientation parallel to the major and intermediate principal stresses due to the Poisson effect. Due to the wide existence of tensile stress in CPB subjected to field loading conditions [12], mode-I (opening) fracture toughness plays a key role in the analysis of mechanical stability of CPB structure. As a type of cementitious materials, the cement hydration contributes directly to the development of cohesive strength component [13]. Therefore, CPB demonstrates strongly time-dependent behavior [14]. Consequently, as the key factors associated with cement hydration, the effect of curing time and cement content on the fracture toughness are required to be identified. Moreover, the pore water is consumed during the cement hydration [15], and thus results in the transition of CPB from a fully saturated state to an unsaturated state (i.e., self-desiccation process) [16]. Correspondingly, the development of matric suction affects the effective stress carried by the solid phase [17] and thus the resistance to fracture initiation and propagation in CPB. Hence, the curing time, cement content and saturation state play crucial roles in the development of fracture toughness in CPB.

Based on the literature review, only one study [12] was conducted to investigate the fracture behavior of CPB subjected to high-temperature heat treatment. Xu and Cao [12] found that high-temperature thermal treatment can considerably reduce Mode-I fracture toughness of CPB. However, no studies have been conducted to systematically investigate the evolution of Mode-I fracture toughness and associated factors including curing time, cement content and saturation state. Therefore, the main objective of the present study is to experimentally investigate the effects of curing time (3, 7, 28, and 90 days), cement content (C_c = 2, 4.5, and 7%), and saturation state (saturated and unsaturated state) on the evolution of mode-I fracture toughness of CPB through single-edge-notch bending (SENB) tests. To obtain the re-saturated CPB specimen in a short time and thus eliminate the effect of re-saturation process on the curing time, a vacuum-based rapid re-saturation approach was developed. Moreover, to interpret the mechanisms responsible for the evolution of mode-I fracture toughness, the change of matric suction and electrical conductivity in CPB was measured through a mold-based monitoring program. In addition, a series of auxiliary analysis including measurement of chemical shrinkage, scanning electron microscope (SEM) observation, and dry density was conducted in this study to further analyze the changes of CPB matrix structure at the micro- and macro-scale.