Mechanical properties and damage evolution law of cemented-gangue−fly-ash backfill modified with different contents of recycled steel fibers

Abstract

The cemented-gangue −fly-ash backfill (CGFB) prepared from coal-based solid waste materials commonly exhibits high brittleness, leading to an increased susceptibility to cracking. Uniaxial compressive strength (UCS), acoustic emission (AE), and scanning electron microscopy tests were conducted on CGFB samples with recycled steel fiber (RSF) contents of 0, 0.5%, 1.0% and 1.5% to assess the mechanical properties and damage evolution law of the CGFB. The research findings indicate that: 1) When RSF contents were 0.5%, 1%, and 1.5%, respectively, compared to samples without RSF, the UCS decreased by 3.86%, 6.76%, and 15.59%, while toughness increased by 69%, 98%, and 123%; 2) The addition of RSFs reduced the post-peak stress energy activity and increased the fluctuations in the b-value; 3) As the RSF dosage increased from 0 to 1.5%, the per unit dissipated strain energy increased from 5.84 to 21.51, and the post-peak released energy increased from 15.07 to 33.76, indicating that the external energy required for the CGFB sample to fail increased; 4) The hydration products, such as C-S-H gel, ettringite, and micro-particle materials, were embedded in the damaged areas of the RSFs, increasing the frictional force at the interface between the RSF and CGFB matrix. The shape variability of the RSFs caused interlocking between the RSFs and the matrix. Both mechanisms strengthened the bridging effect of the RSFs in the CGFB, thereby improving the damage resistance capability of CGFB. The excellent damage resistance occurred at an RSF content of 0.5%; thus, this content is recommended for engineering applications.

1. Introduction

Using coal gangue (CG), fly ash (FA), and other coal-based solid waste to prepare underground cemented-gangue−fly-ash backfill (CGFB) for backfilling goaf is an important approach to address the ecological environmental problems such as surface subsidence and underground water resource loss faced by coal mining, as well as the rapid consumption of solid wastes [1, 2]. To improve backfilling efficiency and enable long-term utilization of underground space, a constructional backfill technique employing CGFB-made point columns and strip columns to control the roof structure is proposed, as illustrated in Figure 1 [3]. However, CGFB generally exhibits unfavorable mechanical properties [4], such as high brittleness and susceptibility to cracking. These properties cause sudden fractures and instability of CGFB in the goaf under the pressure of the overlying strata, posing a safety threat in underground mines [5, 6]. Therefore, reducing the brittleness of CGFB and improving its damage resistance are required to ensure the CGFB’s stability.

Currently, research on improving the brittleness of backfill is mostly focused on cemented tailings backfill (CTB), with the primary measure being the addition of fibers. ZHAO et al [7] utilized uniaxial compressive strength (UCS) tests and acoustic emission (AE) monitoring to investigate the mechanical properties, AE characteristics, and the precursor damage effects of polypropylene fiber-reinforced CTB with different ash-sand ratios. YIN et al [8] conducted UCS tests to study the mechanical properties, failure modes, and damage evolution of sulfur-containing CTB reinforced with polypropylene fibers. SUN et al [9] characterized and optimized the mechanical properties of fiber-reinforced slag CTB using response surface analysis. XUE et al [10] ex...