Analysis of internal structure behavior of fiber reinforced cement-tailings matrix composites through X-ray computed tomography
Introduction
The mining sector plays a vital role in the production of goods, services and infrastructure that ensure the sustainable development of countries [1]. However, mining can create large amounts of tailings and waste rocks [2]. Mine tailings comprise ground-up rock, water and chemical residues that remain after valuable minerals have been removed from the run-of-mine ore [3]. These tailings need to be safely stored to prevent the damage to the environment from leaching, land instability, and other hazards [3,4]. With the introduction of the corresponding national policies and continuous improvement of people's awareness of environmental protection, filling mining method has been widely and intensively used in the global mining industry because of its advantages in controlling ground pressure, realizing waste-free mining, and efficiently utilizing mineral resources [[5], [6], [7], [8]]. Mine backfill technology plays a significant role in environmental protection and the prevention of water resource pollution [9]. In addition, backfill can realize several roles at underground mines and used as a construction material, a ground support tool, and a tailings disposal technique [10].
Cement-tailings matrix composites (CTMC) or cemented paste backfill (CPB) has been widely accepted by modern mines as a preferred mine backfill technique mainly due to reduced bulkhead failure risk, faster backfilling rates, increased ore recovery, and improved flow conditions in the pipeline [[11], [12], [13]]. Many laboratory and field researches have been so far conducted to study physical [14], chemical [[15], [16], [17], [18]], and mechanical properties [[19], [20], [21], [22]] of CTMC, using special techniques and innovative experimental tools. The pore or internal structure of the backfill is also a subject of great interest in relation to its strength and stability. Nevertheless, the actual pore assembly inside of a cementitious matrix is not yet completely understood since there are still the uncertainties linked with the lack of a universal definition of the primary structural elements, such as apparent porosity of C–S–H gels [23,24]. Despite these difficulties, several works have already been undertaken on microstructural properties of CTMC [[25], [26], [27]]. Mineral and chemical additives are also added to the backfill mixtures for improving the internal structure of CTMC [28,29]. Internal micro-cracks can cause the failure of CTMC samples over curing time. Addition of fibers prevents the failure of backfill by mobilizing tensile strengths along the failure planes [30].
In the research field of traditional cement-based composites, different kinds of fibers have been successfully used to effectively control crack propagation and thus improve its strength, toughness, ductility, and crack resistance [[31], [32], [33], [34]]. The commonly used fiber types include polypropylene fiber, carbon fiber, and steel fiber [35,36]. Fiber reinforced CTMC comprises tailings, cementation, fiber, and water, and it is a multiphase composite material with a complex structure [37]. As the tailings, concrete and soil feature different physical structures and composition, their particles are fine and less active. Consequently, the limited research is available regarding the effect of fiber on physical behavior of cemented fill [38,39]. Given the different initial defects, such as pores and microcracks, in the internal microstructure of CTMC samples [40,41], blending of fibers affects the uniformity and continuity of the microscopic structure of backfilling. In the course of crack evolution, internal pore defects contribute considerably to the crack development, and when subjected to external loadings, macroscopic deformation is realized by adjusting internal microstructural parameters [42,43]. Thus, all the above factors greatly affect the working performance and macroscopic mechanical behavior of CTMC samples. It is also necessary to investigate the intrinsic relationship between the macroscopic and micromechanical behavior of CTMC from the perspective of meso-structure.
Nowadays, three methods are widely used to study the meso-structure of rock and earth mass: X-ray computed tomography (CT) scanning technology [[44], [45], [46]], acoustic emission (AE) technology [[47], [48], [49]], and nuclear magnetic resonance (NMR) technology [[50], [51], [52]]. Among these methods, X-ray CT scanning technology, a non-invasive, nondestructive imaging technology, is lengthily used for detecting the internal structure of materials and reconstructs high-resolution images [[53], [54], [55]]. Indeed, X-ray CT technology has been well used for many years in the field of medicine and geotechnical research [56,57]. However, the research on cemented backfill is rarely reported in the literature. To reveal the mechanism of the internal failure, Yi et al. [58] examined the crack growth within CTMC samples with and without fiber in the compressive strength test by X-ray CT. Besides, the images captured using the X-ray CT scanning technology can also be converted into numerical models or 3D images by various image analysis software for examining the internal structure of cementitious materials more quantitatively and visually [59,60]. Hence, X-ray CT technique is an operative tool to explore changes in the internal structure of cement-based materials.
The originality of this paper consists in the evaluation of the effect of fiber types and dosages on mechanical and microstructural properties of CTMC samples through X-ray CT technology and scanning electron microscopy (SEM) tests before and after uniaxial compression. The macroscopic mechanical characteristics embodied in uniaxial compression are analyzed and combined with 2D image processing software and 3D reconstruction technology, the internal structure of CTMC with different fiber types and dosages were studied. To provide intentional reference values for research and application of fiber reinforced CTMC samples, relationship between the internal structure and mechanical strength properties was also analyzed from the mesoscopic point of view.
Section snippets
Material characterization
The raw materials used in this study included tailings (without removal of fine contents) sampled from a gold mine located in Shandong (China), ordinary Portland cement 42.5R (‘R’ stands for the high-strength cement with a strength acquisition of 22 and 42.5 MPa after curing of 3 and 28 days, respectively) and three different types of fibers. The particle size distribution (PSD) analysis (Fig. 1) showed that approximately 27% of the tailings sample is finer than 20 μm, with only 5.3% clay-sized
Effect of fiber type and amount on backfill strength
Table 3 shows results of the uniaxial compressive strength testing performed on CTMC samples subjected to X-ray CT scan. According to Table 3, the compressive strength gain of fiber-reinforced CTMC samples is higher than that of non-reinforced CTMC sample N-1:6, indicating that the three different types of fibers have a certain strengthening effect on CTMC samples, which restricts the expansion of the crack and improves its uniaxial compressive strength. Compared to non-reinforced CTMC sample
Conclusions
This investigation examined the effects of fiber types and dosages on mechanical properties and internal structure of CTMC samples by studying the changes in pore space using X-ray CT scanning technology and SEM techniques. Through SEM and X-ray CT tests, combined with the 2D image processing software, 3D reconstruction model technology was used to better study the relationship between internal microscopic structure and macroscopic mechanical properties. From the laboratory tests performed, the
Acknowledgements
This research was financially supported by grants from China Postdoctoral Science Foundation (No. 2018M631341), Open Fund of the Key Laboratory of Ministry of Education for Efficient Mining and Safety of Metal Mines (No. USTBMSLAB2018049), National Natural Science Foundation of China (No. 51804017), and Fundamental Research Funds for Central Universities (No. FRF-TP-17-075A1). The experimental works described in this study was conducted at the Key Laboratory of High-Efficient Mining and Safety
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