J. Cent. South Univ. (2020) 27: 3531-3543

DOI: https://doi.org/10.1007/s11771-020-4568-7

Strength and deformation behaviors of cemented tailings backfill under triaxial compression

XU Wen-bin(徐文彬)^{1, 2}, LIU Bin(刘斌)¹, WU Wei-lü(吴蔚律)¹

1. School of Energy and Mining Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China;

2. School of Resources and Environment Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: It is of great significance for safety reason to obtain the triaxial compressive properties of cemented tailings backfill(CTB). The influence of cement content, curing age and confining pressure on strength and deformation properties of CTB was examined and discussed. Results indicate that the triaxial compressive and deformation behavior of CTB is strongly affected by the cement content, curing age and confining pressure. The increase in cement content, curing age and confining pressure leads to a change in stress-strain behavior and an increase in the axial strain at failure and post-peak strength loss. The cohesion of CTB rises as the curing age and cement content increase. However, the enhancement in internal friction angle is trivial and negligible. It should be noted that the failure pattern of CTB samples in triaxial compression is mainly along a shear plane, the confining pressure restrains the lateral expansion and the bulging failure pattern is dominantly detected in CTB samples as curing age length and cement content increase. The results will help to better understand the triaxial mechanical and deformation behavior of CTB.

Key words: cemented tailings backfill; triaxial compressive strength; volumetric strain; elastic modulus; cohesion; friction angle

Cite this article as: XU Wen-bin, LIU Bin, WU Wei-lü. Strength and deformation behaviors of cemented tailings backfill under triaxial compression [J]. Journal of Central South University, 2020, 27(12): 3531-3543. DOI: https://doi.org/10.1007/s11771-020-4568-7.

1 Introduction

The exploitation and utilization of mineral resources provide raw materials for the development of industry, but also cause a lot of environmental problems, such as surface subsidence and a large number of solid wastes [1]. Tailings are by-product from ore processing, and most are discharged into the surface tailings impoundment, apart from a small amount for comprehensive utilization in construction. According to statistics, the discharge quantity of tailings has totally exceeded 18 billion tons in China, and increases at a rate of 800 million tons per year [2]. A large mass volume of discharged tailings not only occupies valuable land, but also results in water pollution and damage to the environment of human beings [3-5]. Traditional tailings management in surface storage facilities is defective because of the potential risk of tailings dam break, debris flow and other dam instability [6]. Thus, cemented tailings backfill (CTB), an economic, eco-friendly and safe tailings disposal method, has been progressively utilized by mine managers and scholars around the world [7-9]. CTB is primarily composed of tailings, cement, water and additives. It is well mixed at surface plant with a desired slump before being transported into underground stope through pipeline [10-12]. One of the advantages of CTB is to provide support for secondary stope and improve ground pressure [13]. Therefore, the mechanical strength is particularly important to keep CTB structure stable in field stope. For this reason, numerous researches have already been conducted to study the mechanical behaviors of CTB in different conditions, and plentiful valuable results have been obtained [14]. CAO et al [15] investigated the influence of the cement-tailings ratio and the solid concentration on the uniaxial compressive strength (UCS) of CTB, and found that the UCS rises with the higher cement-tailings (c/t) ratio and solid concentration. XU et al [16] also found that CTB samples under higher temperature for longer time show prominent enhancement in UCS. CAO et al [17, 18] investigated the UCS of CTB was significantly affected by filling times, interval times and layers angle. Other results have revealed that moderate early strength agent and water reducer have positive effect on the UCS improvement of CTB [19-21]. In addition, it was reported that curing temperature and binder have significant impact on the UCS development of CTB [22, 23].

The abovementioned researches mainly focus on the UCS evolution of CTB. In general, triaxial compressive strength (TCS) and UCS are both important strength indices of CTB. Although it is easy and cost saving to obtain UCS, the stress state in underground stope is triaxial after being discharged. Hence, it is of great importance to investigate the triaxial compressive strength and deformation behavior of CTB. So far, few scholars have investigated the mechanical properties of CTB under triaxial compression. FALL et al [24] obtained that the stress-strain behavior of CPB is greatly affected by the confining pressure, curing age, strength and components and the change of the failure mode, stiffness and strength results from the increase of confinement level. CHEN at al [25] indicated that the deformation modes of tailings specimens are affected by different intercalation angles. KUTANAEI et al [26] studied that the elastic modulus, peak strength and initial stiffness of the fiber-reinforced cemented sand in triaxial compression were enhanced with the increase of cement dosage. SIMMS et al [27] found that CPB showed an obvious dilatant behavior at an early age. BELEM et al [28] carried out triaxial compression test on CPB with medium term and long-term curing time, and observed that the internal friction of CPB decreases with the increasing binder content but cohesion increases. Despite researches have investigated the mechanical properties of cemented backfill under different conditions, a better understanding of triaxial compressive strength and deformation behavior of CTB with different cement contents are also worth of further exploration.

In this paper, the triaxial compression tests were executed to study the stress-strain curves, strength parameters and failure modes of CTB with different cement content (2%, 3% and 5%) after 3, 7 and 28 d respectively. The main objectives are to obtain the mechanical properties, axial and volumetric deformation characteristics of CTB samples under triaxial compression (i.e., 50, 100, 150 and 200 kPa confining pressure). Brittleness index of CTB under triaxial compression is defined, and the strength difference coefficient of CTB samples with different cement contents under different confining pressures are investigated. Furthermore, scanning electron microscopy (SEM) tests were conducted to study the effect of curing age and cement content on internal microstructure of CTB samples. The results may be helpful to illustrate the mechanical response of CTB in triaxial compression and provide reference for safe tailings management in mine.

2 Materials and methods

2.1 Test materials

The tailings used in the preparation of CTB samples were taken from an iron mine, Shandong Province, China. The chemical component was measured using X-ray fluorescence spectrometry (XRF), as illustrated in Table 1. The result displays that the tailings are mainly made up of SiO₂ (62.38 wt %), apart from small quantities of Al₂O₃ (3.68 wt %), CaO (5.13 wt %) and MgO (2.93 wt %). The particle size distribution of tailings measured using laser particle size analyzer (S3500, Microtra, America) is presented in Figure 1, which shows that more than 50% of the tailing particles are smaller than 34.3 μm (D₅₀), and the fine particles content (≤20 μm) exceeds 25%, which is beneficial for keeping fresh CTB mixtures homogeneous.

Table 1 Main chemical components (wt%) of tailings

Figure 1 Particle size distribution curves of tailings

Ordinary Portland cement (P.O. 42.5R) with specific gravity of 3.0-3.2 was used to prepare fresh CTB mixtures, with chemical compositions listed in Table 2. It shows that the cement principally consists of CaO (64.78%), SiO₂ (20.34%) and Al₂O₃ (5.02%). Distilled water in the laboratory was used in preparation of CTB samples for avoiding other mixtures effect in water.

2.2 Mix proportions and specimen preparation

To illustrate the effect of different cement contents and curing ages on the mechanical strength, deformation characteristics and failure modes of CTB specimens in triaxial compression, three groups of 36 specimens in total were prepared. The cement content was designed to 2%, 3% and 5% by mass, and the solid concentration was chosen as 75%. The tailings, cement and distilled water were blended evenly in a mixer and then cast into a cylindrical plastic mold. The height of the plastic mold was 80 mm and the diameter was 39.1 mm. Afterwards, the CTB specimens were placed in a chamber with (20±1) °C and 95%±5% relative humidity, and then cured for 3, 7 and 28 d, respectively. The recipe details of all CTB samples are given in Table 3.

2.3 Triaxial test apparatus and instrumentation

Triaxial compressive tests were conducted using a strain-controlled 600 kN-capacity loading machine and confining pressure between 0 and 2 MPa. Test specimen was placed between porous stone caps, and then a latex membrance was installed around the specimen. Every specimen was applied at a deformation rate of 0.5 mm/min during testing process, and the corresponding data were automatically recorded by computer. When conducting triaxial compression, the confining pressure was firstly applied to the designed values, and then axial strain loading was uploaded until CTB specimen failure was complete in accordance with ASTM D4767-11 [29]. In this study, confining pressure was set to 50, 100, 150 and 200 kPa, respectively.

Table 2 Primary chemical compositions of cement (wt%)

Table 3 Preparation details of CTB samples

2.4 Definitions for strength and deformation index

Deviatoric stress (q) is defined by the difference between axial stress (σ_a or σ₁) and radial stress (σ_r or σ₃), as shown in Eq. (1). Volumetric strain (ε_v) was calculated as the sum of the total radial strain (ε_r) and axial strain (ε_α), as shown in Eq. (2).

                              (1)

                             (2)

The brittleness index is used to describe the post-peak strength loss of CTB samples in triaxial phase. As a generate note, an increasing post-peak strength loss will result in increasing brittleness [24, 30]. It is defined as:

                             (3)

where I_b is brittleness index; σ_p is peak deviatoric stress; σ_s is residual deviatoric stress.

The elastic modulus is calculated based on the deviatoric stress-axial strain curves [31]. The initial 1% of the axial strain was applied to compute the elastic modulus of softening materials [32-34]. The elasticity modulus (E) is equal to the increment of deviatoric stress (q_1%) to the increment of axial strain (ε_1%) ratio when the axial strain reached 1%, as shown in Eq. (4).

                                (4)

2.5 Microstructural analysis

Scanning electron microscopy (SEM) was resorted to compare the influence of curing age and cement content on the internal microstructure and morphology of CTB samples. In this test, the texture and size of hydration products in CTB was scanned using a Quanta 250 FEG apparatus with an accelerating voltage of 25 kV and a resolution of 1.0 nm. The samples for SEM tests were dried and coated with conductive carbon powder before testing. The 3 d and 28 d CTB specimens with 5% cement as well as the 28 d CTB specimens with 2% and 5% cement were used for SEM tests to explore the effect of curing age and cement dosage on the mechanical performance of CTB.

3 Results and discussion

3.1 Stress-strain behaviors

3.1.1 Stress-axial strain behavior

The deviatoric stress difference vs axial strain curves of CTB specimens under 50, 100, 150 and 200 kPa confining pressure are indicated in Figure 2. It can be observed that the deviatoric stress increases linearly with the axial strain at the initial deformation stage for all the CTB samples. Beyond a certain value at which the deviatoric stress and axial strain curves significantly change, the slope of the curves varies from gentle to steep. The deviatoric stress reaches a plateau strength as strain increases and subsequently exhibits sharply reduction in strength. The peak strength and axial strain at failure of CTB specimens increase with increasing confining pressure, irrespectively of curing age and cement content. The enhancement becomes prominent with the increasing curing age and cement content. There is an obvious strain- softening response (post-peaks stress decay of stress-strain curves) for all CTB samples at low confining pressure (i.e. 50 kPa). With the confining pressure increasing, the train-softening behavior is mitigated and changes gradually into strain- hardening. Furthermore, strain-hardening effect induced by longer curing age (i.e. 28 d) is more pronounced than the CTB samples cured at an early age of 3 and 7 d. It shows that the strain-softening response of CTB is more sensitive to early curing age. In addition, it is also shown in Figure 2 that with the cement content increasing from 2 % to 5%, the stress strain curves of CTB samples gradually change from strain-softening into strain-hardening while CTB samples with 5% cement content at 28 d show a strain-hardening response from beginning to end. Therefore, detailed results were supplemented in Figures 3 and 4 to illustrate deeper investigation.

Brittleness index reflects the brittle failure feature and loss of post-peak strength of the CTB specimens. As can be seen from Figure 3, with the confining pressure increasing from 50 to 200 kPa, the brittleness index gradually decreases regardless of cement content, indicating that post-ductility behavior of CTB is easier to be restricted by higher confining pressure. However, the brittleness index increases with the increasing cement content and curing age. It means that the rising curing age length and cement content has enhanced the brittleness of CTB.

Figure 2 Deviatoric stress-axial strain curves of CTB specimens cured for 3 (a, d, g), 7 (b, e, h) and 28 d (c, f, i): (a, b, c) With 2% cement; (d, e, f) With 3% cement; (g, h, i) With 5% cement

Figure 3 Brittleness index of CTB specimens with different cement contents cured for 3 d (a), 7 d (b) and 28 d (c)

Elastic modulus is another index showing the capacity of resisting deformation of CTB under diverse conditions. The elastic modulus evolution of CTB samples is shown in Figure 4. As depicted in the figure, the elastic modulus of CTB samples is significantly improved with the growth of confining pressure, and when the confining pressure reaches a constant value, the increasing trend gradually flattens out. Higher confining pressure contributes to the closure of cracks, and improves the stiffness of CTB samples and then enhances the elastic modulus of CTB. Meanwhile, because of the confining pressure, friction force is generated between the fracture surfaces, which increases the bearing capacity of the cracks, and the slippage is also affected by friction force, so the elastic modulus of CTB samples is increased. In addition,it can be obtained that higher cement content leads to the growth of the elastic modulus. And as curing time elapses, the elastic modulus of CTB with different cement contents increases. It is mainly attributed to that the amount of hydration products in CTB increases with the increase of cement content and curing age, and the generated hydration products fill the pore structure in samples, and then the ability to resist shear failure and elastic modulus is enhanced. The conclusions are supported by the following SEM results.

Figure 4 Elastic modulus of CTB specimens cured for 3 d (a), 7 d (b) and 28 d (c)

Figure 5 Deviatoric stress-volumetric strain curves of CTB specimens (2% cement content) cured for 3 d (a), 7 d (b) and 28 d (c)

For all CTB samples, the triaxial compressive strength is the largest for CTB with 5% cement content, and the strength of CTB samples shows an anisotropy. The strength difference coefficient of CTB samples induced by cement content under different confining pressure can be defined as:

                        (5)

where R_s is the strength difference coefficient of CTB samples induced by cement content; is the largest deviatoric stress of CTB samples; is the smallest deviatoric stress of CTB samples.

The strength difference coefficients of CTB samples with different curing ages and under different confining pressures are shown in Table 4. It can be seen for all CTB samples cured for 3, 7 and 28 d, the strength difference coefficient decreases as the confining pressure increases regardless of curing age, and the strength difference coefficient of CTB at different curing ages presents little variation when the confining pressure is fixed at the same level. It means that the strength gap of CTB induced by increasing cement content is declining with the increasing depth of mine. It can be inferred that the triaxial strength of CTB with different cement contents in underground mining stope is weakly different as the density of the CTB below the cap depth of 10 m is approximately 2.0 g/cm³.

Table 4 Strength difference coefficient of CTB samples under different confining pressures

3.1.2 Stress-volumetric strain behavior

Figure 2 presents the deviatoric stress and volumetric strain curves of CTB samples with different conditions as well. To explain the relationship between deviatoric stress and volumetric strain, the deviatoric stress vs volumetric strain curves of CTB specimens with 2% cement content cured for 3, 7 and 28 d are used for the more detailed and specific analysis, as shown in Figure 5. It is important to note that for volumetric strain, positive values represent contractive and negative values represent dilative. As can be obtained from Figure 5, all the deviatoric stress-volumetric strain of CTB samples demonstrated contractive behavior at initially loading phase, and with higher axial loading applied, the behavior transforms into dilative, which is also supported by the results reported by other researchers [35, 36]. The volumetric strain firstly shows positive depending on the compaction of tailings particles in CTB matrix. In this period, the original pores and cracks in CTB matrix are compressed and some micro-cracks occur, and the axial deformation (shrinkage) of the CTB specimen is greater than the transverse deformation (expansion), resulting in increasing volumetric strain values, as shown in Figure 5(a) (section a-b). As the axial loading increases, the volumetric strain keeps constant, presenting that the axial deformation is equal to the radial deformation, as shown in Figure 5(a) (section b-c), which can be regarded as the critical phase on the transition from contractive to dilative. Then (c point), the axial deformation is less than radial deformation, resulting in the volume expansion at higher axial stress. Finally, the volumetric deformation of CTB sample changes from compression to expansion [37]. In addition, the dilative behavior of CTB samples is aggravated with increasing confining pressures and curing ages.

It is also worth of noting in Figure 5 that the maximum reduction in volumetric strain of CTB specimens varies with the curing ages, cement contents and confining pressures. The maximum reduction in volumetric strain of CTB specimens is listed in Table 5. It can be known from Table 5 that with the confining pressure increasing, the maximum reduction in volumetric strain of samples increases. It is mainly due to that higher confining pressures restrain the lateral deformation, and the lateral deformation becomes more difficult [24]. From Table 5, we can also find that for the CTB specimens with the same curing age, there is an incremental impact of cement contents on the maximum volume reduction when the confining pressure is equal.

3.2 Internal friction angle and cohesion

The internal friction angle and cohesion are the key properties of cement-based material. In this study, the failure envelope of CTB samples under different condition was expressed by Mohr circle depending on the envelope theorem as follows:

   (6)

According to Eq. (6), the failure envelopes of CTB samples were calculated, as shown in Figure 6. It can be found from Figure 6 that there is a good correlation between and for all CTB samples and the correlation coefficients of fitted equations are higher than 0.9, which means that calculated envelopes fit well with the Mohr circles.

Figure 7 illustrates the development in internal friction angle and cohesion of CTB sample with different curing time and cement content. As shown, the internal friction angles of CTB samples change from 41.23° to 44.92°. And the cohesion varies drastically from 31.42 kPa (minimum) to 109.11 kPa (maximum). It is apparent that with increasing cement content, the internal friction angles decrease within a narrow range, but the cohesions increase almost linearly, which is similar to the findings of other researchers [28, 38]. In addition, the cohesion of the CTB specimens is enhanced by the prolonging of curing time, and the change in internal friction angle caused by curing age is trivial and can be neglected. The variations of cohesion and friction on account of the changes in curing age and cement content are analyzed and explored as follows: 1) the cohesion increases by the growth of cement content and curing age, because the amount of hydration products increases with increasing cement content and curing age through hydration reaction, and the adhesion of hydration products on tailings particles surface leads to increase of the bonding ability of CTB samples [39-41]. The microstructures and morphologies of the CTB specimen with different cement contents (2% and 5%) and cured for different ages (3 d and 28 d) are revealed in Figure 8. Observations can be obtained from Figures 8(a) and (b) that a number of pores and cracks were filled with hydration products, and the microstructure in CTB sample at 28 d is much denser than that at 3 d. From Figures 8(b) and (c), it can be found that the tailings particles enclosed by connecting hydration products in the CTB sample with 5% cement are more homogeneous than those with the 2% cement, and the quantity and scale of pores and cracks in CTB specimen with 5% cement decrease significantly. Therefore, it is concluded that with the increase of curing age and cement content, the internal structure of CTB sample gets more compact, which contributes to enhancement of cohesion [42, 43]. 2) The internal friction angle caused by the increasing in curing age and cement content is trivial. The phenomenon is mainly attributed to two aspects. On one hand, the interaction sliding force between the tailings particles is lubricated by the precipitated hydration products on the surface of tailings when subjected to shear stress. On the other hand, the stiffness of hydration products is too tiny to resist particles sliding and to change the failure plane as failure occurs.

Table 5 Maximum reduction in volumetric strain of different CTB samples

Figure 6 Failure envelopes for CTB samples cured for different time:

Figure 7 Changes in shear strength parameters of CTB samples

Figure 8 SEM micrographs of specimens:

3.3 Failure pattern

As discussed in section 3.1, CTB in triaxial compression exhibits brittle (50 kPa), quasi-brittle (100 and 150 kPa) and ductile behavior (200 kPa) at different confining pressures. The failure patterns of CTB samples with different cement contents are illustrated in Figure 9. As seen from the images, the deformation and failure patterns of these

samples are little diverse. The three types of failure pattern are observed in the CTB samples, such as, shear slipping, bulging and a hybrid failure of bulging and shear slipping. The shear slipping means that the failure of the CTB samples is controlled by a weak and straight plane in the sample. A shear banding is produced in the weak plane in the CTB samples with strain-softening response, which are mainly along a shear plane with an angle approximately at 45°+φ/2. The failure pattern of CTB samples cured for 3 d at low confining pressure are characterized by shear slipping, as shown in Figure 9(a). With increasing confining pressure, the failure mode gradually changes from a shear plane to bulging. Bulging failure pattern occurs in CTB samples with an expansion outward. In comparison with the CTB samples with shear slipping, the bulging failure pattern dominantly occurs in the CTB samples with higher cement dosage (i.e. 5% and 3%) than those with low cement at high lateral confining. The CTB samples cured for 7 d also show a shear-slip failure mode when confining pressure is low, with the increase of the confining pressure, the failure mode gradually changes into bulging failure, and the 7 d CTB samples with a higher cement content show a more significant bulging failure, which is similar to the CTB samples cured for 3 d, as shown in Figure 9(b). But, the bulging failure mode of 7 d CTB is relatively more obvious. The CTB samples cured for 28 d mainly exhibit shearing banding along a single plane down to the bottom of the samples with the ductile deformation. The crack propagation path in CTB samples becomes more zigzag with increasing curing time. It means that the cracks should extend longer distance and the rupture failure needs more energy to occur in the CTB, as shown in Figure 9(c).

Figure 9 Failure pattern images of CTB specimens under different conditions:

4 Conclusions

In this paper, a series of triaxial compression tests were performed on CTB samples to obtain their deformation behavior and assess parameters that affect the mechanical behavior of CTB including curing age, cement content and confinement levels. The conclusions are summarized as follows.

1) As the confining pressure increases, the train-softening behavior of CTB is mitigated and gradually changes into strain-hardening. The peak strength, brittle index, elastic modulus and axial strain at failure increase with the increase of curing age length and cement content.

2) The volumetric deformation of CTB exhibits different behavior at different loading stages. CTB shows contractive behavior at low axial strains due to the compressibility of discrete tailings particles, while the behavior at high axial strain reverses from contractive to dilative.

3) Increasing cement content and curing age length significantly enhances cohesion of CTB, but the enhancement in internal friction angle caused by curing age and cement content is trivial and can be neglected.

4) The strength difference coefficient of CTB samples cured for 3, 7 and 28 d decreases as the confining pressure increases regardless of curing age, and the strength difference coefficient of CTB at different curing ages presents little variation when the confining pressure is fixed at the same level.

5) The failure pattern of CTB samples in triaxial compression is mainly along a shear plane. As curing age and cement content increase, the confining pressure restrains the lateral expansion and the bulging failure pattern dominantly occurrs in CTB samples.

Contributors

XU Wen-bin provided the concept and edited the draft of manuscript. LIU Bin conducted the literature review and wrote the first draft of the manuscript. WU Wei-lü edited the draft of manuscript.

Conflict of interest

XU Wen-bin, LIU Bin and WU Wei-lü declare that they have no conflict of interest.

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(Edited by FANG Jing-hua)

中文导读

三轴压缩条件下胶结尾砂充填体的强度和变形特性

摘要：胶结充填体的三轴压缩特性对采场充填体的稳定性具有重要意义。本文研究了水泥掺量、养护龄期和围压对胶结充填体在三轴压缩条件下的强度和形变性能的影响。结果表明：水泥掺量、养护龄期和围压对胶结充填体的三轴压缩变形及力学特性有较大影响。随着水泥掺量、养护龄期和围压的增加，应力-应变行为发生变化，峰值点处轴向应变增加，且峰值后强度损失增加。充填体的粘聚力随着养护龄期和水泥掺量的增加而增大，水泥掺量对充填体的内摩擦角影响小。充填体试样主要沿主剪切面发生破坏，围压抑制充填体侧向膨胀；随着养护龄期和水泥掺量的增加，充填体试样主要呈现膨胀破坏形态。研究结果将有助于更好地理解胶结充填体的三轴力学和变形行为。

关键词：胶结充填体；三轴抗压强度；体积应变；弹性模量；内聚力；内摩擦角

Foundation item: Projects(2018YFC0808403, 2018YFE0123000) supported by the National Key Technologies Research & Development Program of China; Project(800015Z1185) supported by the Yueqi Young Scholar Project, China; Project(2020YJSNY04) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2020-04-30; Accepted date: 2020-09-03

Corresponding author: XU Wen-bin, PhD, Associate Professor; Tel: +86-18612987658; E-mail: xuwb08@163.com; ORCID: https://orcid.org/ 0000-0002-0365-3619