The Effect of Acid Mine Drainage Corrosion on the Creep Deformation of Cemented Gangue Backfill

Acid mine drainage (AMD) is a hazardous by-product of coal mining, which may intensify the creep deformation of cemented gangue backfill (CGB) in solid backfill mining. In this study, the long-term stability and creep property of CGB soaked in AMD were quantitatively measured by experimental and theoretical analysis. -e uniaxial compressive strength, elastic modulus, and mass of CGB soaked in AMD, with pH of 5.56 and soaking time of 30 to 360 days, were tested. -e change trend of compressive strength, elasticity modulus, and mass first increased, then decreased, and finally ended at 94.37%, 96.68%, and 98.00%, respectively. -e respective creep processes of CGB soaked in AMD and untreated CGB were similar, initial creep stage lasted only for a short time, and no creep deformation occurred in the steady creep stage. Moreover, the long-term strength of CGB soaked in AMDwas 98.6% of the untreated CGB.-e creep law of the CGB soaked in AMDwas accurately fitted by using the Kelvin model. -e theoretical analysis revealed that the final creep deformation of the CGB column soaked in AMDwas only 4.4mm.-e results of this paper show the possibility of using CGB in AMD-accumulating goaf.


Introduction
China is the world's largest producer and consumer of coal due to its coal-rich and oil-poor energy reserves [1,2]. Till date, several measures have been introduced to expand China's share of renewable and carbon-efficient fossil-fuel energies so as to reduce coal production and consumption [3], yet coal will remain a crucial source of energy in China for at least several decades [4][5][6]. e underground mining method is the most common method to mine coal in China [7], and many mined-out goaf, coal gangue, and acid mine drainage (AMD) are created during the underground coal excavation [8][9][10][11]. e mined-out goaf under railways, roads, buildings, and surface water is very prone to surface subsidence and water inrush accidents [12]. Coal gangue is often discarded and accumulated in large heaps due to its inherent low heat and utilization value [13]. ese gangue heaps account for a significant number of lands and cause potential hazards, such as spontaneous combustion accidents, air contamination, and soil heavy metal pollution [3]. AMD always contains metals, sulfate, and acidity [14], which is an important area of environment impact [15,16]. In coal mine goaf, AMD can contaminate groundwater resources and corrode buildings [17,18]. e use of coal gangue as the main component of backfill material of solid backfill mining (SBM) can aid in the efficient disposal of coal gangue as well as decrease the influence of surface subsidence [19][20][21]. Some constituents of cemented gangue backfill (CGB) may react with AMD, which leads to the decrease in the support strength and stability of backfill material in SBM [22]. Creep property is a key parameter of the backfill material of SBM, which directly affects the longterm stability of supported goaf [23]. erefore, examination of the influences of AMD corrosion on the creep deformation of CGB is significantly important for the application and long-term stability of SBM. e mechanical properties of CGB have increasingly attracted widespread attention [24,25]. Yin et al. [7] investigated the characteristics of hydration kinetics and microstructure of fly ash cemented backfill materials based on the thermodynamic and electrical perspectives. Wu et al. [9] studied the uniaxial compressive strength of cemented gangue backfill by means of experiments. Fall and Pokharel [26] studied the strength development and microstructure of coal gangue concrete at different initial sulfate contents and temperatures. Vegas et al. [27] studied the compressive strength of the cement matrices of blended thermally activated coal gangue via experiments. Wu et al. [28] studied the influence of cement types and mixing water quality on cemented paste backfill properties. Dong et al. [29] investigated the compressive strength and flexural strength of concrete mortar under different coal gangue fine aggregate contents. Li et al. [30] used ANSYS to investigate the flexural properties of reinforced spontaneous combustion gangue concrete beams. Cihangir et al. [31] investigated the influence of the silicate modulus on the strength and stability of cemented paste backfill. Guan et al. [32] studied the effects of low temperature on the mechanical properties of coal gangue concrete through freeze-thaw cycle experiments. e long-term acid corrosion resistance of CGB soaked in AMD is the key step toward ensuring long-term support in wateraccumulating goaf of coal mines [33][34][35]. However, all of the aforementioned research studies have mainly focused on the influence of components and their contents on the mechanical properties of CGB, while very few researchers have investigated the effects of AMD corrosion on the creep deformation of CGB in SBM.
In this study, the mechanical properties of CGB prepared by cement, fly ash, and coal gangue were tested. Further, multistage loadings were adopted to test the creep property of CGB, which was soaked in AMD gathered from the goaf of a coal mine in China. Next, the creep law of the CGB soaked in AMD was fitted by using the Kelvin model based on the soak experimental creep curves. Finally, the creep deformation of the CGB column under AMD corrosion was investigated by using a generalized Kelvin model for SBM.  Tables 1-4. In order to guarantee the quality of aggregate, the carbonaceous mudstone and arenaceous shale were removed before crushing. e coarse aggregate was the crushed coal gangue of which the grain size was 4.75-20 mm, while the fine aggregate was those of which the grain size was 0.15-4.75 mm. e physical appearance of the coarse aggregate and the fine aggregate is shown in Figure 1. e main parameters of the fine aggregate are shown in Table 5. e cement was pc32.5 cement. e fly ash was obtained from the power plant near Silaogou Coal Mine. e water was tap water.

Materials and Experiment Methods
e admixture was mainly composed of water reducer, and it was added to reduce the dosage of water from 345 to 231 kg/m 3 and to improve the slump of the CGB from 140 mm to 260 mm.

AMD.
e AMD was obtained from the old mine goaf at No. 14 −3 coal seam of Silaogou Mine. e main sources of the AMD were water-bearing surrounding rocks and underground human activities. e chemical properties of AMD are shown in Table 6, and its chemical composition of the AMD is shown in Table 7. e pH of most AMD in Shanxi Province is more than 7 [36,37]; therefore, the AMD with a pH of 5.56 represents a relatively bad circumstance of goaf in SBM.     Table 8. First, cement, fly ash, coarse aggregate, and fine aggregate were weighed using an electronic balance and then placed in a blender. Next, the mixture was whisked until the contents were evenly mixed. Tap water and admixture were added before stirring. e mixture was shaken, and the initial slump was measured. After one hour, the slump was measured again, and then the concrete was placed into a mold within 15 min. After 24 h, the concrete in the mold was demolded in a standard curing room, where the temperature was 18−22°C and the relative humidity was greater than 95%. In the following 28 days, the specimens were placed in the standard curing room, apart from each other by more than 10 mm, and the surface was kept moist [38]. e mechanical properties of CGB are listed in Table 9. According to the aforementioned procedure, the CGB specimen with the dimension of 100 mm × 100 mm × 100 mm was prepared for the CGB soaking test, and cylindrical specimens with the size of Φ 25 mm × 50 mm were produced by a core-drilling machine for the creep test.

e Medium-Grained Sandstone Specimen.
e medium-grained sandstone test specimen was taken from the roof of Silaogou Coal Mine. e medium-grained sandstone is distributed in multiple layers in the overlying strata of the No. 14 −3 coal seam, with the thicknesses of 7.3, 4.0, 6.2, and 5.3 m, respectively, and a cumulative thickness of 22.8 m, accounting for 24% of the total thickness of the overlying strata in the No. 14 −3 seam. It was relatively hard and complete in this set of formations, which controlled the deformation of the overlying formations. e thickest (7.3 m) mediumgrained sandstone, found only 0.9 m away from the No. 14 −3 coal seam, was the key layer in the mining process. e large rock samples of medium-grained sandstone that were collected from the Silaogou Coal Mine were sealed with paraffin and transported to the laboratory. According to the recommended method for rock mechanics testing by the International Rock Mechanics Institute, three specimens were processed with a size of V25 mm × 50 mm [39].    A control group, containing specimens soaked in distilled water, was also investigated for comparative analysis. When the soaking time had come to an end, the masses of CGB specimens were observed, and a uniaxial compression test was performed to measure the compressive strength and elastic modulus.

Creep Test.
ree YDY-100 high-temperature uniaxial rock creep testers were adopted to test the creep characteristics of specimens, as shown in Figure 2(a). A pressure sensor with the maximum range of 200 kN and measurement accuracy of 0.01 kN was used to measure the pressure. e real-time displacement was measured using a linear variable displacement transducer sensor with the measuring accuracy of 10 −3 mm and range of 10 mm. e pressure, displacement, and temperature were automatically and directly measured in real time using a computer system, as shown in Figure 2(b). A multistage loading axial compression loading method was adopted, and the load application increment was determined according to the creep condition during the experiments. e specific steps of the uniaxial creep experiment under multistage loading are as follows.
First, the specimen was placed in the storehouse of the rock creep tester. e first stage of load was applied slowly, and the displacement was measured every minute. Second, after entering the steady-state creep stage, the interval of displacement measuring was adjusted to 20 min. If the deformation of the specimens did not change over a period of more than at least 48 h, it was considered that the specimens were no longer undergoing creep under the load and the next stage of load would be applied. ird, after entering the accelerated creep stage, the interval of displacement measuring was adjusted to 1 min, until the specimen was broken. ree creep tests were conducted. e first was a multistage loading uniaxial creep test on medium-grained sandstone specimens. e second was a multistage loading uniaxial creep test on CGB specimens in a dry state. e last was multistage loading uniaxial creep tests on CGB which was soaked in AMD, with specific steps as mentioned above, except for the fact that the storehouse was filled with AMD.

Mechanical Properties.
e variation of compressive strength and elasticity modulus of CGB soaked in AMD is shown in Figure 3. e compressive strength of CGB increases at the beginning of the immersion. is result could be attributed to the following two reasons. First, CGB was soaked in water and cement hydration continued to occur, which resulted in an increase in the compressive strength and elastic modulus [40][41][42][43]. Second, the expansion stress inside CGB led to the decrease in the pore diameter or increase in the compactness because the SO 4 2− ions in AMD reacted with the CGB to form ettringite crystals [44,45]. is in turn resulted in the increase in the compressive strength and elastic modulus of the CGB [40]. After soaking for 60 days, the compressive strength of CGB began to decrease, as the H + ions in AMD reacted with CGB, which reduced the components such as CaO, Fe 2 O 3 , and Al 2 O 3 [46][47][48][49][50]. e SO 4 2− ions could only decelerate the action of H + in the initial stage, and its effect was limited. e corrosion of H + ions on CGB gradually became the dominant factor influencing the effect of AMD on CGB. Moreover, the volume expansion of the SO 4 2− ion reaction products caused internal stress in CGB, which generated tiny holes and exacerbated the CGB corrosion [51]. With the gradual decrease in the compressive strength, after 120 days, it began to be lower than the original value, and after 300 days, it reached a minimum of 26.8 MPa, which was 94.37% of the original value, yet it still met the demand for CGB in SBM. After 360 days of soaking, the compressive strength was 0.3 MPa higher than that at 300 days, which was likely caused by individual differences in the specimens.
Variation of the elasticity modulus basically followed the same trend as the compressive strength, as shown in Figure 3. Before the 90-day point, the elasticity modulus of the CGB increased, and it began to decrease further. After soaking for 360 days, the elastic modulus was the smallest, at 871.26 MPa, which was 96.68% of the original value. e corrosive effects of AMD with a pH of 5.56 for CGB were found to be negligible, as the CGB is a typical heterogeneous body and the performance of the same batch of concrete in laboratory tests and industrial production often fluctuates in this manner [52]. Figure 4 exhibits that the mass of CGB soaked in AMD continued to increase until 90 days and then decreased, but it was still higher than the mass before soaking. e  Advances in Materials Science and Engineering cause of this phenomenon was a combination of the hydration of cement and ettringite crystals produced by the SO 4 2− ions. In order to determine the effects of acidic substances in AMD on the mass of the CGB, the influence of cement hydration was removed by eliminating the mass change of CGB soaked in distilled water [53]. e mass of CGB soaked in AMD still increased, but after 90 days, it began to decrease. It was less than the original mass at 120 days when the corrosion effect of H + ions in AMD dominated. Further, the mass of CGB continued to decrease, but the decrease rate decelerated, and at 300 days, it was basically stable. e mass of CGB soaked in AMD remained the same after 300 days and was reduced by 0.199% compared to the original mass. is revealed that AMD with a pH of 5.56 showed little effect on mass of CGB.

Creep Property Analysis.
Various creep properties of the specimens soaked in AMD are shown in Figure 5. e loading stages of the given specimens are shown in Table 10.
It can be observed that when the stress was lower than 10 MPa, the initial creep time lasted for a very short period, usually several hours. When the stress was higher than 20 MPa, the duration of the initial creep stage was prolonged, typically by 1-3 days. e duration of steady-state creep was prolonged while the stress increased. e initial and steady-state creep stages of both untreated and soaked CGB were longer than those of the medium-grained sandstone. When the stress was loaded to 40 MPa, the medium-grained sandstone specimen quickly entered the accelerated creep stage and broke soon. When the stress of 28.1 MPa was applied, the untreated CGB specimen underwent a steady-state creep of 17 days and developed into the accelerated creep stage, eventually resulting in damage. When the stress of 25 MPa was applied, the soaked CGB specimen underwent accelerated creep after a 12-day steadystate creep stage and eventually ruptured. erefore, the long-term strength of medium-grained sandstone, untreated CGB, and CGB soaked in AMD was confirmed to be 30, 25, and 24 MPa, respectively. e values of long-term strength of the medium-grained sandstone, CGB, and CGB soaked in AMD are presented in Table 11. After soaking in AMD for 110 days, the long-term strength of CGB was 23.17 MPa,  Advances in Materials Science and Engineering which was 98.6% of normal CGB. It was proven that only minimal corrosion occurred on the CGB by AMD.

Fitting Analysis of Creep Curve.
In order to judge the long-term stability of SBM with CGB support, the creep model was adopted to fit the results of the creep experiment.
e Kelvin model was adopted through a comparison of multiple creep models, as its regularity and agreement with the experimental results were relatively good.
e Kelvin model consists of a spring connected in parallel with a damper, as shown in Figure 6. e fitting parameters of the Kelvin model are listed in Table 12.
e constitutive equation of the Kelvin model is as follows: where σ represents the stress, Pa; E k denotes the elasticity modulus of the spring in the Kelvin body, Pa; η represents the viscosity coefficient of paste in the Kelvin body, Pa·s; and t is the time, s. e generalized Kelvin model curves of medium-grained sandstone, CGB, and CGB soaked in AMD are shown in Figure 7. e fitting curve was basically consistent with the experimental data. Figure 7(b) demonstrates that when the stress was 22.5 MPa, CGB produced a strain of only 0.002, and creep deformation did not occur after 40 days. Figure 7(c) illustrates that even if it was soaked in AMD, the CGB still produced only 0.0017 strain under the stress of 22.5 MPa, and after 30 days, no creep occurred. is shows that the CGB soaked in AMD could still effectively support the overburdened strata. According to the experiment results of the mechanical properties, the compressive strength and elastic modulus of CGB remained at 98.24 and 97.65%, respectively, of its original value when the CGB was soaked in AMD for 110 days.

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Long-Term Effects of AMD Corrosion on the Creep Deformation of the CGB Column.
e deformation characteristics of the CGB columns over time are critical to the application of the CGB support column under actual stress conditions and chemical corrosion of AMD in SBM.
Two key problems are involved: first, whether long-term accumulated deformation would cause a large uneven settlement on the ground, in turn destroying the column; and second, the deformation evolution of the CGB column.
e design of the support scheme is as follows: the CGB column with the side length of 5 m and height of 3.4 m was adopted to support the roof area with the side length of 15 m in the No. 14 −3 coal seam of Silaogou Mine. A section of the CGB column was taken for analysis, as shown in Figure 8.
e model can be simplified as a cylinder with undeformed bottom surface whose top surface is subjected to load. It is assumed that the load of the model of the CGB column is a uniform load of 12 MPa and is evenly distributed. In consideration of long-term AMD corrosion, a spring E 1 was introduced to the rheological model, in a series with the Kelvin model, as shown in Figure 9 [54,55]. Such a simplified model was adopted to perform the calculation, and the result of deformation might be greater than the actual value, signifying that it was much safer. e constitutive equation of the rheological model of the CGB column soaked in AMD is as follows: where E 1 represents elasticity modulus of the series spring, Pa. e acid corrosion law of the elastic modulus of CGB under action of AMD in the goaf is shown in Figure 3. e power function was used to fit Figure 3, and the fitting formula for chemical corrosion is represented as follows:  Figure 6: Illustration of the Kelvin model.  Advances in Materials Science and Engineering 7 e failure law of the CGB column considering the longterm AMD corrosion and creep coupling of the hydrochemical interaction in the goaf is as follows: 1 − e − 0.25t + 12 1.006t −0.0065 .
(4) Figure 10 shows the calculation result of equation (4). It is demonstrated that the creep deformation of the CGB column soaked in AMD mainly occurred within one month. en the creep of the column entered the steady creep state, deformation was 2.7 mm. Owing to the chemical action of AMD in the goaf, the CGB support column was slightly corroded, and its elastic modulus was reduced, which in turn resulted in acid corrosion deformation. e change law of deformation was that in the first month, the deformation was severe, reaching 0.75 mm. From 30 to 360 days, the deformation then increased to 1.5 mm. Finally, during the period of 360 to 720 days, the deformation increased from 1.5 to 1.7 mm; thus, the deformation was 0.2 mm throughout the year, which was extremely weak. Certainly, the deformation would be slower after 720 days, after which it ceased. e deformation of CGB was only 4.4 mm after two years; then, it stabilized when two types of aging deformations were superimposed. is is the effect of AMD corrosion

Conclusions
e experimental CGB was soaked for 360 days in AMD with pH of 5.56. e combined action of cement hydration and acid corrosion caused the mechanical properties to increase first and then decrease slightly. e results indicate that under SBM conditions, the AMD corrosion on CGB was negligible. e long-term strength of the CGB soaked in AMD was 98.6% of the untreated CGB. AMD showed little effect on the long-term strength of the CGB. e creep of CGB could be accurately fitted by the Kelvin model. According to the hydrochemical interaction rheological model of creep and effect of AMD, the creep deformation of CGB under AMD corrosion in SBM stabilized toward a negligible value in two years.
e results indicate that CGB could meet the requirements of SBM under the action of AMD.

Data Availability
e data used to support the findings of this study are included within the article, and they are fully available without restriction.    Advances in Materials Science and Engineering 9