Experimental Study on the Dynamic Mechanical Properties and Crashing Behaviors of Limestone Under High Temperatures in Real-Time

Abstract

In this study, a split Hopkinson pressure bar (SHPB) test system with real-time temperature control was developed, and dynamic tests on limestone taken from deep coal mines within real-time temperatures of 25 to 800 °C were carried out. Additionally, the scanning electron microscope (SEM), X-ray diffraction (XRD), and energy dispersion spectrum (EDS) tests were conducted to analyze the fracture mechanism of limestone at real-time temperatures. The results reveal that the dynamic compressive strength of limestone linearly declines with increasing temperatures; due to not being affected by thermal shock damage, its strength degradation is not significant after cooling to room temperature, whereas the dynamic elastic modulus exhibits a negative exponential nonlinear decrease with the increase in temperatures. The average strain rate has a positive correlation with the dynamic compressive strength of limestone, while the dynamic elastic modulus exhibits variations in accordance with the Boltzmann function and its relationship with the strain rate. The combined influence of strain rate and temperature on the dynamic compressive strength of limestone can be accurately described by a binary quadratic function. The mechanism of real-time action on limestone can be divided into three stages: when the temperature is between 25 and 200 °C, crystal micro-expansion leads to the densification of micropores, which leads to the increase in limestone strength. When the temperature is between 200 °C and 600 °C, the formation of microcracks induced by thermal stress and intergranular expansion results in a reduction in limestone strength. When the temperature is between 600 and 800 °C, in addition to the continued expansion of the intergranular resulting in the increase in the number of micro-cracks, the decomposition of dolomite at high temperatures leads to chemical deterioration and further reduction in the strength of limestone.

1. Introduction

The depletion of shallow coal resources results in a gradual increase in mining depth. Currently, deep mining encounters numerous engineering problems and technical challenges, including low mining efficiency, inadequate security measures, significant ecological damage, suboptimal resource recovery rate, and high energy loss [1]. Deep coal gasification mining is an efficient solution to these challenges, simultaneously mitigating the emission of pollutants during solid coal treatment and promoting the efficient and environmental utilization of coal [2,3]. However, the high-temperature condition required in the gasification stope significantly affects the mechanical and physical properties of surrounding rocks, thereby exerting a substantial influence on mine stability [4,5,6,7]. For example, under the influence of high temperatures, Carboniferous rocks, especially clay shales, lose their properties, their structure changes, and their strength and deformation parameters decrease [8]. Under high-intensity fluidized mining, the performance deterioration of surrounding rocks under the high temperature and strong disturbance results in frequent disasters of the surrounding rock, posing significant challenges for controlling and preventing instability and disasters in the surrounding rock [7,9,10]. Therefore, the investigation of the dynamic characteristics and fracture mechanisms of coal rock under high temperatures and impact loading is of great significance.

The current research focuses on investigating the impact of elevated temperatures on the mechanical characteristics of rocks [11,12,13,14,15]. However, the research results exhibit significant variability, which can be ascribed to the intricate nature of mineral compositions and microstructures. At high temperatures, the original cracks, pores, and other microscopic structures in the rock are changed and rearranged. When the temperatures reach critical values, the mineral grains undergo physical and chemical transformations such as expansion, melting, and thermal decomposition, leading to variations in the mechanical properties of the rock mass [16,17,18,19,20,21,22]. The strong impact disturbance in the deep underground further exacerbates the challenge of the accurate evaluation of the complex mechanical characteristics of the rock mass under such a high-temperature environment. A series of advanced split Hopkinson pressure bar (SHPB) test systems, including confined pressure [23,24] and static–dynamic combined loading control modules, have been designed to explore the mechanical characteristics of the rock mass under different stress conditions [25]. These research results are conducive to the safe development of deep coal resources [26,27,28,29,30,31,32,33,34,35,36,37]. However, the superposition of temperature and stress in fluidized mining cannot be fully illustrated in the existing studies, and the coupling effect of temperature and stress on the mechanical characteristics of the rock mass during high-intensity fluidized mining still requires further investigation [6,38,39,40,41,42,43,44] in the existing studies. It should be noted that dynamic mechanical characteristics of the rock mass in this special engineering are the result of the co-evolution of their internal crystal phases, pore structures, and stress fields under real-time high temperatures. Additionally, the melting or expansion of crystal phases at real-time high temperatures, as well as the formation of cracks induced by thermal stress, can further complicate the dynamic mechanical characteristics of the rock mass under external impact [45,46,47,48].

In this study, the dynamic tests on limestone samples taken from a deep coal mine at the real-time temperatures of 25 to 800 °C were conducted by an SHPB test system with a real-time temperature control, and the coupling effects of real-time high temperature and impact loading on the dynamic stress–strain curve, compressive strength, deformation, and macro-fracture characteristics of limestone samples were analyzed. Additionally, the scanning electron microscope (SEM), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS) were carried out to investigate the fracture mechanism of limestone at real-time temperatures.

2. Materials and Methods

The preparation method of the limestone sample and its micromorphology are introduced. The structure, working principle, and test method of SHPB with real-time temperature control system are described in detail.

2.1. Materials and Specimen Preparation

The limestone was taken from the Jiagou coal mine in Xuzhou, Jiangsu Province, China. The results of XRD analysis showed that the limestone was mainly composed of calcite (CaCO₃, 79.1%), dolomite (CaMg(CO₃)₂, 19.6%), and white mica (KAl₂(Si₃AlO₁₀)(OH)₂, 1.3%) at the normal temperature. All limestone specimens utilized in the experiments were obtained from one complete block. As shown in Figure 1a, each specimen was a cylinder shape with a height of 50 mm and a diameter of 50 mm. The parallelism and perpendicularity of the specimen were processed to be less than 0.02 mm according to the International Society for Rock Mechanics (ISRM) standard.

Figure 1a–c illustrate the SEM and EDS results of limestone specimens. As indicated by point a in Figure 1b, the morphological features are prismatic, angular, and smooth. The mass proportions of C, O, Mg, Ca, Al, and Si elements at point a are 8.4%, 54.9%, 7.9%, 24.6%, 1.6%, and 2.6%, respectively (Figure 1c). Thus, it can be concluded that the area around point a is mainly composed of dolomite. However, the morphology of point b is rough, which is similar to the cement mortar during mixing. The mass proportions of C, O, Ca, Al, and Si elements at point b are 2.7%, 49.0%, 40.7%, 1.6%, and 3.6%, respectively (Figure 1d). Therefore, the area around point b is composed of calcite with certain impurities.

2.2. SHPB with Real-Time Temperature Control System

Figure 2 shows the developed SHPB test system with real-time temperature control. This system consists of the gas pressure loading module, the rod module, the information acquisition and analysis module, and the high-temperature control module. The gas pressure loading module is composed of a gas cylinder, a gas gun, and a gas pressure controller. The rod module encompasses a damper, a transmitted bar, an incident bar, a striker bar, an absorber bar, and two preloading devices. Moreover, the information acquisition and analysis module contains two strain gauges, a dynamic strain gauge, a tachometer, and a data processing system. The high-temperature treatment module consists of a high-temperature heating furnace and a temperature controller. During the test, the specimen was placed in the high-temperature control module and simultaneously connected to the incident bar and transmitted bar for real-time impact testing at elevated temperatures. It should be noted that the absorber bar, striker bar, transmitted bar, and incident bar were fabricated using high-strength and high-temperature-resistant materials with a diameter of 50 mm, a density of 7580 kg/m³, a P-wave velocity of 5190 m/s, an elastic modulus of 210 GPa, and an ultimate strength of more than 500 MPa.

2.3. Test Process

The temperatures in the SHPB tests at real-time temperature states were set as 25, 100, 200, 300, 400, 500, 600, 700, and 800 °C, respectively. The impact velocity and strain rate were controlled by the impact pressure, which was set to be 0.5, 0.6, 0.7, 0.8, and 0.9 MPa.

In the test process, the specimen was placed in the high-temperature heating furnace at the heating rate of 10 °C/min until it reached the desired target temperature. After the temperature reached the pre-set value, the specimen was subjected to a constant temperature for 4 h to ensure thermal equilibrium of the specimen [49,50]. Subsequently, the high-temperature heating furnace was quickly opened, followed by the swift alignment and clamping of the incident bar and transmitted bar with the specimen using two preloading devices.

Open the dynamic strain gauge and check the connection with the strain gauge. The connection mode is Wheatstone half-bridge circuit. The dynamic strain gauge was balanced and zeroed, the parameters of strain gauge were input into the signal acquisition and analysis system, and the sampling frequency was set to 10 M.

The specimen was adjusted to the pre-set impact pressure, and the projectile was fired using the launching controller of the impact test. Meanwhile, the velocity detector and dynamic strain acquisition systems were used for data collection. In this process, the heating device is continuously operated to maintain a constant temperature of the specimen. After the test, the fractured specimen was collected when the temperature decreased to the normal temperature.

2.4. Principles and Methods

The strain rate is calculated by a three-wave method, as presented in Equation (1).

$$\begin{array}{l}\dot{\epsilon }\left(t\right)={\displaystyle \frac{C_0}{L_{\mathrm{S}}}}\left[{\epsilon }_{\mathrm{I}}\left(t\right)-{\epsilon }_{\mathrm{R}}\left(t\right)-{\epsilon }_{\mathrm{T}}\left(t\right)\right]\\ \epsilon \left(t\right)={\displaystyle \frac{C_0}{L_{\mathrm{S}}}}{\displaystyle {\int }_0^t\left[{\epsilon }_{\mathrm{I}}\left(t\right)-{\epsilon }_{\mathrm{R}}\left(t\right)-{\epsilon }_{\mathrm{T}}\left(t\right)\right]}dt\\ \sigma \left(t\right)={\displaystyle \frac{A_0}{2A_{\mathrm{S}}}}{E}_0\left[{\epsilon }_{\mathrm{I}}\left(t\right)+{\epsilon }_{\mathrm{R}}(t)+{\epsilon }_{\mathrm{T}}\left(t\right)\right]\end{array}$$

(1)

where E₀, A₀, and C₀ denote the elastic modulus, the cross-section area, and the wave velocity of the bar; L_S and A_S are the original lengths and the cross-sectional area of the specimen; ε_I(t), ε_R(t), and ε_T(t) represent the strain histories of the incident bar, reflected bar, and transmitted bar at a time t.

To evaluate the accuracy of the SHPB test, the incident reflected wave and transmitted wave were extracted, superimposed, and compared. It can be concluded that the incident wave and reflected wave align with the transmitted wave upon superposition. In addition, the Stress Equilibrium (SE) and Constant Strain Rate (CSR) conditions in the SHPB test were evaluated by analyzing the strain time record and strain rate, and the equilibrium index R(t) was defined in Equation (2). As described in Equation (2), R(t) ≤ 5%, and the total equilibrium time is more than 4 times the time required to pass the stress wave through the specimen, indicating that both CSR and SE conditions are satisfied [51].

$$R\left(t\right)=2\left|{\displaystyle \frac{{\epsilon }_I+{\epsilon }_R-{\epsilon }_T}{{\epsilon }_I+{\epsilon }_R-{\epsilon }_T}}\right|\le 5\%$$

(2)

3. Results and Interpretations

Based on SHPB with real-time temperature control system, the dynamic stress–strain curves of limestone under real-time high-temperature environment were obtained, and the changes in dynamic peak strength, dynamic elastic modulus, and macroscopic fracture characteristics of limestone with temperature and strain rate were analyzed.

3.1. Dynamic Stress–Strain Behavior

Figure 3a–c show the dynamic stress–strain curves of the limestone specimen under varying temperatures and impact velocities V of 6–7, 7–8, and 8–10 m/s. This curve can be classified into three stages: the elastic stage, the plastic stage, and the fracture stage. However, as the temperature increases, the curve does not show an obvious compaction stage, which is different from the dynamic compression characteristics of limestone after high temperature. The abovementioned is in good agreement with the existing literature. For instance, Ping et al. reported that the limestone under high temperatures (>300 °C) appears to have an obvious compaction stage, and it is mainly caused by the closure of existing micro-cracks in the limestone specimen [49], and these micro-cracks are induced by the thermal impact of high temperature and cooling. However, under real-time high-temperature conditions, no thermal damage occurs in the specimen, and thermal expansion occurs in the internal mineral crystals; thus, there are no obvious compaction characteristics. With the increasing temperature, the slope of the stress–strain curve in the elastic stage decreases significantly, and that in the plastic stage increases, while that in the fracture stage with cliff drop features slows down. This indicates that the specimen undergoes a transition from brittle failure to brittle–ductile failure as the temperature increases. Additionally, the peak stress initially rises and then declines in response to an initial increase in real-time temperature. When the temperature is between 25 °C and 200 °C, with the increase in temperature, the thermal stress causes the expansion of mineral particles inside the limestone, and the volume increases, resulting in the closure of the original cracks inside the sample, and the integrity of the sample is enhanced. In addition, the evaporation of water within the sample leads to increased friction between mineral particles, resulting in increased peak strength of the limestone. However, when the temperature exceeds 200 °C, the increase in thermal stress leads to the initiation and gradual expansion of cracks in the sample, the internal damage of the sample is intensified, and the strength of the limestone is gradually reduced.

Figure 3d–e show the influence of the impact velocity on the dynamic stress–strain curve of the limestone specimen at different real-time temperatures T of 25, 400, and 800 °C. With the increase in the impact velocity, the slope of this curve increases during the elastic stage and plastic stage. Therefore, an increase in the impact velocity enhances the deformation resistance of the specimen in the elastic stage and increases the plastic deformation. The average strain rate can be calculated by averaging the entire process of impact loading in this study.

3.2. Dynamic Compressive Strength

Figure 4 presents the influence of real-time temperature on the dynamic compressive strength of the limestone specimen. When the dynamic strain rate is 40–60 s⁻¹, 60–80 s⁻¹, and 80–100 s⁻¹, the dynamic compressive strength of the specimen exhibits a linear decrease as the temperature increases. Within the range of 40–60 s⁻¹, the dynamic compressive strength of the specimen is approximately 190.5 MPa at a temperature of 25 °C and 72.6 MPa at a temperature of 800 °C, exhibiting a reduction of 61.9%. This finding is consistent with the results obtained by Ping et al. [49] when the heated specimen underwent natural cooling to room temperature. The difference lies in that the specimen in our study at real-time high temperature is also affected by thermal shock at the same time. In addition, the strength exhibits a negative correlation with temperature across different strain rates, with descent rates of −0.15 MPa/°C, −0.15 MPa/°C, and −0.13 MPa/°C under the strain rates of 40–60 s⁻¹, 60–80 s⁻¹, and 80–100 s⁻¹, respectively.

Figure 5 illustrates the influence of strain rate on the dynamic compressive strength of the limestone specimen. The strength of the limestone specimen increases linearly with the strain rate within a real-time temperature range of 25–800 °C, which is different from the findings obtained by Wang et al. [50]. At the real-time temperature of 25 °C, the dynamic compressive strength of the limestone specimen is 112.6 MPa under the strain rate of 31.0 s⁻¹. However, when the strain rate is increased to 48.1 s⁻¹, the dynamic compressive strength significantly rises to 208.8 MPa, indicating an impressive increase of 85.4%. When the real-time temperature reaches 800 °C, the dynamic compressive strength of the limestone specimen is 72.6 MPa under a strain rate of 46.7 s⁻¹, while at a higher strain rate of 95.8 s⁻¹, the strength increases to 148.2 MPa, representing a significant improvement of 104.1%.

The temperature sensitivity coefficient η is defined in Equation (3) to characterize the effect of temperature on the dynamic compressive strength of the limestone specimen under different strain rates in Figure 6.

$$\eta =\eta (T)={\displaystyle \frac{d{\sigma }_{\mathrm{d}}}{d\dot{\epsilon }}}$$

(3)

When the real-time temperatures are set as 25, 100, 200, 300, 400, 500, 600, 700, and 800 °C, the temperature sensitivity coefficients η are 5.4, 3.6, 1.6, 1.7, 1.2, 1.4, 1.5, 0.8, and 1.5 MPa/s, respectively. Moreover, the coefficient η presents a non-linear decrease and tends to a stable value as the temperature increases, as illustrated in Figure 6. Through fitting calculation, the expression of η about temperature T is obtained, as presented in Equation (4).

$$\eta =5.36e^{(-T/103.02)}+1.26$$

(4)

Figure 7 presents the coupling effect of the temperature and strain rate on the dynamic compressive strength of the specimen. It is evident that elevated temperatures have a detrimental impact on the dynamic properties of the specimen, leading to a decrease in its dynamic compressive strength. However, an increase in strain rate results in an enhancement of its dynamic compressive strength. The quantitative relationship can be expressed as follows:

$${\sigma }_{\mathrm{d}}=1.1\times {10}^{-4}{T}^2+4.3\times {10}^{-3}{\dot{\epsilon }}^2-2.3\times {10}^{-3}T\dot{\epsilon }-9.5\times {10}^{-2}T+2.0\dot{\epsilon }+85.5,R^2=0.811$$

(5)

3.3. Dynamic Elastic Modulus

Figure 8 shows the influence of the real-time temperature on the dynamic elastic modulus of the limestone specimen. The static elastic modulus of limestone specimen at room temperature (25 °C) is only 65 GPa. As the temperature increases, the elastic modulus of the specimen presents a nonlinear variation with a negative exponent. As the temperature increases from 25 to 800 °C, the dynamic elastic modulus of the limestone specimen under the strain rate of 40–60 s⁻¹ declines from 193.8 to 31.8 GPa, decreasing by 83.6% at a decrease rate of 0.26 GPa/°C. As the temperature increases from 200 to 800 °C, the dynamic elastic modulus of the limestone specimen under the strain rate range of 60–80 s⁻¹ decreases from 184.4 to 33.4 GPa, reducing by 81.9% at a rate of 0.26 GPa/°C. As the temperature increases from 300 to 800 °C, the dynamic elastic modulus of the limestone specimen under the strain rate of 80–100 s⁻¹ decreases from 210.8 to 72.8 GPa, decreasing by 65.5% at a decrease rate of 0.24 GPa/°C. The fitting curve in Figure 8 illustrates the variation of the dynamic elastic modulus of the specimen under different strain rates with the temperature. It is evident that the elastic modulus exhibits a similar decreasing trend as the temperature within the strain rate ranges of 60–80 s⁻¹ and 80–100 s⁻¹.

Figure 9 shows the influence of strain rate on the dynamic elastic modulus of the limestone specimen. The dynamic elastic modulus exhibits a nearly nonlinear increase with the strain rate, and the Boltzmann function is employed to analyze the results within the temperature range of 25 to 800 °C, as depicted in Figure 9. At the real temperature of 25 °C, the dynamic elastic modulus of the specimen is 73.5 GPa under a strain rate of 31.0 s⁻¹, while it increases to 142.9 GPa under a strain rate of 48.10 s⁻¹, exhibiting a significant enhancement of 94.4%. For the real-time temperature of 800 °C, the dynamic elastic modulus is 31.84 GPa at the strain rate of 46.74 s⁻¹, which subsequently increases to 92.09 GPa under the strain rate of 95.8 s⁻¹, demonstrating an impressive increase of up to 189.2%. In comparison with those under a temperature of 25 °C, the deformation resistance of the specimen is weakened.

Figure 10 presents the coupling effects of the strain rate and temperature on the dynamic elastic modulus of the limestone specimen. It can be observed that the temperature weakens the dynamic elastic modulus of the limestone specimen. With the increase in temperature, its dynamic elastic modulus decreases. The quantitative relationship can be described as follows:

$$E_{\mathrm{d}}=9.7\times {10}^{-5}{T}^2+3.7\times {10}^{-2}{\dot{\epsilon }}^2-4.1\times {10}^{-3}T\dot{\epsilon }-2.8\times {10}^{-2}T-1.1\dot{\epsilon }+120.4,R^2=0.720$$

(6)

3.4. Macro-Fracture Characteristics

Figure 9 shows the failure characteristics of coal-series limestone after impact at real-time temperature. It can be found that when the real-time temperature is fixed, the degree of macroscopic fragmentation of the sample increases with the increase in impact velocity (strain rate). When the impact velocity is similar (i.e., V = 7.0–8.0 m/s), the degree of macroscopic fragmentation of the sample increases as the real-time temperature increases. When the impact velocity is 6.0–9.0 m/s and the temperature is between 25 °C and 500 °C, the coal-series limestone samples are mostly fractured into several parts from the center after being subjected to impact load, and the shape of the fragments is prismatic. When the temperature is between 600 and 800 °C, the degree of fragmentation of the sample increases, and the shape of the fragments is wedge-shaped. It can be obtained that when the temperature is greater than 600 °C, the initial damage of coal-series limestone increases significantly. When the impact velocity is between 9.0 and 10.0 m/s, the degree of fragmentation of the sample is significantly higher than that at the impact velocity of 6.0–7.0 m/s under the same temperature. From a macroscopic perspective, as the temperature increases, discerning the level of sample fragmentation becomes increasingly challenging. In this case, particle size screening can be used to calculate the average particle size of the broken fragments and the fractal dimension for quantitative analysis.

Table 1. Variation in the macro-fracture characteristics with temperature T (V = 7.0–8.0 m/s).

T = 25 °C	T = 100 °C	T = 200 °C
T = 300 °C	T = 400 °C	T = 500 °C
T = 600 °C	T = 700 °C	T = 800 °C

shows the fracture characteristics of the limestone specimen under the impact loading (V = 7.0–8.0 m/s) at real-time temperatures from 25 °C to 800 °C. As the real-time temperature increases, the macro-fracture degree of the limestone specimen increases. Meanwhile, the specimen is predominantly fragmented into multiple large sections from its core; the fragment shape is prismatic at the real temperature of 25–500 °C. The fragments become smaller and adopt a wedge-shaped morphology at temperatures ranging from 600 to 800 °C, attributed to the significant increase in initial specimen damage.

Table 2. Variation in the macro-fracture characteristics with the impact velocity V (25 °C and 100 °C).

T	Macro-Fracture Characteristics
	V = 6.0–7.0 m/s	V = 7.0–8.0 m/s	V = 8.0–9.0 m/s	V = 9.0–10.0 m/s
25 °C
800 °C

shows the fracture characteristics of the limestone specimen under the impact loading at different impact speeds under the temperature of 25–800 °C. We found that the macro-fracture degree increases as the impact velocity increases. When the temperature is 25 °C, the specimen is mostly fractured into several big blocks from the core, and the fragment shape is prismatic under the impact velocity ranging from 6.0 to 8.0 m/s, and the shape of fragments is wedge-shaped and becomes smaller under the impact velocity ranging from 8.0 to 10.0 m/s. When the temperature is 800 °C, the specimen is mostly fractured into several big blocks under the impact velocity ranging from 6.0 to 7.0 m/s, and the amount of debris further increases, while the shape is smaller under the impact velocity ranging from 7.0 to 10.0 m/s.

4. Discussions

By means of scanning electron microscopy, energy spectrum analysis and mineral component testing techniques, the microscopic fracture morphology, elemental composition and mass ratio of limestone were analyzed, and the microscopic mechanism of strength variation of limestone was summarized.

4.1. Effects of High Temperatures on the Mineral Composition

In order to make the test results easily comparable with each other, the average values of σ_d and E_d of limestone at different temperatures and strain rates are given in

Table 3. Average values of σ_d and E_d of limestone at different temperatures and strain rates.

Mechanical Parameter	Strain Rate (s−1)	Temperatures (°C)
		25	100	200	300	400	500	600	700	800
σd/MPa	40~60	190.8	192.9	165.6	142.1	129.8	119.9	106.7	103.4	72.1
	60~80	-	-	197.1	173.1	160.7	140.5	152.1	-	92.7
	80~100	-	-	-	205.7	180.5	182.6	178.5	145.1	138.1
Ed/GPa	40~60	193.9	182.6	150.9	105.8	106.9	69.8	46.2	40.9	31.7
	60~80	-	-	183.7	138.6	103.1	65.6	55.9	-	32.8
	80~100	-	-	-	209.9	135.3	125.1	99.4	83.3	72.5

. It is well known that dolomite is a complex salt that consists of MgCO₃, CaCO₃, and other impurities, and its thermal decomposition reaction can be expressed by Equations (7) and (8) [51]. Relevant studies have shown that the temperature for the thermal decomposition of MgCO₃ in dolomite is approximately 600 °C [52]. The main component of calcite is CaCO₃, and its decomposition temperature is about 1000 °C. Moreover, its thermal decomposition reaction can be represented by Equation (9). Hence, it can be inferred that calcite does not undergo decomposition in this study.

$${\mathrm{MgCO}}_3\cdot \mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3\to \mathrm{M}\mathrm{g}\mathrm{O}\cdot \mathrm{C}\mathrm{a}\mathrm{O}+2\mathrm{C}{\mathrm{O}}_2$$

(7)

$${\mathrm{MgCO}}_3\to \mathrm{M}\mathrm{g}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2$$

(8)

$${\mathrm{CaCO}}_3\to \mathrm{C}\mathrm{a}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2$$

(9)

Compared with the results obtained at a real-time temperature of 25 °C (Figure 11a), there is a significant increase in the proportion of CaCO₃ in the rock specimen when the temperature reaches 800 °C (Figure 11b). In contrast, there is a notable decrease in the proportion of dolomite. This indicates that a portion of the dolomite undergoes decomposition, leading to alterations in its microstructure and intensifying the initial damage caused by heating [53,54].

Figure 12a–d show the micro-structural characteristics of dolomite in limestone at 800 °C. Compared with Figure 1a, thermal decomposition occurs in dolomite after calcination at high temperatures. The thermal decomposition degree and micro-structural characteristics of dolomite vary across different regions due to the heterogeneity of the specimen and non-uniform heating. Under the action of high temperature, the dolomite undergoes passivation at certain edges and corners, as depicted in Figure 12a,b. Additionally, some crystals become granulated, resulting in the formation of pores and micro-cracks, as presented in Figure 12c,d. This phenomenon is a result of the thermal decomposition of the dolomite accompanied by CO₂ release, which disrupts the original structure of the dolomite and diminishes its mechanical properties. Figure 12e presents the EDS analysis results for the dolomite featured in Figure 12b. Compared with Figure 1c, it is evident that the dolomite undergoes partial thermal decomposition following the high-temperature treatment at 800 °C, as indicated by a decrease in the mass proportions of C and O elements and an increase in those of Ca and Mg elements.

Figure 13a shows the micro-structural characteristics of calcite at 800 °C, and Figure 13b shows the EDS analysis results. It can be observed that the mass proportions of C, O, Ca, and Si elements are 11.0%, 49.8%, 37.2%, and 2.0%, respectively. Under high-temperature conditions, the mass proportions of the C, O, and Ca elements in calcite undergo slight changes, suggesting that there is no thermal decomposition of calcite. However, the calcite of the rock specimens calcined at 800 °C has many microcracks in its mesoscopic characteristics; most of them are intergranular cracks, as indicated by arrow a in Figure 13a, while some are transgranular cracks, as indicated by arrow b in Figure 13a.

4.2. Effects of Temperature-Impact Loading on Microstructure

Figure 14 shows the micro-characteristics of the limestone specimen after impact loading at different real-time temperatures and impact velocities within the range of 9.0–10.0 m/s. Based on the macro-mechanical properties, macro-micro-fracture morphology, and structural characteristics, the action mechanism of real-time temperature on the limestone can be divided into three stages.

When the temperature reaches 25 °C, a few cracks are generated on the fracture surface, showing the brittle fracture characteristics of transgranular cracks and intergranular cracks. The presence of dolomite crystals on the fracture surface suggests that the fracture propagates along the interface of these crystals, as depicted in Figure 14a. The fracture surface becomes relatively smooth with only a few visible cracks as the real-time temperature increases to 100 °C and 200 °C. The stepped brittle fractures are generated on the calcite, and the characteristic morphology of the calcite fracture is marked in the box, as illustrated in Figure 14b,c. The limestone is mainly composed of calcite, dolomite, muscovite, and clay mineral impurities. After heating, crystal expansion is caused, and primary voids are filled to a certain extent, resulting in the densification of the microstructure. Therefore, the temperature has a reinforcement effect on the mechanical properties of the specimen within the real-time temperature range of 25–200 °C.

When the real-time temperature increases to 300–500 °C, the temperature stress inside the specimen gradually increases, and the initial damage becomes more severe. Therefore, the severity of microstructure fracture intensifies under impact loading, leading to an increase in microcracks, as depicted in Figure 14d–f. The mineral components continue to expand during this stage but are limited by the surrounding minerals, and the thermal expansion characteristics of each component are different. The occurrence of extrusion stress is prominent among the components, particularly for prismatic grains like dolomite, resulting in the generation of concentrated stress during extrusion (thermal stress). When the thermal stress reaches the fracture threshold of each component, the failure of the primary structure occurs, and new micro-cracks are generated, which gradually weakens the mechanical properties of the specimen. Consequently, elevated temperatures induce a deterioration in the mechanical characteristics, and the strength of the specimen significantly decreases.

When the real-time temperature reaches 600–800 °C, the high stress generated by the temperature significantly impairs the microstructure, resulting in extensive formation of cracks on the fracture surface. Moreover, the quantity, width, and length of cracks escalate with the elevating temperature, as shown in Figure 14g–i. The physical properties of the mineral components in the specimens are changed due to high-temperature treatment, resulting in plastic fracture features during impact loading, as shown in the box of Figure 14h,i. It can be found that fracture features in Figure 14h,i are different from that in Figure 14b,c. In addition, dolomite also undergoes thermal decomposition and experiences a weakening of the crystal structure, causing fractures and destruction during impact loading, as shown in Figure 14i. This behavior is notably distinct from that illustrated in Figure 14a,d. The thermal expansion causes the weakening of the physical structure of the minerals. Meanwhile, dolomite minerals undergo thermal decomposition due to chemical reactions during this stage. Under the combined action of the physical and chemical effects, thermal damage in limestone rapidly increases, and the performance deterioration of the specimen increases greatly.

Figure 15 shows the variation in the microfracture patterns under different strain rates at a real-time temperature of 800 °C. The microscopic fracture characteristics of the specimen exhibit greater destructiveness as the strain rate increases. Additionally, the impact loading induces an increased number and width of cracks with higher strain rates.

4.3. Challenges of Deep Mining Operations Under High-Temperature Conditions

Safe and green mining of deep coal resources is a strategic development direction for China’s coal industry economy. At present, underground coal mining has entered depths below 1000 m. In complex geological environments, the frequency and intensity of dynamic disasters such as rock bursts and gas outbursts have significantly increased, posing severe challenges to the safe mining of coal resources. Underground Coal Gasification, as a coal in situ chemical mining method, converts coal into combustible gas or raw material gas through scientifically controllable combustion, solving the ecological and environmental damage and pollution problems caused by traditional coal resource mining and utilization from the source [55,56].

In the process of underground coal gasification, the complex load environment is the biggest challenge facing the stability of the overlying rock in the combustion zone. Firstly, during the combustion process of the gasification working face, the ambient temperature remains above 1000 °C. Under the action of high temperature load, the physical and mechanical properties of the overlying rock medium undergo significant changes, manifested as significant thermal damage characteristics such as decreased strength and increased porosity [57]. Secondly, during the process of coal seam gasification, high temperature and high ground stress can cause dynamic disasters such as rock bursts inside the surrounding rock, and the impact stress waves induced by the burst have a direct dynamic load on the overlying rock of the combustion zone [58]. With the advancement of the coal seam gasification working face, the overlying rock in the combustion zone continues to be subjected to high temperatures, and internal thermal damage gradually accumulates. The bearing capacity of the rock layer gradually weakens, and the impact stress wave produces significant impact disturbance in the thermal damage zone of the rock layer, which easily leads to regional sudden changes, instability, and fracture of the overlying rock, thereby inducing large-scale collapse of the overlying rock in the combustion zone. Therefore, the accumulation of thermal damage in the overlying rock medium under high temperature and explosive impact and the disturbance of impact load are the fundamental reasons for the collapse of the overlying rock layer in the combustion zone.

5. Conclusions

In this study, a split Hopkinson pressure bar (SHPB) test system with real-time temperature control was developed, and dynamic tests on limestone taken from deep coal mines were carried out in real-time temperatures from 25 to 800 °C. The XRD, SEM, and EDS tests were conducted to investigate the fracture mechanism of the limestone specimen at real-time temperatures. The main conclusions are as follows:

(1)

Under the coupling effect of real-time temperature and impact loading, the dynamic stress–strain curve of the limestone specimen has no evident compaction stage, while the strain rate demonstrates a nearly linear correlation with the impact velocity across different temperatures. As the temperature increases, the dynamic compressive strength of the limestone specimen decreases linearly at a rate of 0.15 MPa/°C within the strain rate interval of 40–60 s⁻¹, 60–80 s⁻¹, and 80–100 s⁻¹. Additionally, a negative exponential nonlinear relationship is established to describe the variation of elastic modulus with temperature.

(2)

The dynamic compressive strength increases linearly with strain rate at different temperature ranges of 25–800 °C. The temperature sensitivity coefficient η is defined to quantify the influence of temperature on the variation of dynamic compressive strength with strain rate, and a quantitative relationship is established accordingly. Moreover, the correlation between dynamic elastic modulus and strain rate can be characterized by a Boltzmann function. The macroscopic fracture characteristics of the limestone specimen under real-time temperature and the impact load are also obtained.

(3)

The real-time high temperature leads to a decrease in dolomite content and an increase in calcite content due to the thermal decomposition of dolomite, resulting in a significant reduction in the mechanical properties of the limestone specimen. With the increasing real-time temperature, the number of micro-cracks on the fracture surface of the specimen under impact loading is greater, the crack width is wider, and the breakage degree is higher. As a result, the fracture characteristics of the calcite mineral components change from brittle failure to brittle–ductility failure. The action mechanism of real-time high temperatures on the physical and chemical properties of limestone can be categorized into three characteristics: physical reinforcement, physical degradation, and physical and chemical deterioration.