1. Introduction
Cold regions are widely distributed worldwide, accounting for approximately 75% of China’s land area [
1]. Rocks are formed by mineral grains, pores, and microcracks [
2,
3], and the characteristics of rocks are greatly influenced by the environment they sustain [
4,
5,
6]. With the development of transportation, infrastructure, and energy facilities, numerous rock engineering projects, such as tunnels, highways, slopes, and open pit mines, are planned or constructed in cold regions [
7,
8,
9]. The temperature in the cold areas fluctuates around 0 °C with the change in seasons and the transition of day and night, leading to periodic freezing and thawing of water in the rock, and the physical and mechanical properties of rocks can be irreversibly damaged under freeze–thaw cycles [
10,
11,
12,
13]. Thus, rock engineering located in cold areas is exposed to a freeze–thaw environment, which is a severe threat to the safe construction and healthy operation of the engineering project [
14,
15]. As a result, the multi-scale physical and mechanical characteristics of rocks, such as strength, durability, and porosity, have been investigated by researchers for decades [
13,
16,
17].
The effects of freeze–thaw cycles on the mechanical characteristics of rocks were investigated through a series of laboratory experiments [
18,
19,
20,
21]. For example, Mousavi et al. [
22] measured the physical and mechanical properties of calc-schist rock samples from an open-pit mine under frozen and thawed processes. They indicated that the main mechanical properties of calc-schist rock, such as cohesion, internal friction, and triaxial compression strength, decrease with freeze–thaw cycles. Zhang et al. [
23] tested the mechanical properties of sandstones that experienced different freeze–thaw cycles with four kinds of confining pressures and analyzed the whole process characteristics of the deformation and failure of sandstone. Wang et al. [
24] selected fine sandstone and coarse sandstone as representatives of hard rocks and studied physical and triaxial compression mechanical properties under the freeze–thaw environment. Fang et al. [
25] investigated the coupling effects of chemical corrosion and freeze–thaw cycles on the mechanical characteristics of yellow sandstone. They concluded that both chemical corrosion and freeze–thaw processes adversely impact the rock’s mechanical characteristics. Mu et al. [
26] conducted shear tests on three typical types of jointed rocks under the freeze–thaw environment and examined the degradation characteristics through the evolution of shear parameters. Luo et al. [
27] analyzed the dynamic mechanical properties of rock under dynamic loading and freeze–thaw action and obtained the static and dynamic mechanical parameters of sandstone. Martinez-Martinez et al. [
28] conducted a long-term test to investigate the resistances of six different limestone types, and the evolution of their properties under the freeze–thaw weathering process was obtained.
The microstructure of rocks directly impacts the mechanical characteristics. Thus, the micropores and cracks induced by freeze–thaw cycles can also be detected through nondestructive tests such as Nuclear Magnetic Resonance (NMR) and Scanning Electron Microscope (SEM) [
29,
30]. Liu et al. [
31] used the NMR technique to study the freeze–thaw damage degradation of sandstone with initial damage, and the porosity, T
2 spectrum distribution, and the T
2 spectral area were obtained. Tian et al. [
32] used NMR and SEM techniques to monitor the micropore evolution of three soils under repeated freeze–thaw cycles. Martínez-Martínez et al. [
28] conducted SEM tests to study the microstructure evolution of polished rocks after freeze–thaw cycles. Ke et al. [
33] conducted NMR tests and impact loading experiments on sandstone under different freeze–thaw cycles. They indicated that with the increase in freeze–thaw processes, the pore expands, and the pore size tends to be uniform. Park et al. [
34] conducted artificial weathering tests in the laboratory and SEM images were obtained to investigate the microstructure evolution of diorite, basalt, and tuff specimens with freeze–thaw cycles.
The previous articles from the literature presented the physical and mechanical deterioration of various rocks exposed to freeze–thaw cycles. However, previous research has mainly concentrated on sandstone, granite, marble, limestone, quartz, shale, amphibolite, diorite, basalt, tuff, etc. [
16,
19,
35,
36]. Few studies have involved the multi-scale deterioration mechanisms of anhydrite rock under a freeze–thaw environment. It has been reported that anhydrite rock will swell and dissolve when encountering water, causing a decrease in its bearing capacity and durability [
37]. Consequently, it is determined that the freeze–thawed damage mechanisms of anhydrite rock are different from other rocks. Thus, in this paper, a series of multi-scale laboratory tests were conducted systematically to reveal the deterioration mechanisms of anhydrite rock exposed to freeze–thaw cycles. The multi-scale physical and mechanical evolution characteristics were obtained, and the relationship between the pore structure and mechanical strength was established. Finally, the deterioration mechanisms of anhydrite rock exposed to the freeze–thaw environment were discussed.
4. Multi-Scale Deterioration Mechanism of Anhydrite Rock under Freeze–Thaw Environment
4.1. Correlation Analysis between Pore Structure and Macroscopic Mechanical Parameters
Porosity is a comprehensive parameter that reflects the pore structure of porous media. However, different pore structures influence the macroscopic mechanical parameter evolution of anhydrite rocks differently.
The relationship between pore area and mechanical strength is depicted in
Figure 21 and
Figure 22. As can be seen with the increase in the area of micropores, the mechanical strength of anhydrite rock increases exponentially. Meanwhile, with the increase in the area of mesopores and macropores, the mechanical strength decreases exponentially.
The mean correlation coefficient between the compression strength and the area of micropores, mesopores, and macropores is 0.75, 0.74, and 0.97, respectively. The correlation coefficient between the tension strength and the area of micropores, mesopores, and macropores is 0.75, 0.76, and 0.99, respectively. Thus, it can be concluded that macropores play the most significant role in the evolution of the mechanical parameters of anhydrite rock under the freeze–thaw environment.
4.2. Deterioration Mechanism Analysis
The mechanical loss rate is used to evaluate the deterioration of mechanical parameters of rocks that experience different freeze–thaw cycles. The mechanical loss rate can be calculated by Equation (4):
where
is the mechanical loss rate,
is the mechanical parameters of the untreated samples, and
is the mechanical parameters of samples treated with
N freeze–thaw cycles.
Combined with the previous literature, it is found that the mechanical loss rate of anhydrite rock is higher than that of rocks with a similar porosity under the same freeze–thaw conditions. For example, when the number of freeze–thaw cycles is 60, the UCS loss rate of anhydrite rock and medium-grained feldspathic sandstone [
42] is 27.23% and 6.6%, and the elastic modulus of anhydrite rock and sandstone [
24] is 38.65% and 6.76%, respectively. When the number of freeze–thaw cycles is 120, the UCS loss rate of anhydrite rock and granite [
43] is 46.54% and 13.9%, and the cohesion loss rate of anhydrite rock and sandstone [
44] is 52.48% and 36.02%, respectively.
The above analysis indicates that when the initial porosity is similar, the damage to anhydrite rock under freeze–thaw cycles is more severe than that of other rocks. The freeze–thaw damage mechanism of rocks includes the volume expansion mechanism, hydrostatic pressure mechanism, capillary mechanism, and crystallization pressure mechanism. Zhou et al. [
45] point out that the damage caused by capillary and crystallization pressure dominates when the rock freezing rate is low. On the contrary, when the freezing rate is high, the volume expansion and hydrostatic pressure mechanisms dominate. The damage mechanism of anhydrite rock is different from other rocks due to the complex water–rock interaction effect. It is believed that in addition to the freeze–thaw deterioration mechanisms mentioned above, water–rock expansion and water–rock dissolution deterioration effects also exist in the damage process of freeze–thawed anhydrite rock. The water–rock expansion effect is due to the volume expansion caused by the formation of dihydrate gypsum (CaSO
4·2H
2O) when anhydrite (CaSO
4) meets water. The water dissolution effect is caused by the dihydrate gypsum (CaSO
4·2H
2O) dissolved in water. The connection between the mineral crystals is hydrolyzed, and the cohesion is reduced. The water–rock expansion and water–rock dissolution reaction equations are as follows:
During the freeze–thaw process, different deterioration mechanisms promote each other, leading to the deterioration of the physical and mechanical properties of anhydrite rock. Therefore, when the porosity and saturation are similar, the freeze–thaw damage to anhydrite rock is more severe than other rocks, leading to a higher proportion of macropore structures under freeze–thaw action.
5. Conclusions
In this work, comprehensive multi-scale laboratory technologies involving mass variation, uniaxial compression, triaxial compression, Brazilian splitting, NMR, and SEM tests were performed to study the deterioration mechanisms of anhydrite rock under a freeze–thaw environment. As a result, the mass variation, main mechanical characteristics, porosity, and microstructure evolution of anhydrite were obtained. Then, the relationships between the microstructure and the mechanical characteristics were established, and the freeze–thawed deterioration mechanisms of anhydrite rock were revealed. The main conclusions can be summarized as follows:
- (1)
The macroscopic test results indicate that the mass variation increases exponentially with the increase in freeze–thaw cycles, while the mechanical strength, elastic modulus, and cohesion decrease exponentially. Meanwhile, as the freeze–thaw cycles increase, the frost resistance coefficient decreases, while the damage variable increases.
- (2)
The porosity of anhydrite rock increases with the freeze–thaw cycles, and the mean porosity increases by 66.27% after 120 cycles. With the increase in freeze–thaw cycles, the area of micropores (r ≤ 0.1 μm) and PT-Ipore throat (0–0.1 μm) decreases exponentially. In comparison, the area of mesopores (0.1 μm < r < 1 μm), macropores (r ≥ 1 μm), and PT-II pore throat (0.1–4 μm) increases exponentially. Under the freeze–thaw treatment, the roughness of the sample gradually increases, and for the samples treated with 120 cycles, there is a significant honeycomb and pitted surface phenomenon.
- (3)
The correlation analysis between microstructure and macroscopic mechanical parameters shows that macropores play the most significant role in the mechanical parameters evolution of anhydrite rock under the freeze–thaw environment.
- (4)
It is found that the mechanical loss rate of anhydrite rock is higher than that of rocks with a similar porosity under the same freeze–thaw conditions. Finally, it is revealed that the water–rock expansion and water dissolution effects play a crucial role in the multi-scale damage of anhydrite rock under a freeze–thaw environment.