5.1 Three micro-mechanisms of hygroscopic fissure propagation in red mudstone
The crack propagation mechanism of red mudstone primarily includes hydration-swelling and softening, adsorption-wedge failure, and humidity stress field. Hydration-swelling and softening are chemical processes of hydration, expansion, and exfoliation of clay minerals. Clay minerals in red mudstone, such as hydrophilic montmorillonite, easily absorb water, resulting in the formation and expansion of the hydration of the electric double layer (Chenevert et al.,1970; Fairhurst et al.,2002). The clay particles then increase in size owing to crystal expansion and osmotic expansion (Miller et al., 1990; Fukue et al., 1999). Consequently, the resulting expansion pressure reduces the attractive force between the structural connections and the bonds between the particles are broken, leading to the development of mudstone cracks.
Adsorption-wedge failure (Fig. 10) is a mechanical process involving crack initiation, propagation, and coalescence (Terzaghi et al., 1967). Mudstone absorbs water under the action of matric suction. Simultaneously, cracks and pores are conducive to water absorption and are the dominant channels for water migration, resulting in increased capillary pressure and gas pressure. In addition, both pressures tend to promote crack propagation and coalescence (Liu et al., 2020). Consequently, the bond between carbonate and clay minerals is broken by the two pressures, which causes the red mudstone to crack. Thus, water enters the pores and fissures and results in the formation of a strength damage zone with the contacted matrix (Fig. 10), which causes stress concentration at the junction of the strength damage zone and the intact bedrock. Subsequently, under the interaction of capillary pressure and gas pressure, the stress concentration area promotes the generation and propagation of cracks.
The theory of the humidity stress field was proposed by Professor Miao Xiexing (1993; 1995), who highlighted that the diffusion of water in a swelling rock mass is coupled with water content, water absorption, volume deformation, etc. The red-bed mudstone in central Sichuan is a heterogeneous body composed of different minerals. Thus, the expansion coefficients of various minerals are different under the influence of the humidity field, such that the deformation of various mineral particles is different after mudstone encounters water. as mudstone is a continuum, to ensure the continuity of deformation, various mineral particles in the mudstone cannot freely deform according to their inherent expansion coefficient with the change in humidity. Therefore, there exist mutual constraints among clay mineral particles, and those with large deformations are compressed, while those with small deformations are stretched. Consequently, a type of humidity structural stress was formed in the mudstone. Moreover, maximum stress often occurs at the junction of the mineral particles, which upon reaching or exceeding the tensile strength of mudstone, results in the connection between the mineral particles being broken, thereby resulting in cracks (Ji et al., 2009; Xu et al., 2018).
When mudstone is self-absorbed, hydration expansion and softening reduce the strength of the mudstone itself, thereby rendering it easier for adsorption-wedge pressure and humidity stress to break the bond between mudstone particles and form cracks. Consequently, the appearance of cracks provides an advantageous channel for the migration of water and enhances the adsorption-wedge failure of mudstone and the humidity stress field effect. Moreover, in the process of mudstone self-absorption, the three mechanisms complement each other and work together to accelerate the fracture of the connection between mudstone particles, as well as the generation, expansion, and penetration of cracks.
5.2 Four influences of pH value on the propagation of mudstone fissures
The influence of acidic and alkaline solutions on the fracture expansion of red-bed mudstone primarily includes the following four aspects: mineral dissolution, ion absorption and exchange, particle association, and pore changes. The dissolution of minerals affects the chemical decomposition process of the red-bed mudstone in central Sichuan, which mainly manifests as dissolution and erosion. The dissolution equation of mudstone clay minerals in an acid-base environment is shown in Table 2. As shown in Table 2, clay minerals are more soluble in acidic environments, illite and montmorillonite are dissolved and transformed into kaolinite in an acidic environment, and mudstone contains kaolinite. Therefore, in acidic environments, the kaolinite content in mudstone is significantly greater than that in neutral and alkaline environments. Further, kaolinite exhibits high hygroscopicity, thereby promoting hydration swelling and softening (Yong et al., 1979), which causes the final water absorption rate of mudstone in acidic environments to be higher than that in neutral and alkaline environments. In addition, H+ dissolves haematite and breaks the carbonate cementation in calcite (Zhang et al., 2018), which promotes the generation, expansion, and penetration of cracks in acidic environments.
Table 2
Dissolution reaction equation of red-bed mudstone clay minerals (Li et al., 2016; Feng et al., 2010; Yuan et al., 2019)
solutions | minerals | chemical reaction equations |
---|
acid | Quartz | SiO2 + 2H2O = H4SiO4 |
| Albite | NaAlSi3O8 + 4H++4H2O = Na++Al3++3H4SiO4 |
| Calcite | CaCO3 + 2H+=Ca2++H2O + CO2 |
| Hematite | Fe2O3 + 6H+=2Fe3++3H2O |
| Montonorillonite | Nax(H2O)4{(Al2 − xMgx)[Si4O10](OH)2}+H2O + H+→Na++Mg2++Al2O3(2SiO2)(2H2O) + H4SiO4 |
| Illite | K{Al2[AlSi3O10](OH)2}+H2O + H+→Al2O3(2SiO2)(2H2O) + K+ |
Neutral | Quartz | SiO2 + 2H2O = H4SiO4 |
| Calcite | CaCO3 = Ca2++CO32− |
| Albite | NaAlSi3O8 + 5.5H2O = 0.5Al2Si2O5(OH)4+K++OH−+2H4SiO4 |
Alkaline | Quartz | SiO2 + 2OH−=H2SiO42− |
| Albite | NaAlSi3O8 + 6OH−+2H2O = Na + + Al(OH)−4+3H2SiO42− |
| Calcite | CaCO3 = Ca2++CO32− |
Ion exchange is an exchange process between ions and molecules adsorbed to mineral particles via physical and chemical forces and water (Kozaki et al., 2005). Owing to the large specific surface area of clay minerals, a large amount of colloidal substances are adsorbed on the surface, and ion exchange with ions in water is easier. Further, when mudstone is self-absorbed in an acidic solution, H+ can be replaced by cations such as K+, Al3+, Ca2+, and Mg2+, which may cause the crystal lattice to separate from the water phase, which results in the dissolution of montmorillonite (Townsend et al., 1984). In addition, in the acidic environment of mudstone, bond breaks are usually formed on the edges or non-cleavage surfaces of clay mineral particles in contact with water after the minerals are dissolved, and bond breaks increase with a decrease in clay minerals. As both sides of the broken bond are charged positively and negatively, this effect promotes the ion exchange capacity. Consequently, the ion exchange adsorption results in the transformation of minerals in the mudstone self-absorption process, and thus the cementation between particles is destroyed, thereby promoting the formation of mudstone fissures.
The association of particles affects the chemical decomposition of clay minerals in the mudstone of the central Sichuan red bed when it comes in contact with water, thereby affecting the generation, expansion, and penetration of moisture-absorbing fissures in the mudstone. In addition, the combination of the two particles reflects the dominant interparticle force. There exist three common types of minimum energy particle flocculation: edge-to-face, edge-to-edge, and face-to-face (Olphen et al., 1977). When H+ and OH− concentrations are increased various charges are generated on the exposed edges of mineral particles, resulting in a gradual increase in positive or negative charges on the outer surface of the particles. Therefore, when the diffusing double layer and the force between the particles change, the charges between the particles form different combinations (Santamarina et al., 2002). Under acidic conditions, the Coulomb force between the positive and negative sides results in flocculation between the sides of the particles. However, this flocculation state is unstable, the structure is loose, and there exist sufficient space between the particles, which is good for adsorbing more water and promoting the moisture absorption of mudstone. In an alkaline environment, negatively charged particles form face-to-face flocculation through osmotic repulsion and van der Waals forces, and the structure formed is relatively stable, with the particles being closely arranged. However, this is not conducive to water adsorption. Under neutral conditions, owing to the decrease in the Coulomb repulsive force from edge to edge and the repulsive force from face to face, edge to edge flocculation can be observed. In addition, the stability of this state is between that of the previous two (Vanolphen et al 1951).
Pore changes cause a change in the water migration rate of the red-bed mudstone, thereby affecting the rate of crack expansion in the red-bed mudstone. The pore size classification in coal is based on the Hodot pore classification method (Li et al 2015), and are classified as follows: micropores (D < 10 nm), transition pores (10 ≤ D < 100 nm), mesopores (100 ≤ D < 1 000 nm), and macropores (≥ 1 000 nm). Figure 12, 14, 16, and 13, 15, and 17 show the pore size-pore volume distribution and pore size-specific surface area distribution of mudstone under different pH conditions, respectively. The pore structure and pore size of mudstone are shown in Figure (Wang et al 2021). Combined with Table 3, it is evident that when pH = 3, the mudstone pore size-pore volume distribution curve has a peak when the pore size is 17.11 nm. Moreover, micropores and transitional pores contributed more to the total pore volume, with the values being 0.0942 and 0.1620 mL·g− 1, respectively, at a contribution rate of 46.29%. Simultaneously, the specific surface area curve of mudstone has a single peak at a pore size of 6.48 nm, where the micropores and transition pores contributed the most to the total surface area, with surface area values of 18.5659 and 9.3206 m2·g− 1, respectively, at a contribution rate of 99.7%. Further, when pH = 7, the mudstone pore size-pore volume distribution curve has a peak when the pore size is 17.11 nm, with micropores and transitional pores contributing more to the total pore volume having values of 0.1 and 0.1678 mL·g− 1, respectively, and a contribution rate of 60.37%. Moreover, the mudstone specific surface area curve has a single peak at a pore size of 13.73 nm, wherein micropores and transition pores contributed the most to the total surface area, with specific surface area values of 19.9116 and 11.6297 m2·g− 1, respectively, at a contribution rate of 99.76%. Furthermore, when pH = 11, the mudstone pore size-pore volume distribution curve has a peak when the pore size is 17.11 nm. Herein, the micropores and transitional pores contributed more to the total pore volume, with values of 0.1163 and 0.2222 mL·g− 1, and the contribution rate reached 61.88%. Simultaneously, the specific surface area curve of mudstone has a single peak at 6.03 nm pore size, with the micropores and transition pores contributing the most to the total surface area, with values of 20.9757 and 12.6972 m2·g− 1, respectively, at a contribution rate of 99.71%. Thus, the analysis showed that with an increase in the pH value, the pore volume and the proportion of the pore volume of the micropores and transition pores in the mudstone gradually increased, which indicates that the acidic environment promotes the transformation of mudstone micropores and transitional pores into mesopores and macropores. Therefore, an acidic environment is conducive to the generation, expansion, and penetration of mudstone water and moisture absorption fissures.
Table 3
Distribution results of pore size-pore volume/specific surface area of mercury intrusion experiments under different pH conditions
| Pore size d(nm) | d < 10 | 100 > d > 10 | 1000 > d > 100 | 100000 > d > 1000 | d > 100000 | dHg (Full pore size) |
PH = 3 | Pore volume(10− 4mL/g) | 0.0942 | 0.1620 | 0.1620 | 0.1268 | 0.0084 | 0.5534 |
Pore volume ratio(%) | 17.020 | 29.267 | 29.267 | 22.921 | 1.525 | 100 |
Specific surface area(10− 4m2/g) | 18.5659 | 9.3206 | 0.0667 | 0.0173 | 0.0001 | 27.9706 |
Ratio of specific surface area(%) | 66.37655 | 33.32290 | 0.23834 | 0.06171 | 0.00051 | 100 |
PH = 7 | Pore volume(10− 4mL/g) | 0.1000 | 0.1678 | 0.0534 | 0.1143 | 0.0081 | 0.4436 |
Pore volume ratio(%) | 22.540 | 37.826 | 12.042 | 25.777 | 1.815 | 100 |
Specific surface area(10− 4m2/g) | 19.9116 | 11.6297 | 0.0627 | 0.0134 | 0.0001 | 31.6175 |
Ratio of specific surface area(%) | 62.97652 | 36.78254 | 0.19829 | 0.04223 | 0.00042 | 100 |
PH = 11 | Pore volume(10− 4mL/g) | 0.1163 | 0.2222 | 0.0944 | 0.1069 | 0.0072 | 0.5471 |
Pore volume ratio(%) | 21.257 | 40.622 | 17.257 | 19.541 | 1.323 | 100 |
Specific surface area(10− 4m2/g) | 20.9757 | 12.6972 | 0.0856 | 0.0112 | 0.0001 | 33.7698 |
Ratio of specific surface area(%) | 62.11384 | 37.59916 | 0.25351 | 0.03313 | 0.00036 | 100 |
Thus, the four processes of mineral dissolution, ion absorption and exchange, particle association, and pore change synergistically affect the chemical decomposition and physical decomposition of the red mudstone in central Sichuan through hydration, expansion, dissolution, expansion, and exfoliation. This results in three distinct mechanisms in different pH solutions having different effects. Therefore, the final moisture absorption rate and final fracture rate of mudstone at different pH values are quite different.