As mentioned above, macroscopic mechanical parameters (strength, elastic modulus) and microscopic test parameters (T2 spectral distribution, T2 spectral area, and porosity) were utilised to explain the damage and deterioration effect of rock samples under different pH values and dry-wet cycles. To further grasp the changes in the composition and microstructure of rock samples during the dry-wet cycle more intuitively, rapidly, and precisely, to explain the relationship between chemical composition changes and damage, and to summarize and analyse the physical and chemical damage laws. Scanning electron microscopy (SEM) and X-ray diffraction(XRD) were employed in this part to investigate changes in the structure and composition of the surface of rock samples before and after the dry-wet cycle, yielding complete information on the microstructure, structure, and composition of rock samples (Huang et al., 2020).
4.1 Mechanism of microstructure damage
After dry-wet cycles, scanning slices of a specific size were collected from the same position of the rock sample for the SEM test (Yang et al., 2019). SEM images of rock samples with various pH values after 1, 6, and 16 dry-wet cycles are shown in Fig. 9. It can be clearly seen from the figure that the dry-wet cycle has a considerable effect on the micro-morphology (grains, pores, and cracks) of rock samples with varying pH values.
Minerals in rock samples are primarily degraded by hydrolysis in a neutral environment when pH = 7. When the number of cycles is 1 and 6, the microstructure of the sandstone is uniform and compact, with no obvious cracks and a small amount of dispersed natural pores. This microscopic phenomenon can partially explain the rock sample integrity in Section 3.3 (lower porosity) and the mechanical properties in Section 3.4 (higher peak stress and elastic modulus) under the same conditions. However, as the number of cycles reaches 16, the number of pores and cracks in the scanned image increases, and the particle boundaries are more visible. It shows that the internal organizational structure of rock samples changes dramatically as the number of cycles even in a neutral solution.
When pH = 5, the number of pores rises, the fractures lengthen, and the particle boundary gradually becomes visible, which is relatively substantial even after 6 cycles, as shown in Fig. 9(b). When pH = 3, the reaction between acidic ions and minerals inside the rock sample is intensified, and the dissolution of cement is gradually accelerated, resulting in significant cement loss between particles and the creation of pores or cracks. When the number of cycles reaches 16, the macro-pores expand and the cracks become more visible. This is because when the acidity of the solution grew, the acid solution progressively corroded the interior of the sample, the pore radius gradually increased, and the sample's connectivity improved, allowing the acid solution to enter the interior of the sample. Finally, under the recurrent action of dry-wet cycles, the pore structure of rock samples changes fast(Huang et al., 2020).
From a microscopic point of view, it is further demonstrated that the combined action of acidic solution and the dry-wet cycle has a major influence on the microstructure of sandstone and that the damage and deterioration of the microstructure of sandstone in an acidic environment is greater than in a neutral environment.
4.2 Deterioration mechanism of acid solution
Under long-term cycles, the rock minerals are prone to react with H+ ions in solution, generating a large number of free ions and altering the occurrence state of mineral particles. Moreover, the dissolution of certain minerals in the rock can promote the development of fractures, and the combined action of physics and chemistry accelerates rock failure (Matsumoto et al., 2017). Furthermore, different pH samples after dry-wet cycles were selected to analyze the changes in minerals and chemical components and evaluate their potential contribution to rock degradation(Zhang et al., 2021).
The mineral composition analysis reveals that the rock sample is mostly quartz, albite, kaolinite, biotite,and calcite, all of which are susceptible to dissolving reactions with H+ ions in acidic solutions. The XRD pattern of the rock sample in Fig. 10 shows that calcite and biotite have vanished, and new products calcium silicate and gypsum have formed, although the predominant substance on the sample's surface remains quartz, which is almost insoluble in acidic solution. The following are the primary ion reactions:(Feng and Ding, 2011)。
\(N{a^+}\) 、\(A{l^{3+}}\)and precipitated protosilicate are formed when albite combines with \({H^+}\). The following is the particular reaction equation:
\(NaAlS{i_3}{O_8}+4{H^+}+4{H_2}0=A{l^{3+}}+N{a^+}+3{H_4}Si{O_4}\)
The specific reaction equation of kaolinite with\({H^+}\)is as follows, and forms silicic acid:
\(A{l_2}S{i_2}{O_5}{(OH)_4}+6{H^+}=2A{l^{3+}}+2{H_2}Si{O_3}+3{H_2}O\)
It can be seen from the figure that the calcite content reduced most significantly, especially in the acidic solution, indicating that the dissolution rate of calcite between particles is much higher than that of other particles. The specific reaction equation of calcite and\({H^+}\)is as follows:(Lee et al., 2011)
\(CaC{O_3}+2{H^+}{\text{=}}C{a^{2+}}+C{O_2}+{H_2}O\)
\({\text{KF}}{{\text{e}}_{\text{3}}}{\text{AlS}}{{\text{i}}_{\text{3}}}{{\text{O}}_{{\text{10}}}}{{\text{(OH)}}_{\text{2}}}{\text{ +10}}{{\text{H}}^{\text{+}}}{\text{ =}}{{\text{K}}^{\text{+}}}{\text{ +3F}}{{\text{e}}^{{\text{2+}}}}{\text{ +A}}{{\text{l}}^{{\text{3+}}}}{\text{ +3}}{{\text{H}}_{\text{4}}}{\text{Si}}{{\text{O}}_{\text{4}}}\)
In addition, free are combined\(S{O_4}^{{2 - }}\)to form gypsum, and free\(C{a^{2+}}\)ions combined with \(Si{O_3}^{{2 - }}\)to form calcium silicate:
\(C{a^{2+}}+S{O_4}^{{2 - }}=CaS{O_4}\)
\(C{a^{2+}}+Si{O_3}^{{2 - }}=CaSi{O_3}\)
When the results of the NMR test, the mechanical test, and the deterioration mechanism analysis are combined, it can be concluded that the mineral particles of sandstone undergo dissolution and hydrolysis under the combined action of acid solution and dry-wet cycles, resulting in the gradual loosening of the organizational structure between the particles, making the pores continue to expand and evolve into large pores, and the damage deterioration effect to become apparent. Furthermore, the mineral debris transforms into clay minerals and a variety of free products, which are lost over time, causing sandstone mechanical parameters and porosity degradation to continue to rise(Huang et al., 2022). And the degree of deterioration is positively correlated with the degree of acidity.
4.3 Damage variable
Damage is the deterioration of a material due to internal faults under load or in the environment. As a result, damage to rock caused by long-term dry-wet alternation is referred to as dry-wet cycle damage. The acid solution's corrosion may lead to the formation of fissures, and crack propagation exacerbates the acid solution's penetration into the rock sample. Corrosion and fracture propagation enhance each other and accelerate the damage rate of rock samples when dry and wet cycles are repeated. The concept of continuity \(\psi\) is proposed to describe the gradual decay of materials(Kachanov, 1958) :
$$\psi =\frac{{\tilde {A}}}{A}$$
2
where: is the cross-sectional area before damage; \(\tilde {A}\) is the effective bearing area after damage.
The porosity measured by NMR technology is defined as continuity in this research. The initial porosity \({n_0}\) of rock samples are obtained by saturating for 24 hours prior to dry-wet cycles, measuring the porosity of rock samples after the dry-wet cycle. Because the dry-wet cycle has little influence on the total volume \({V_0}\) of the rock sample, assuming that the total volume \({V_0}\) of the rock sample is constant before and after the dry-wet cycle, there are:
$$\tilde {V}={V_0}(1 - n)$$
3b
Combining equations (2) to (3), the expression of the degree of continuity may be found from the definition of the degree of continuity:
$$\phi =\frac{{\tilde {V}}}{V}=\frac{{{V_0}(1 - n)}}{{{V_0}(1 - {n_0})}}=\frac{{1 - n}}{{1 - {n_0}}}$$
4
The damage variable D was proposed by (Rabotnov, 1963) when studying the creep constitutive relation:
$$D={\text{1}} - \psi$$
5
Where: D is the scalar, D = 0 is the initial undamaged state, and D = 1 is the complete damage state.
$${D_n}={\text{1}} - \frac{{\tilde {V}}}{{{V_0}}}=1 - \frac{{1 - n}}{{1 - {n_0}}}$$
6
Equation (6) is the relationship between the porosity of the rock sample and the damage variable after the dry-wet cycle.
The initial porosity\({n_0}\)of rock sample is 9.60%. The initial porosity and the fitting equation obtained in Fig. 5 were substituted into Eq. (6), yielding a functional relationship between the number of cycles and the damage variable with different pH values. The functional relationship is shown in Fig. 11.
The amount of sandstone damage caused by various acidic solutions grows steadily as the number of cycles increases, whereas the growth rate reduces gradually. The curve climbs approximately linearly at the stage of 0–6 dry-wetting cycles, showing that the pore cycle in the rock sample structure grows rapidly at the first stage and the damage growth rate is relatively significant. After 6 dry-wet cycles, the slope of the curve progressively reduces, suggesting that the sandstone porosity expands and the evolution rate falls continually under the action of dry-wet cycles, indicating that the damage amount growth rate gradually declines. As the number of cycles rises, the curve eventually flattens out. This is because, under the action of long-term dry-wet cycles, the pores inside the rock sample structure have been fully developed, the initiation of new pores and the expansion of primary pores have progressively stabilized, and the damage amount has gradually diminished. According to the curve in Fig. 10, the damage degree of pH = 3 is always higher than that of pH = 7 under the same number of dry-wet cycles, and the damage degree of pH = 3 is the highest after 16 cycles, which further indicates that the acidic solutions have a significant impact on the damage and deterioration of sandstone.