# Experimental Study of the Pore Structure Deterioration of Sandstones under Freeze-Thaw Cycles and Chemical Erosion

Authors: Jielin Li; Rennie B. Kaunda; Longyin Zhu; Keping Zhou; Feng Gao
Journal: Advances in Civil Engineering
 (2019)

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## Abstract

The issue of rock deterioration in chemical environments has drawn much attention in recent years in the rock engineering community. In this study, a series of 30 freeze-thaw cycling tests are conducted on sandstone samples soaked in H2SO4 solution and in pure water, prior to the application of nuclear magnetic resonance (NMR) on the rock specimens. The porosity of the sandstone, the distribution of transverse relaxation time T2, and the NMR images are acquired after each freeze-thaw cycle. The pore size distribution curves of the sandstone after freeze-thaw cycles, four categories of pore scale, and the features of freeze-thaw deterioration for pores of different sizes in H2SO4 solution and pure water are established. The result shows that, with the influence of the acid environment and the freeze-thaw cycles, the mass of the samples largely deteriorates. As the freeze-thaw cycles increase, the porosity of rocks increases approximately linearly. The distribution of the NMR T2 develops gradually from 4 peaks to 5 or even to 6. Magnetic resonance imaging (MRI) dynamically displays the process of the freeze-thaw deterioration of the microstructure inside the sandstones under acid conditions. The results also show pore expansion in rocks under the coupling effects of chemistry and the freeze-thaw cycles, which differ largely from the freeze-thaw deterioration of the rock specimens placed in pure water.


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## Body

## 1. Introduction

The issue of rock deterioration in chemical environments has drawn much attention in recent years in the rock engineering community due to hazards and risks posed to engineered structures. Corrosion due to hydrochemical solutions can cause changes in mineralogical composition and structure of rocks leading to the transformation of the physical mechanical properties of rocks. Therefore, the hydrochemically altered state of rock can pose a great threat to the long-term stability of rock engineering structures [1, 2]. The chemical elements in solution can drive the expansion of cracks and the rate of the crack expansion. In addition, the stress strength factor and the stress strength coefficient of quartz materials are influenced by deionized water, HCl, NaOH, and other acids and bases [3].The chemical atmospheres may cause damage to all types of stones, especially calcite stones, and the effect of acid atmospheres is related with porosity [4]. In the chemical environment, the mineral components in rocks tend to react with solutions, via complex processes and mechanisms. Previous studies have been carried out to investigate the characteristics of chemical deterioration of rocks; for instance, Feucht and Logan [5] evaluated the effect of chemical solutions (NaCl, CaCl2, and Na2SO4 solutions) on the frictional properties of quartz-rich sandstones with triaxial compression tests which were made with varying ionic strengths and pH values. Feng et al. [6–8] conducted a good number of laboratory experiments and theoretical analyses, some advanced instruments were used to study the microstructural damage of rocks in the chemical environment, such as CT, SEM, and X-ray analysis, and the research ideas for studying the fracture criteria of fractured rock considering the effect of chemical solution were provided. Other scholars also conducted investigations on the rock damage constitutive model under the effect of chemical corrosion [9–11]. The results from these previous achievements have provided references for studies of the long-term stability of rock engineering in chemical environments.As the research continues to advance, the issues of rock deterioration under the multifactor coupling conditions of pressure, chemical, and other factors have drawn increasingly more attention, such as the rock creep behaviour in a chemical environment [12], the damage of physicomechanical parameters of rocks under the coupling effects of chemical environment and stress loading [13, 14], and the mechanical properties of triaxial compression of rocks immersed in different water chemical solutions [15]. However, besides the influence of the complex hydrochemical environment on rock, there is an essential need to study the damaging effect of the freeze-thaw cycles in cold regions on a rock mass [16]. In mines located in cold regions, for example, the freeze-thaw disasters of the rock mass in mines can occur concurrently with the corrosion of the acid waste water, acid rain, or the deicing saline solution used at the mines, whose influences have been known to interact in certain cases [17]. Some research has been performed on the deterioration of the rock mechanics under the decoupling influence of chemistry and the freeze-thaw. Ning et al. [18] conducted the splitting tests of the concrete under the coupling effect of the acid corrosion and the freeze-thaw cycles, studied the features of the surface erosion and the mechanical properties of the concretes that are of different pH values and being processed by different numbers of freeze-thaw cycles, and analyzed the splitting process of the concrete corroded by acid and the freeze-thaw. They found that the acid erosion of concrete is inversely proportional to the pH value, and the erosion begins on the surface of the concrete. The coupling effects of acid erosion and freeze-thaw cycles accelerate the deterioration of mechanical properties of concrete. In the weak acid environment, freeze-thaw cycles play a leading role in concrete erosion. As the pH value decreases, the effect of acid on concrete gradually increases. Shi et al. [19] studied the changes of different chemical diluents in the concrete of Portland in the freeze-thaw environment and adopted SEM to study the mechanism of formation of pores and cracks in the test specimens, and the results show that NaCl, the NaCl-based deicer, and the K-acetate-based deicer were the most deleterious to the concrete. Lu et al. [20] carried out the indoor simulation tests of concretes under the conditions of the dual corrosion of acid-base and freeze-thaw, and he found that both strong acid and alkali are corrosive to concrete. The corrosion of acid, alkali, and freeze-thaw cycle dominates at different times, accelerating the deterioration of mechanical properties concrete. Gao et al. [21] conducted a freeze-thaw cycle experiment on rock samples which were soaked in four different solutions: sulfuric acid solution, sodium hydroxide solution, sodium chloride solution, and pure water, and the microstructural changes of each group of rock samples were tested by nuclear magnetic resonance (NMR). The results have shown that the influences of sodium hydroxide and sodium chloride solutions are the most significant, and the influence of sulfuric acid solution is relatively weak. In the meantime, NMR results show the chemical solutions have great effects on the microstructure of rocks.In the previous studies, the research objectives are mainly aimed at the physical and mechanical properties of buildings in cold regions under chemical conditions. Therefore, the reported previous studies tend to take concrete as the research objective when studying the mechanical properties of the rocks under the influence of chemistry and freeze-thaw. Given that the focus is on macroscopic fracture features, very few of the studies have evaluated mechanisms related to the structural deterioration of pores; therefore, the results are somewhat limited in terms of mechanisms and characteristics of the deterioration of the rock mass. In this study, with the impact of acid waste water at mines as the engineering background, rock is selected as the research object. The study discusses freeze-thaw cycling tests of sandstone in the acid environment, the acquisition of the mass of the rock specimens, and observed changes, such as the exterior alteration. It also analyzes and discusses the characteristics of the freeze-thaw deterioration of the microstructure of the rocks in the acid environment with nuclear magnetic resonance technique.


## 2. Materials and Methods

### 2.1. Materials

The rock specimens consisted of fleshy red, sandstone with a dense-and-fine fragment texture. The specimens were prepared from natural and fresh rocks which had formed at a comparatively deeper level, anisotropic, and without any traces of cracks developed on the joints. According to the results of the X-ray diffraction test, the red sandstone is mainly composed of quartz, albite, aerinite, sanidine, microcline, laumontite, and chabazite. The bulk density of rock was 2.42 g/cm3. The water absorption test showed that the percent absorption of the rock specimens was 1.65%, the water saturation ratio was 1.74%, and the water saturation coefficient was 0.95. The rocks were prepared as specimens of standard cylinders 50 ± 1 mm in diameter by 30 ± 1 mm tall, according to “The Codes of Rock Tests in Water Conservancy and Hydroelectrical Projects (SL 264-2001)” standards.


### 2.2. Experimental Setup and Test Procedures

The rock specimens were numbered C1–C9 and F1–F9, respectively, to match with the H2SO4 solution group and the pure water group. First, prepare a H2SO4 solution with initial pH = 1.5 (the mass concentration of sulfuric acid is 1.5 g/L), according to the lowest pH value in the mine waste water where the sampling site is located. Then, the rock specimens were separately immersed in the H2SO4 solution and pure water for 48 hours to reach a state of natural saturation. The volume of the sulfuric acid solution and pure water is based on the thoroughly immersed specimens. It ranges from 2 L to 3 L. Next the rock specimens were placed in a TDS-300 freeze-thaw testing machine for the freeze-thaw cycling tests. According to the requirements of the freeze-thaw test, the parameters of the freeze-thaw were the freezing temperature was −20°C, the thawing temperature was 20°C, and both the time for freezing and the time for thawing were 4 hours. There were 30 freeze-thaw cycles to undergo, and the rock specimens were numbered 1–3, 4–6, and 7–9 corresponding with 0, 10, 20, and 30 freeze-thaw cycles, respectively. At the end of every 10 freeze-thaw cycles, the rock specimens were extracted to conduct the NMR tests using an AniMR-150 MRI analyzing system (Suzhou Niumag Electronic Technology Co., Ltd.). As shown in Figure 1, in the NMR porosity test, the rock specimens that have completed the freeze-thaw cycle test are taken out from pure water or H2SO4 solution, and the surface moisture (or solution) is wiped with a towel, wrapped with a plastic wrap (to prevent evaporation of water), and then placed in the MRI analyzing system.Figure 1
Sequence showing experimental setup and test procedure requiring rock specimen preparation, the TDS-300 freeze-thaw testing machine, and the AniMR-150 MRI analyzing system.The AniMR-150 MRI analysis system is mainly composed of the spectrometer system, RF unit, magnet cabinet, industrial personal computer, and inversion software. The device is developed on the basis of a1H relaxation response. The main magnetic field of the MRI system is 0.3 ± 0.05 Tesla, the diameter of the probe coil is 150 mm, the main frequency of instrument is 12.8 MHz, the gain of the receiver is greater than 40 dB, and the maximum sampling bandwidth is greater than 300 kHz; the RF power is 300 W, and the maximum resolution of MRI is 100 μm. The number of sampling peaks can reach 18,000, the echo time is less than 150 ms, and the sample relaxation time is measured from 80 μs to 14 s [22].In the NMR test, the transverse relaxation time (T2) was retrieved by using the NMR rock-core analyzing software (Version 1.1.0) [23], which could obtain the intensity of nuclear magnetic signals and T2 spectra and composite them as the transverse relaxation time of the rock specimen. During the test, one specimen was placed at the center of the radio frequency (RF) coil for CPMG pulse sequence experiments. The experimental parameters were as follows: receiver bandwidth SW = 200 kHz; frequency main value SF = 12 MHz; center frequency O1 = 531730.99 Hz; sampling points TD = 331208; waiting time Tw = 3000 ms; number of echo cycles NECH = 6000; TE = 0.276 ms; coil waiting time RFD = 0.06 ms; analog gain RG1 = 20 db; digital gain RG2 = 4; cumulative sampling NS = 32.After the test, the decay curve between echo signal and sampling time can be obtained by the AniMR-150 MRI analysis system (Figure2(a)) and the relationship between porosity increment and T2 relaxation time distribution (Figure 2(b)) can be derived from the decay curve by inversion methods. The shaded area between the curve and horizontal axis is termed as the spectra area and measured porosity. A final porosity of 3.6% can be obtained from the accumulated porosity curve.Figure 2
Results of NMR analysis: (a) echo signal decay curve; (b)T2 relaxation time distribution curve.
(a)
(b)


## 2.1. Materials

The rock specimens consisted of fleshy red, sandstone with a dense-and-fine fragment texture. The specimens were prepared from natural and fresh rocks which had formed at a comparatively deeper level, anisotropic, and without any traces of cracks developed on the joints. According to the results of the X-ray diffraction test, the red sandstone is mainly composed of quartz, albite, aerinite, sanidine, microcline, laumontite, and chabazite. The bulk density of rock was 2.42 g/cm3. The water absorption test showed that the percent absorption of the rock specimens was 1.65%, the water saturation ratio was 1.74%, and the water saturation coefficient was 0.95. The rocks were prepared as specimens of standard cylinders 50 ± 1 mm in diameter by 30 ± 1 mm tall, according to “The Codes of Rock Tests in Water Conservancy and Hydroelectrical Projects (SL 264-2001)” standards.


## 2.2. Experimental Setup and Test Procedures

The rock specimens were numbered C1–C9 and F1–F9, respectively, to match with the H2SO4 solution group and the pure water group. First, prepare a H2SO4 solution with initial pH = 1.5 (the mass concentration of sulfuric acid is 1.5 g/L), according to the lowest pH value in the mine waste water where the sampling site is located. Then, the rock specimens were separately immersed in the H2SO4 solution and pure water for 48 hours to reach a state of natural saturation. The volume of the sulfuric acid solution and pure water is based on the thoroughly immersed specimens. It ranges from 2 L to 3 L. Next the rock specimens were placed in a TDS-300 freeze-thaw testing machine for the freeze-thaw cycling tests. According to the requirements of the freeze-thaw test, the parameters of the freeze-thaw were the freezing temperature was −20°C, the thawing temperature was 20°C, and both the time for freezing and the time for thawing were 4 hours. There were 30 freeze-thaw cycles to undergo, and the rock specimens were numbered 1–3, 4–6, and 7–9 corresponding with 0, 10, 20, and 30 freeze-thaw cycles, respectively. At the end of every 10 freeze-thaw cycles, the rock specimens were extracted to conduct the NMR tests using an AniMR-150 MRI analyzing system (Suzhou Niumag Electronic Technology Co., Ltd.). As shown in Figure 1, in the NMR porosity test, the rock specimens that have completed the freeze-thaw cycle test are taken out from pure water or H2SO4 solution, and the surface moisture (or solution) is wiped with a towel, wrapped with a plastic wrap (to prevent evaporation of water), and then placed in the MRI analyzing system.Figure 1
Sequence showing experimental setup and test procedure requiring rock specimen preparation, the TDS-300 freeze-thaw testing machine, and the AniMR-150 MRI analyzing system.The AniMR-150 MRI analysis system is mainly composed of the spectrometer system, RF unit, magnet cabinet, industrial personal computer, and inversion software. The device is developed on the basis of a1H relaxation response. The main magnetic field of the MRI system is 0.3 ± 0.05 Tesla, the diameter of the probe coil is 150 mm, the main frequency of instrument is 12.8 MHz, the gain of the receiver is greater than 40 dB, and the maximum sampling bandwidth is greater than 300 kHz; the RF power is 300 W, and the maximum resolution of MRI is 100 μm. The number of sampling peaks can reach 18,000, the echo time is less than 150 ms, and the sample relaxation time is measured from 80 μs to 14 s [22].In the NMR test, the transverse relaxation time (T2) was retrieved by using the NMR rock-core analyzing software (Version 1.1.0) [23], which could obtain the intensity of nuclear magnetic signals and T2 spectra and composite them as the transverse relaxation time of the rock specimen. During the test, one specimen was placed at the center of the radio frequency (RF) coil for CPMG pulse sequence experiments. The experimental parameters were as follows: receiver bandwidth SW = 200 kHz; frequency main value SF = 12 MHz; center frequency O1 = 531730.99 Hz; sampling points TD = 331208; waiting time Tw = 3000 ms; number of echo cycles NECH = 6000; TE = 0.276 ms; coil waiting time RFD = 0.06 ms; analog gain RG1 = 20 db; digital gain RG2 = 4; cumulative sampling NS = 32.After the test, the decay curve between echo signal and sampling time can be obtained by the AniMR-150 MRI analysis system (Figure2(a)) and the relationship between porosity increment and T2 relaxation time distribution (Figure 2(b)) can be derived from the decay curve by inversion methods. The shaded area between the curve and horizontal axis is termed as the spectra area and measured porosity. A final porosity of 3.6% can be obtained from the accumulated porosity curve.Figure 2
Results of NMR analysis: (a) echo signal decay curve; (b)T2 relaxation time distribution curve.
(a)
(b)


## 3. Macroscopic Observation and Pore Structure Alteration Based on Quantification of Water Imbibition

The appearance of the rock specimens soaked in H2SO4 and pure water after the freeze-thaw tests is as shown in Figures 3 and 4, respectively. A comparison of the appearances of the frozen rock specimens shows that, after 30 freeze-thaw cycles, the surface of the sandstone specimens which were placed in the acid environment was seriously eroded, and the particles on the surface peeled off in large quantities. In contrast, the surface of the sandstone specimens placed in pure water did not show significant changes.Figure 3
Specimens macroscopic observation after different numbers of freeze-thaw cycles in H2SO4 solution: (a) 10 cycles; (b) 20 cycles; (c) 30 cycles.
(a)
(b)
(c)Figure 4
Specimens macroscopic observation after different numbers of freeze-thaw cycles in pure water: (a) 10 cycles; (b) 20 cycles; (c) 30 cycles.
(a)
(b)
(c)In the following experiment, after every round of freeze-thaw cycles (the number is predetermined), the rock specimens were taken out and wiped dry, and then a mass test was conducted. Therefore, the change in the mass of the rock specimens is also a useful indicator of deterioration. The result of the mass of the rock specimens which changed with the number of freeze-thaw cycles in the acid environment is shown in Figure5. It is clear from Figure 5 that, after 10 freeze-thaw cycles, the average mass of the rock specimens of the H2SO4 solution group and the pure water group increases by 0.20% and 0.16%, respectively. The observed changes in the average mass indicate that the 10 freeze-thaw cycles had led to the expansion of minute cracks in the sandstones to various degrees, and that water had entered the pores, which caused the increase of the mass of rock.Figure 5
Curve of the changes of the mass of the rock specimens. Two different kinds of solution are labelled by ① H2SO4 solution and ② pure water.After 20 freeze-thaw cycles, the mass of the specimens begins to show a decrease at various degrees. The mass of the specimens of the H2SO4 solution group and the pure water group shows a drop of 0.63% and 0.16%, respectively. By observing the appearance of the specimens, one can find free particles in the solution. Therefore, the 20 freeze-thaw cycles cause the surface of the specimens to undergo macroscopic fracture, resulting in the peeling off of the particles from the rock surface, and the mass of those particles is apparently larger than that of the increase of the pore water. In other words, at the midstage of the freeze-thaw deterioration, the acid environment will influence the freeze-thaw deterioration of the rocks at a comparatively larger scale.After 30 freeze-thaw cycles, the mass of the rock specimens in pure water almost remains nearly unchanged; however, the surface quality of the rock specimens placed in the H2SO4 solution continues to worsen. The implication is that, under the acid conditions, the particles peeled off from the surface of the rock specimens at a higher rate, and the number of free particles in the solution had increased significantly. There is a large number of solid sediments in the solution, leading to the deterioration of the overall quality and integrity of the rock specimens.


## 4. Pore Structure Alterations Characterized by NMR Response

### 4.1. Porosity

An assessment of porosity could directly reflect the condition of deterioration within the rock specimens. The AniMR-150 MRI analyzing system was used to measure the porosity of the sandstone specimens after every 10 freeze-thaw cycles, and the results are as shown in Table1.Table 1
Result of the measurement of the porosity of the sandstones (porosity (%)).
H2SO4 solution
Pure water
Specimens
Freeze-thaw cycles
Specimens
Freeze-thaw cycles
0
10
20
30
0
10
20
30
C7
3.6
4.1
4.5
5.0
F7
3.5
4.0
3.9
4.2
C8
4.0
4.6
5.0
5.4
F8
3.3
3.8
3.7
3.9
C9
3.9
4.3
5.0
5.6
F9
3.2
3.6
3.6
3.8
Average
3.8
4.3
4.8
5.3
Average
3.3
3.8
3.7
4.0Table1 shows that, with the addition of the freeze-thaw cycles, the porosity of the sandstones generally increases, but the scale of increase of the porosity of the sandstones in H2SO4 solution is relatively larger than that of the specimens placed in pure water. It is thus clear that, for the same number of freeze-thaw cycles, the minerals (such as quartz, albite, and microcline) in the sandstones had reacted with H2SO4 solution chemically. Therefore, the mineral elements of the rocks changed and the links between particles were weakened, which led to the increase in porosity. This result is in agreement with the changes in the mass and surface appearances of the specimens discussed in Section 3, which shows that the interior and exterior deterioration of rock occurred under the coupling effect of chemistry and the freeze-thaw.The mean values of the porosity of every group of rock specimens were analyzed and are shown in Table1. It can be seen that, under the same conditions of freeze-thaw cycling, H2SO4 solution apparently accelerated the deteriorating process within the rocks, especially after 10 freeze-thaw cycles. While the porosity of sandstones in pure water changed little, the mass change is also small. However, the porosity of sandstones in H2SO4 solution increased linearly with the increased number of the freeze-thaw cycles. After 30 freeze-thaw cycles, the porosity increased by 39.5% in H2SO4 solution and only by 21.2% in pure water. Under the influence of freeze-thaw cycles, the pores inside the rock in pure water continue to develop and expand, and the external moisture enters the newly created or expanded pores, resulting in changes in mass and porosity. In H2SO4 solution, freeze-thaw also leads to changes in pores of rock samples. The observed discrepancies in increased porosity indicate that H2SO4 solution had entered the newly produced pores of rock and had undergone additional chemical reactions incessantly with some of the activated minerals in the rocks. Therefore, the particles within the rocks corroded continuously, and the mineral elements of the rocks changed. The solutes were drifted out of the rock in water to produce sediments and then resulted in the decrease of mass of the rock specimens (Figure 5). However, the separation of solutes inside the rocks resulted in the formation of cavities, karst voids, and cracks of corrosion. Thus, the acid environment could produce a significant influence on the freeze-thaw deterioration of rock.


### 4.2.T2 Spectral Distribution of NMR

Previous studies [24, 25] have shown that the T2 spectral distribution reflects the size and distribution of the pores of rock specimens. The size of pore is related to the position of the spectral peak; in particular, the group of the small size of pores corresponds to smaller T2 values, while the group of large size of pores corresponds to larger T2 values. Figure 6 shows the distribution curves of NMR transverse relaxation time T2 of the sandstone rock specimens after different numbers of freeze-thaw cycles in H2SO4 solution and in pure water. It can be seen that there were mainly four T2 spectral peaks for the sandstones. With the increase of the number of freeze-thaw cycles and under the influence of freeze-thaw and the acid solution, new pores have developed within the rock, and tiny pores have gradually expanded into large-sized pores. And the number of peaks increased from 4 to 5 or even to 6. It can be seen from Figure 6(b) that, after 30 freeze-thaw cycles, the pores in the rock expand and the large-sized pores increase under the action of freeze-thaw cycles. The water enters the newly created large-sized pores and caused the large pores to “collapse,” causing the large-sized pores to be packed and their sizes to be changed. Thus, 30 freeze-thaw cycle samples have shorter T2 values than after 20 cycles.Figure 6
T
2 distribution of sandstones after different numbers of freeze-thaw cycles: (a) H2SO4 solution; (b) pure water.
(a)
(b)Table2 shows the peak areas of the NMR T2 spectra of specimens under different conditions. It can be seen from Figure 6 and Table 2 that the individual T2 spectral distribution of the sandstones in H2SO4 solution and in pure water differed largely. After 30 freeze-thaw cycles, the total spectral area of the rock specimen #C7 (in H2SO4 solution) increased by 35.6% and the number of peaks increased from 4 to 6. In the meantime, the total spectral area of the rock specimen #F7 (in pure water) increased by 21%, and the number of peaks increased from 4 to 5.Table 2
Statistics of the proportion of each peak area in theT2 curve.
Solution
Specimen
Proportion of each peak area (vol.%)
Total peak area
1st
2nd
3rd
4th
5th
6th
H2SO4
C7-0
48.37
28.27
17.00
6.36
0
0
12467
C7-10
54.62
19.99
21.46
3.94
0
0
14417
C7-20
40.12
28.09
20.52
9.89
1.38
0
14378
C7-30
43.25
18.11
14.93
18.07
4.84
0.80
16903
Pure water
F7-0
51.76
33.24
10.89
4.10
0
0
11614
F7-10
32.50
40.00
22.46
5.04
0
0
13111
F7-20
42.87
28.81
20.94
5.82
1.56
0
12835
F7-30
41.51
29.73
20.44
6.36
1.95
0
14058In addition, the trend of each spectra peak is also different. The proportion of the 4th, 5th, and 6th spectra peaks in H2SO4 solution is gradually increasing; that is, the increase of large-sized pores is obvious, especially the 4th spectra peak, which increases by 2.8 times. However, in H2O solution, it mainly shows that the proportion of the first spectra peak has decreased, and the proportion of the 3rd and 5th spectra peaks has increased. Given that, during the freezing and thawing processes, the moisture within the pores freezes, and the bulk volume expands as a result [26, 27]. Thus, the development and expansion of the pores are accelerated to introduce chemical solution into the new pores. The entry of chemical solution into the new pores would be expected to corrode and lubricate the particles within the rock material. Therefore, the cohesion between the mineral particles decreased, the pores grew larger, and the frictional force was reduced. Under the repeated influence of freezing and thawing, the chemical solution in the pores would be expected to incessantly corrode the minerals in the rock and to destroy the friction and the mineral elements between the mineral particles. Furthermore, the pressure created by the volume expansion after the freezing of moisture would easily reach the extreme tensile strength of the walls of the pores. These changes would lead to a rapid expansion of the tiny pores and cause the larger pores to collapse, which leads to a macroscopic fracturing of the specimens. This chain of events ultimately leads to a macroscopic fracturing, which also explains why the general porosity of rocks increases seemingly linearly.Table2 shows that, under the coupling effects of freeze-thaw cycles and chemical conditions, the changing trend of different size pores in rock which has been immersed in pure water and H2SO4 solution is different. It also shows that, after the different number of freeze-thaw cycles, the proportion of the second spectrum peak in pure water is significantly higher than the proportion of the second spectrum peak in H2SO4. Therefore, the porosity of the component of T2 spectrum in pure water is higher than that in H2SO4, but the total spectrum area is still large in H2SO4 solution.


### 4.3. MRI Results

Figure7 displays the magnetic resonance imaging (MRI) of the rock specimens after 0, 10, 20, and 30 freeze-thaw cycles in the H2SO4 solution. The maximum resolution of the MRI system used in the experiment was 100 μm, and the image was automatically generated by the NMR test system. The illuminated areas in the image were the places where the moisture was present, and the dark areas represent the background color. The brightness of the image shows the quantity of water content, i.e., the lighter the image, the greater the amount of moisture present in that area, and the larger the pore size in that area. At freeze-thaw cycle at time 0, the light dots in the MRI are well-distributed, which indicates that the interior of the sandstone specimens was perfectly isotropic. After 10 freeze-thaw cycles, the number of light dots increases, which indicates that the number of pores in the rocks had increased. There is a circle of light dots around the contour of the sandstones which had been placed in H2SO4 solution, which shows that, with the dual effect of acid corrosion and the freezing and thawing, small cracks on the outer layer of the rock specimens had kept expanding, while the rock specimens in pure water had not shown similar changes (Figure 8(b)).Figure 7
Images of sandstones of the H2SO4 solution group after different numbers of freeze-thaw cycles: (a) 0; (b) 10; (c) 20; (d) 30. Two different kinds of pore deformation are approximately labelled by ① which indicates new pores are generated and ② which indicates pores are deformed to larger size.
(a)
(b)
(c)
(d)Figure 8
Images of sandstones of the pure water group after different numbers of freeze-thaw cycles: (a) 0; (b) 10; (c) 20; (d) 30. Two different kinds of pore deformation are approximately labelled by ① which indicates new pores are generated and ② which indicates pores are deformed to larger size.
(a)
(b)
(c)
(d)After 20 freeze-thaw cycles, the number of light dots continue to increase in the MRI of the sandstones in H2SO4 solution covering almost the entire section. The size of pores of the rock specimens continues to grow, and the number of large-sized pores continues to grow also, while the pores within the rock specimens in pure water did not display similar changes. After 30 freeze-thaw cycles, the number and size of the light dots in the MRI of the sandstones in H2SO4 solution has clearly increased. There is a large quantity of pores on the surface of the rock specimens, and they are surrounded by the pore water. The NMR signals within the rock specimens are very strong, and the areas of light dots are close to connecting with each other. The observed trend shows that the acid solution had affected the interior structure of the sandstone, which had led to the unusual development of the cracks within the rocks and the macroscopic fracture of the appearances of the rock specimens.In contrast, as shown in Figure8, the pores inside the rock specimens in pure water do not display significant development and progression, although the results still show the process of the development of pores from the exterior to the interior of the rock specimens under freeze-thaw effects. After 30 freeze-thaw cycles, there are some light dots nearly in contact inside and outside the specimens; however, the deterioration remains weak. In addition, close inspection shows that there were relatively few macroscopic cracks formed in the surface appearance of the rock specimens. In summary, the chemical solution had reacted with and corroded the mineral structures in the acidic solution, which caused the inner and outer shapes of rocks and the sizes of particles, pores, and cracks to transform. The chemical etching had largely influenced the development and expansion of the cracks within the sandstone.


## 4.1. Porosity

An assessment of porosity could directly reflect the condition of deterioration within the rock specimens. The AniMR-150 MRI analyzing system was used to measure the porosity of the sandstone specimens after every 10 freeze-thaw cycles, and the results are as shown in Table1.Table 1
Result of the measurement of the porosity of the sandstones (porosity (%)).
H2SO4 solution
Pure water
Specimens
Freeze-thaw cycles
Specimens
Freeze-thaw cycles
0
10
20
30
0
10
20
30
C7
3.6
4.1
4.5
5.0
F7
3.5
4.0
3.9
4.2
C8
4.0
4.6
5.0
5.4
F8
3.3
3.8
3.7
3.9
C9
3.9
4.3
5.0
5.6
F9
3.2
3.6
3.6
3.8
Average
3.8
4.3
4.8
5.3
Average
3.3
3.8
3.7
4.0Table1 shows that, with the addition of the freeze-thaw cycles, the porosity of the sandstones generally increases, but the scale of increase of the porosity of the sandstones in H2SO4 solution is relatively larger than that of the specimens placed in pure water. It is thus clear that, for the same number of freeze-thaw cycles, the minerals (such as quartz, albite, and microcline) in the sandstones had reacted with H2SO4 solution chemically. Therefore, the mineral elements of the rocks changed and the links between particles were weakened, which led to the increase in porosity. This result is in agreement with the changes in the mass and surface appearances of the specimens discussed in Section 3, which shows that the interior and exterior deterioration of rock occurred under the coupling effect of chemistry and the freeze-thaw.The mean values of the porosity of every group of rock specimens were analyzed and are shown in Table1. It can be seen that, under the same conditions of freeze-thaw cycling, H2SO4 solution apparently accelerated the deteriorating process within the rocks, especially after 10 freeze-thaw cycles. While the porosity of sandstones in pure water changed little, the mass change is also small. However, the porosity of sandstones in H2SO4 solution increased linearly with the increased number of the freeze-thaw cycles. After 30 freeze-thaw cycles, the porosity increased by 39.5% in H2SO4 solution and only by 21.2% in pure water. Under the influence of freeze-thaw cycles, the pores inside the rock in pure water continue to develop and expand, and the external moisture enters the newly created or expanded pores, resulting in changes in mass and porosity. In H2SO4 solution, freeze-thaw also leads to changes in pores of rock samples. The observed discrepancies in increased porosity indicate that H2SO4 solution had entered the newly produced pores of rock and had undergone additional chemical reactions incessantly with some of the activated minerals in the rocks. Therefore, the particles within the rocks corroded continuously, and the mineral elements of the rocks changed. The solutes were drifted out of the rock in water to produce sediments and then resulted in the decrease of mass of the rock specimens (Figure 5). However, the separation of solutes inside the rocks resulted in the formation of cavities, karst voids, and cracks of corrosion. Thus, the acid environment could produce a significant influence on the freeze-thaw deterioration of rock.


## 4.2.T2 Spectral Distribution of NMR

Previous studies [24, 25] have shown that the T2 spectral distribution reflects the size and distribution of the pores of rock specimens. The size of pore is related to the position of the spectral peak; in particular, the group of the small size of pores corresponds to smaller T2 values, while the group of large size of pores corresponds to larger T2 values. Figure 6 shows the distribution curves of NMR transverse relaxation time T2 of the sandstone rock specimens after different numbers of freeze-thaw cycles in H2SO4 solution and in pure water. It can be seen that there were mainly four T2 spectral peaks for the sandstones. With the increase of the number of freeze-thaw cycles and under the influence of freeze-thaw and the acid solution, new pores have developed within the rock, and tiny pores have gradually expanded into large-sized pores. And the number of peaks increased from 4 to 5 or even to 6. It can be seen from Figure 6(b) that, after 30 freeze-thaw cycles, the pores in the rock expand and the large-sized pores increase under the action of freeze-thaw cycles. The water enters the newly created large-sized pores and caused the large pores to “collapse,” causing the large-sized pores to be packed and their sizes to be changed. Thus, 30 freeze-thaw cycle samples have shorter T2 values than after 20 cycles.Figure 6
T
2 distribution of sandstones after different numbers of freeze-thaw cycles: (a) H2SO4 solution; (b) pure water.
(a)
(b)Table2 shows the peak areas of the NMR T2 spectra of specimens under different conditions. It can be seen from Figure 6 and Table 2 that the individual T2 spectral distribution of the sandstones in H2SO4 solution and in pure water differed largely. After 30 freeze-thaw cycles, the total spectral area of the rock specimen #C7 (in H2SO4 solution) increased by 35.6% and the number of peaks increased from 4 to 6. In the meantime, the total spectral area of the rock specimen #F7 (in pure water) increased by 21%, and the number of peaks increased from 4 to 5.Table 2
Statistics of the proportion of each peak area in theT2 curve.
Solution
Specimen
Proportion of each peak area (vol.%)
Total peak area
1st
2nd
3rd
4th
5th
6th
H2SO4
C7-0
48.37
28.27
17.00
6.36
0
0
12467
C7-10
54.62
19.99
21.46
3.94
0
0
14417
C7-20
40.12
28.09
20.52
9.89
1.38
0
14378
C7-30
43.25
18.11
14.93
18.07
4.84
0.80
16903
Pure water
F7-0
51.76
33.24
10.89
4.10
0
0
11614
F7-10
32.50
40.00
22.46
5.04
0
0
13111
F7-20
42.87
28.81
20.94
5.82
1.56
0
12835
F7-30
41.51
29.73
20.44
6.36
1.95
0
14058In addition, the trend of each spectra peak is also different. The proportion of the 4th, 5th, and 6th spectra peaks in H2SO4 solution is gradually increasing; that is, the increase of large-sized pores is obvious, especially the 4th spectra peak, which increases by 2.8 times. However, in H2O solution, it mainly shows that the proportion of the first spectra peak has decreased, and the proportion of the 3rd and 5th spectra peaks has increased. Given that, during the freezing and thawing processes, the moisture within the pores freezes, and the bulk volume expands as a result [26, 27]. Thus, the development and expansion of the pores are accelerated to introduce chemical solution into the new pores. The entry of chemical solution into the new pores would be expected to corrode and lubricate the particles within the rock material. Therefore, the cohesion between the mineral particles decreased, the pores grew larger, and the frictional force was reduced. Under the repeated influence of freezing and thawing, the chemical solution in the pores would be expected to incessantly corrode the minerals in the rock and to destroy the friction and the mineral elements between the mineral particles. Furthermore, the pressure created by the volume expansion after the freezing of moisture would easily reach the extreme tensile strength of the walls of the pores. These changes would lead to a rapid expansion of the tiny pores and cause the larger pores to collapse, which leads to a macroscopic fracturing of the specimens. This chain of events ultimately leads to a macroscopic fracturing, which also explains why the general porosity of rocks increases seemingly linearly.Table2 shows that, under the coupling effects of freeze-thaw cycles and chemical conditions, the changing trend of different size pores in rock which has been immersed in pure water and H2SO4 solution is different. It also shows that, after the different number of freeze-thaw cycles, the proportion of the second spectrum peak in pure water is significantly higher than the proportion of the second spectrum peak in H2SO4. Therefore, the porosity of the component of T2 spectrum in pure water is higher than that in H2SO4, but the total spectrum area is still large in H2SO4 solution.


## 4.3. MRI Results

Figure7 displays the magnetic resonance imaging (MRI) of the rock specimens after 0, 10, 20, and 30 freeze-thaw cycles in the H2SO4 solution. The maximum resolution of the MRI system used in the experiment was 100 μm, and the image was automatically generated by the NMR test system. The illuminated areas in the image were the places where the moisture was present, and the dark areas represent the background color. The brightness of the image shows the quantity of water content, i.e., the lighter the image, the greater the amount of moisture present in that area, and the larger the pore size in that area. At freeze-thaw cycle at time 0, the light dots in the MRI are well-distributed, which indicates that the interior of the sandstone specimens was perfectly isotropic. After 10 freeze-thaw cycles, the number of light dots increases, which indicates that the number of pores in the rocks had increased. There is a circle of light dots around the contour of the sandstones which had been placed in H2SO4 solution, which shows that, with the dual effect of acid corrosion and the freezing and thawing, small cracks on the outer layer of the rock specimens had kept expanding, while the rock specimens in pure water had not shown similar changes (Figure 8(b)).Figure 7
Images of sandstones of the H2SO4 solution group after different numbers of freeze-thaw cycles: (a) 0; (b) 10; (c) 20; (d) 30. Two different kinds of pore deformation are approximately labelled by ① which indicates new pores are generated and ② which indicates pores are deformed to larger size.
(a)
(b)
(c)
(d)Figure 8
Images of sandstones of the pure water group after different numbers of freeze-thaw cycles: (a) 0; (b) 10; (c) 20; (d) 30. Two different kinds of pore deformation are approximately labelled by ① which indicates new pores are generated and ② which indicates pores are deformed to larger size.
(a)
(b)
(c)
(d)After 20 freeze-thaw cycles, the number of light dots continue to increase in the MRI of the sandstones in H2SO4 solution covering almost the entire section. The size of pores of the rock specimens continues to grow, and the number of large-sized pores continues to grow also, while the pores within the rock specimens in pure water did not display similar changes. After 30 freeze-thaw cycles, the number and size of the light dots in the MRI of the sandstones in H2SO4 solution has clearly increased. There is a large quantity of pores on the surface of the rock specimens, and they are surrounded by the pore water. The NMR signals within the rock specimens are very strong, and the areas of light dots are close to connecting with each other. The observed trend shows that the acid solution had affected the interior structure of the sandstone, which had led to the unusual development of the cracks within the rocks and the macroscopic fracture of the appearances of the rock specimens.In contrast, as shown in Figure8, the pores inside the rock specimens in pure water do not display significant development and progression, although the results still show the process of the development of pores from the exterior to the interior of the rock specimens under freeze-thaw effects. After 30 freeze-thaw cycles, there are some light dots nearly in contact inside and outside the specimens; however, the deterioration remains weak. In addition, close inspection shows that there were relatively few macroscopic cracks formed in the surface appearance of the rock specimens. In summary, the chemical solution had reacted with and corroded the mineral structures in the acidic solution, which caused the inner and outer shapes of rocks and the sizes of particles, pores, and cracks to transform. The chemical etching had largely influenced the development and expansion of the cracks within the sandstone.


## 5. Discussion

### 5.1. The Effects of Freeze-Thaw on the Evolution of Rock Pores

The freezing and thawing cycles of the sandstone specimens in both acidic and nonacidic environments affected the size, number, and distribution of pores within the entire rock material, as discussed in Section4. Consequently, freeze-thaw cycles in such environments have important implications related to the permeability of the sandstones and to the mechanical behaviour of the sandstone. According to the NMR principle, the NMR crosswise relaxation rate 1/T2 is given as follows [28, 29]:(1)1T2=ρ2SVporosity,where S is the surface area of a pore (cm2), V is the pore volume (cm3), and ρ2 is the crosswise surface relaxation strength (μm/ms).If the pore radius is positively related to the throat radius, equation (1) becomes [30, 31](2)1T2=ρ2rcFs,where rc is the pore radius and Fs is a geometrical factor (for the spherical pores, Fs=3; and for the columnar pores, Fs=2).(3)rc=CT2,where C=Fsρ2 and C is called the conversion coefficient.It can be concluded from equations (1)–(3) that the pore radius is in direct proportion to T2: the smaller the T2 is, the smaller the represented pore becomes, and the larger the pore, the larger the T2 becomes.With the scaling and error analysis of the transverse relaxation timeT2 and pore throat radius, the best scale range of the C value of the red sandstone is between 0.3 and 0.5 μm/ms, and the average value is 0.3125 μm/ms [32]. Therefore, equation (4) can be achieved by simplifying equation (3) as follows:(4)rc=0.3125T2.According to equation (4), T2 spectra can be transformed into the pore size distribution curve (Figure 9). There has been no agreed standard about the classification of the pore size of sandstone. Based on the result of the classification of the pore size of sandstone acquired by some scholars [33–35], this paper divides the pore sizes into four categories: micropores (diameter < 0.1 μm), minipores (diameters between 0.1 and 1 μm), mesopores (diameters between 1 and 50 μm), and macropores (diameters ≥50 μm). Figure 9 shows the pore size distribution curve of rock specimens which are soaked in H2SO4 solution and in pure water after 0 and 30 freeze-thaw cycles, respectively. With the segmented statistics of the pore size distribution curves, the proportion of the distribution of different-sized pores soaked in two solutions after 0, 10, 20, and 30 freeze-thaw cycles are obtained (Table 3).Figure 9
Division ofT2 spectra of sandstone specimens according to pore size in different solutions.Table 3
Proportional variations of pore sizes in different solutions.
Pore size
Proportion of pores in cores in H2SO4 solution (%)
Proportion of pores in cores in pure water (%)
0 cycle
10 cycles
20 cycles
30 cycles
0 cycle
10 cycles
20 cycles
30 cycles
Micropores (<0.1μm)
25.5
24.3
15.7
24.8
17.7
20.9
21.1
22.8
Minipores (0.1–1μm)
45.8
43.4
47.4
36.5
48.9
41.6
47.1
43.6
Mesopores (1–50μm)
28.6
32.3
35.6
37.8
33.3
36.4
30.3
31.6
Macropores (≥50μm)
0.0
0.0
1.4
0.8
0.2
1.2
1.6
2.0After 0 and 30 freeze-thaw cycles, the numbers of minipores and mesopores of sandstone in two solutions are dominant. The rock specimen (rock specimen #C7) in H2SO4 solution accounts for 74.4% and 74.3%, respectively, after 0 and 30 freeze-thaw cycles, and the rock specimen (rock specimen #F7) in pure water accounts for 82.2% and 75.2%, respectively, after 0 and 30 freeze-thaw cycles. The results show that there are relatively more minipores and mesopores and less micropores and macropores in the initial sandstones. With the growth of freeze-thaw cycles, the pores develop and expand under the coupling effect of freeze-thaw cycles and chemical reaction, and the pores connect with each other. The pore sizes generally expand, and the micropores develop, and the growth of the number of marcopores is the most clear, leading to the transformation of the pore structure of rock specimens.After 30 freeze-thaw cycles, differences occur in the changes of the pore structure of sandstone in the H2SO4 solution and pure water. Part of the minerals dissolve and are separated out and move out of the rocks in the H2SO4 solution given that chemical reactions occur between the H2SO4 solution and the mineral granules within the rock specimens. Therefore, cavities are formed in the rocks, causing the pore sizes to develop and expand, and the permeability of the rock is improved, which facilitates the movement of the solution and intensifies the chemical reaction. Consequently, the number of mesopores grows by 32.2%, and the number of macropores grows from 0 to 0.8. Conversely, the numbers of micropores and minipores decline, with the decrease ranges of −2.7% and −20.3%, respectively. In contrast, in the pure water where only the influencing factors of frost heave and water movement are significant, the pores develop and expand continuously, which leads to the expansion of pore sizes and the creation of new micropores. Thus, micropores increase by 28.8%, and macropores increase from 0.2 to 2.0; the minipores and mesopores decline, with decreasing ranges of −10.8% and −5.1%, respectively.


### 5.2. Pore Structure Deterioration under the Coupled Effect of Freeze-Thaw and Chemical Solution

The chemical compositions of red sandstone are shown in Table4.Table 4
Chemical composition of red sandstone (%).
SiO2
Al2O3
CaO
MgO
Na2O
Fe2O3
K2O
TiO2
P2O5
52.5
11.4
5.8
3.8
3.2
2.3
1.8
0.5
0.1It can be seen in Table4 that the red sandstone is mainly composed of SiO2, Al2O3, CaO, MgO, Na2O, Fe2O3, and other minor chemical elements. Among the constituents, SiO2 is the major element in the red sandstone. When chemical solutions and rocks were put together during the experiments, a series of complex physical and chemical reactions were bound to occur, involving complex processes and mechanisms [36]. Based on the ability of acid-dissolving sandstone minerals, the minerals could be divided into two classes [37]: fast reacting minerals (with strong acid-dissolving ability), which includes aluminosilicate (authigenic clay, feldspar, and calcite), and slow reacting minerals (clay splinter and quartz). During the acid-rock reaction, the H2SO4 solution provides H+ and H2O.Under the condition of a strong acid,SO42− in the solution slowly reacted or did not react with the major element SiO2, which caused the difficult dissolution of quartz minerals, and thus, the quartz minerals were prevented from moving out of the rocks. In the unlikely event that there were a few quartz minerals that were dissolved and generated free SiO42−, the silicic acid still existed as a single molecule under the strong acid condition where pH = 1–3, and thus, biopolymers that were weak in water solubility would not be produced. Therefore, in the freeze-thaw process, the mass loss of rock specimens occurred slowly. The reason for the mass loss is that other chemical compositions (such as CaO) had dissolved and migrated out of the rocks, explaining why the mass loss of rock specimens was relatively little.The chemical formulae of chemical reactions between elements such as H+ and SO42− in the H2SO4 solution, pure water, Al2O3, CaO, and MgO are as follows:In pure water:(5)CaO+H2O⟶CaOH2(6)Na2O+H2O⟶2NaOH(7)K2O+H2O⟶2KOH(8)P2O5+H2O⟶2HPO3In H2SO4:(9)SiO2+2H2SO4⟶SiSO42+2H2O(10)Al2O3+3H2SO4⟶Al2SO43+3H2O(11)CaO+H2SO4⟶CaSO4+H2O(12)MgO+H2SO4⟶MgSO4+H2O(13)Na2O+H2SO4⟶Na2SO4+H2O(14)Fe2O3+3H2SO4⟶Fe2SO43+3H2O(15)K2O+H2SO4⟶K2SO4+H2O(16)P2O5+3H2SO4⟶2H3PO4+3SO3According to chemical formula (11), slightly soluble element CaSO4 was produced after the chemical reaction between the rock specimens and the H2SO4 solution, which would float on the surface of CaO to prevent the sulfuric acid from contacting CaO, thus interfering in the chemical reaction. This interference explains the observation that, after 30 freeze-thaw cycles, the rock specimens in the H2SO4 solution were not seriously damaged, and only the rock surface demonstrated corrosion and peeling off (Figure 3). Therefore, there were relatively few sediments that could be observed during the experiment.In addition, in the coupling environment of freeze-thaw and chemical effect, the red sandstones treated in H2SO4 solution were clearly affected by the freeze-thaw effect. When the temperature was low, the water or solution in the pores expanded due to freezing and produced pairs of frosting-heaving forces on the sidewalls of the mineral particles, causing the development and expansion of cracks within the rocks, and increase in porosity [38, 39]. When the temperature rose, the frozen water and chemical solution melted and moved into newly formed cracks, and the acid solution and the mineral elements within the rocks reacted with each other, so that part of the minerals were dissolved and separated and migrated out of the rocks. Therefore, cavities were formed in the rock specimens, causing the pores to develop and expand continuously. The number of mesopores and macropores increased, as displayed in Table 3, which enhanced the rock damaging process. With the progressive increase of freeze-thaw cycles, the cracks in the rock fabric kept developing and expanding, leading to the gradual increase of porosity. In the meantime, as the chemical solution from the outside entered the newly formed cracks and reacted with the mineral elements of the rocks, the cohesive force between the mineral particles was weakened periodically, which further accelerated the rock damage leading to macroscopic deterioration of the rock structures.


## 5.1. The Effects of Freeze-Thaw on the Evolution of Rock Pores

The freezing and thawing cycles of the sandstone specimens in both acidic and nonacidic environments affected the size, number, and distribution of pores within the entire rock material, as discussed in Section4. Consequently, freeze-thaw cycles in such environments have important implications related to the permeability of the sandstones and to the mechanical behaviour of the sandstone. According to the NMR principle, the NMR crosswise relaxation rate 1/T2 is given as follows [28, 29]:(1)1T2=ρ2SVporosity,where S is the surface area of a pore (cm2), V is the pore volume (cm3), and ρ2 is the crosswise surface relaxation strength (μm/ms).If the pore radius is positively related to the throat radius, equation (1) becomes [30, 31](2)1T2=ρ2rcFs,where rc is the pore radius and Fs is a geometrical factor (for the spherical pores, Fs=3; and for the columnar pores, Fs=2).(3)rc=CT2,where C=Fsρ2 and C is called the conversion coefficient.It can be concluded from equations (1)–(3) that the pore radius is in direct proportion to T2: the smaller the T2 is, the smaller the represented pore becomes, and the larger the pore, the larger the T2 becomes.With the scaling and error analysis of the transverse relaxation timeT2 and pore throat radius, the best scale range of the C value of the red sandstone is between 0.3 and 0.5 μm/ms, and the average value is 0.3125 μm/ms [32]. Therefore, equation (4) can be achieved by simplifying equation (3) as follows:(4)rc=0.3125T2.According to equation (4), T2 spectra can be transformed into the pore size distribution curve (Figure 9). There has been no agreed standard about the classification of the pore size of sandstone. Based on the result of the classification of the pore size of sandstone acquired by some scholars [33–35], this paper divides the pore sizes into four categories: micropores (diameter < 0.1 μm), minipores (diameters between 0.1 and 1 μm), mesopores (diameters between 1 and 50 μm), and macropores (diameters ≥50 μm). Figure 9 shows the pore size distribution curve of rock specimens which are soaked in H2SO4 solution and in pure water after 0 and 30 freeze-thaw cycles, respectively. With the segmented statistics of the pore size distribution curves, the proportion of the distribution of different-sized pores soaked in two solutions after 0, 10, 20, and 30 freeze-thaw cycles are obtained (Table 3).Figure 9
Division ofT2 spectra of sandstone specimens according to pore size in different solutions.Table 3
Proportional variations of pore sizes in different solutions.
Pore size
Proportion of pores in cores in H2SO4 solution (%)
Proportion of pores in cores in pure water (%)
0 cycle
10 cycles
20 cycles
30 cycles
0 cycle
10 cycles
20 cycles
30 cycles
Micropores (<0.1μm)
25.5
24.3
15.7
24.8
17.7
20.9
21.1
22.8
Minipores (0.1–1μm)
45.8
43.4
47.4
36.5
48.9
41.6
47.1
43.6
Mesopores (1–50μm)
28.6
32.3
35.6
37.8
33.3
36.4
30.3
31.6
Macropores (≥50μm)
0.0
0.0
1.4
0.8
0.2
1.2
1.6
2.0After 0 and 30 freeze-thaw cycles, the numbers of minipores and mesopores of sandstone in two solutions are dominant. The rock specimen (rock specimen #C7) in H2SO4 solution accounts for 74.4% and 74.3%, respectively, after 0 and 30 freeze-thaw cycles, and the rock specimen (rock specimen #F7) in pure water accounts for 82.2% and 75.2%, respectively, after 0 and 30 freeze-thaw cycles. The results show that there are relatively more minipores and mesopores and less micropores and macropores in the initial sandstones. With the growth of freeze-thaw cycles, the pores develop and expand under the coupling effect of freeze-thaw cycles and chemical reaction, and the pores connect with each other. The pore sizes generally expand, and the micropores develop, and the growth of the number of marcopores is the most clear, leading to the transformation of the pore structure of rock specimens.After 30 freeze-thaw cycles, differences occur in the changes of the pore structure of sandstone in the H2SO4 solution and pure water. Part of the minerals dissolve and are separated out and move out of the rocks in the H2SO4 solution given that chemical reactions occur between the H2SO4 solution and the mineral granules within the rock specimens. Therefore, cavities are formed in the rocks, causing the pore sizes to develop and expand, and the permeability of the rock is improved, which facilitates the movement of the solution and intensifies the chemical reaction. Consequently, the number of mesopores grows by 32.2%, and the number of macropores grows from 0 to 0.8. Conversely, the numbers of micropores and minipores decline, with the decrease ranges of −2.7% and −20.3%, respectively. In contrast, in the pure water where only the influencing factors of frost heave and water movement are significant, the pores develop and expand continuously, which leads to the expansion of pore sizes and the creation of new micropores. Thus, micropores increase by 28.8%, and macropores increase from 0.2 to 2.0; the minipores and mesopores decline, with decreasing ranges of −10.8% and −5.1%, respectively.


## 5.2. Pore Structure Deterioration under the Coupled Effect of Freeze-Thaw and Chemical Solution

The chemical compositions of red sandstone are shown in Table4.Table 4
Chemical composition of red sandstone (%).
SiO2
Al2O3
CaO
MgO
Na2O
Fe2O3
K2O
TiO2
P2O5
52.5
11.4
5.8
3.8
3.2
2.3
1.8
0.5
0.1It can be seen in Table4 that the red sandstone is mainly composed of SiO2, Al2O3, CaO, MgO, Na2O, Fe2O3, and other minor chemical elements. Among the constituents, SiO2 is the major element in the red sandstone. When chemical solutions and rocks were put together during the experiments, a series of complex physical and chemical reactions were bound to occur, involving complex processes and mechanisms [36]. Based on the ability of acid-dissolving sandstone minerals, the minerals could be divided into two classes [37]: fast reacting minerals (with strong acid-dissolving ability), which includes aluminosilicate (authigenic clay, feldspar, and calcite), and slow reacting minerals (clay splinter and quartz). During the acid-rock reaction, the H2SO4 solution provides H+ and H2O.Under the condition of a strong acid,SO42− in the solution slowly reacted or did not react with the major element SiO2, which caused the difficult dissolution of quartz minerals, and thus, the quartz minerals were prevented from moving out of the rocks. In the unlikely event that there were a few quartz minerals that were dissolved and generated free SiO42−, the silicic acid still existed as a single molecule under the strong acid condition where pH = 1–3, and thus, biopolymers that were weak in water solubility would not be produced. Therefore, in the freeze-thaw process, the mass loss of rock specimens occurred slowly. The reason for the mass loss is that other chemical compositions (such as CaO) had dissolved and migrated out of the rocks, explaining why the mass loss of rock specimens was relatively little.The chemical formulae of chemical reactions between elements such as H+ and SO42− in the H2SO4 solution, pure water, Al2O3, CaO, and MgO are as follows:In pure water:(5)CaO+H2O⟶CaOH2(6)Na2O+H2O⟶2NaOH(7)K2O+H2O⟶2KOH(8)P2O5+H2O⟶2HPO3In H2SO4:(9)SiO2+2H2SO4⟶SiSO42+2H2O(10)Al2O3+3H2SO4⟶Al2SO43+3H2O(11)CaO+H2SO4⟶CaSO4+H2O(12)MgO+H2SO4⟶MgSO4+H2O(13)Na2O+H2SO4⟶Na2SO4+H2O(14)Fe2O3+3H2SO4⟶Fe2SO43+3H2O(15)K2O+H2SO4⟶K2SO4+H2O(16)P2O5+3H2SO4⟶2H3PO4+3SO3According to chemical formula (11), slightly soluble element CaSO4 was produced after the chemical reaction between the rock specimens and the H2SO4 solution, which would float on the surface of CaO to prevent the sulfuric acid from contacting CaO, thus interfering in the chemical reaction. This interference explains the observation that, after 30 freeze-thaw cycles, the rock specimens in the H2SO4 solution were not seriously damaged, and only the rock surface demonstrated corrosion and peeling off (Figure 3). Therefore, there were relatively few sediments that could be observed during the experiment.In addition, in the coupling environment of freeze-thaw and chemical effect, the red sandstones treated in H2SO4 solution were clearly affected by the freeze-thaw effect. When the temperature was low, the water or solution in the pores expanded due to freezing and produced pairs of frosting-heaving forces on the sidewalls of the mineral particles, causing the development and expansion of cracks within the rocks, and increase in porosity [38, 39]. When the temperature rose, the frozen water and chemical solution melted and moved into newly formed cracks, and the acid solution and the mineral elements within the rocks reacted with each other, so that part of the minerals were dissolved and separated and migrated out of the rocks. Therefore, cavities were formed in the rock specimens, causing the pores to develop and expand continuously. The number of mesopores and macropores increased, as displayed in Table 3, which enhanced the rock damaging process. With the progressive increase of freeze-thaw cycles, the cracks in the rock fabric kept developing and expanding, leading to the gradual increase of porosity. In the meantime, as the chemical solution from the outside entered the newly formed cracks and reacted with the mineral elements of the rocks, the cohesive force between the mineral particles was weakened periodically, which further accelerated the rock damage leading to macroscopic deterioration of the rock structures.


## 6. Conclusions

In summary, the coupling effects of acid solution and freeze-thaw had a significant influence on the microstructure of the red sandstone rock specimens. The coupled effects are supported by observations that rock specimens immersed in the H2SO4 solution were damaged more adversely than rock specimens immersed in pure water. The differences in the preconditioned states of the two categories of specimens prior to exposure to freeze-thaw cycles account for the different types of pore structures observed, with significant implications for bulk permeability and macroscopic mechanical properties. The following conclusions can be made:(1)
Compared with the sandstone specimens in pure water, the freeze-thaw deterioration of the sandstone specimens is more severe under acidic conditions. The mass of the rock specimens decreases evidently with the increasing number of freeze-thaw cycles, and large particles peel off and fragments fall from the surface of the rock specimens.(2)
The H2SO4 solution has a large effect on the changes in porosity of the sandstone specimens. With the increasing number of freeze-thaw cycles, the trend in a linearly increasing porosity becomes clear, given that the acid environment has accelerated the development and expansion of the cracks within the specimens.(3)
As the number of freeze-thaw cycles increases, the pore structure of the rocks changes evidently under the acidic conditions. After 30 freeze-thaw cycles, the total spectral area of the rock specimen #C7 (in H2SO4 solution) increased by 35.6%, and the number of peaks increased from 4 to 6. In the meantime, the total spectral area of the rock specimen #F7 (in pure water) increased by 21%, and the number of peaks increased from 4 to 5. In addition, the trend of each spectra peak is also different.(4)
The MRI of the freeze-thaw deterioration of the sandstones under the acidic environment shows that the freeze-thaw deterioration of the sandstones is a gradual process which develops from the exterior to the interior and is much more severe than observed for the rock specimens in pure water. Therefore, the chemical environment will produce significant effects on the freeze-thaw deterioration of the rock specimens.(5)
Typically the pore sizes can be classified into four categories: micropores, minipores, mesopores, and macropores. After 30 freeze-thaw cycles, differences occur in the changes of the pore structure of sandstones in the H2SO4 solution and pure water: in H2SO4 solution, the numbers of mesopores and macropores grow, while the numbers of micropores and minipores decline. In contrast, in the pure water, micropores and macropores increase, while the minipores and mesopores decline.(6)
In the coupling environment of freeze-thaw and chemical effect, cracks within the red sandstone develop and expand continuously with the influence of the freeze-grow force, which causes the porosity to increase. Furthermore, as H2SO4 solution enters periodically into new cracks and react with mineral elements, part of the minerals are dissolved and separated out, which accelerates the rock damage process. However, given that the major compounds SiO2 and H2SO4 in sandstone react slowly or do not react at all, the mass change of rock specimens is relatively small. Finally, the generation of the slightly soluble compound CaSO4 affects the chemical reaction and slows down the deterioration of the interior structure of the rock specimens.



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*Source: 9687843-2019-01-16.xml*