The Effect of Chloride Ions Morphology on the Properties of Concrete Under Dry and Wet Conditions

Abstract

In order to explore a model for the deterioration rate law and mechanism of concrete performance in salt lake water or sea water, the mixed sand concrete test of different forms of chloride ion erosion under a dry–wet cycle was simulated in the laboratory. The compressive strength and penetration depth were used to characterize the structural degradation degree of mixed sand concrete. The performance degradation of mixed sand concrete was analyzed through field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), thermogravimetry (TG), and nuclear magnetic resonance (NMR) testing. Experimental investigations have revealed that, at an age of 140 days and under alternating wet–dry conditions, liquid chloride ion erosion results in a 17.47% reduction in the compressive strength of blended sand concrete, accompanied by an erosion depth of 28.077 mm. This erosion progresses from the exterior towards the interior of the material. Conversely, gaseous chloride ion erosion exhibits a bidirectional penetration pattern. When subjected to gaseous chloride ion erosion, the compressive strength of blended sand concrete decreases by 31.36%, with an associated erosion depth of 38.008 mm. This exposure subjects the structure to heightened crystalline pressures, leading to severe deterioration of both the micro-porous structure within the concrete and the dense structure of hydration products. Consequently, the overall extent of structural damage is more pronounced, and the rate of degradation progression is accelerated. Under the action of liquid chloride ion erosion, the degradation of mixed sand concrete structure is caused by dry–wet fatigue, crystallization pressure, chloride salt erosion and calcium ion dissolution. Under the action of spray-born chloride erosion, the degradation of the mixed sand concrete structure is caused by dry–wet fatigue, crystallization pressure, chloride salt erosion, and calcium ion dissolution, among which crystallization degradation plays a major role. In line with the engineering standards for the utilization of vast desert resources in Inner Mongolia and the long-term service of concrete in the Hetao Irrigation District, our approach contributes to the achievement of sustainable development.

1. Introduction

With the rapid development of infrastructure in China and increasing environmental protection efforts, high-quality natural river sand resources have become scarce, and are unable to meet the growing demand of large-scale construction projects. This has resulted in a significant supply–demand imbalance for construction sand [1,2,3]. However, China possesses abundant resources of aeolian sand and manufactured sand. If mixed sand composed of aeolian and manufactured sand could be used as a substitute for river sand in construction, it would not only alleviate the supply–demand conflict, but also help mitigate the effects of sand encroachment in certain regions [4,5,6,7]. Nevertheless, in western China’s salt lake regions and coastal areas, high concentrations of corrosive chloride ions are present in lake and seawater. During the service life of mixed sand concrete, the protective layer of concrete at or near the water surface is exposed to chloride ions in different forms [8,9,10,11]. This exposure can lead to differential structural degradation around the water surface, causing more severe degradation and negatively impacting the application of mixed sand concrete in construction.

Currently, both domestic and international scholars have systematically conducted research on the effects of chloride ion corrosion on concrete performance. Xie Jian [12] and colleagues, through interfacial bonding tests between UHPC-NC in chloride environments, discovered that chloride ion corrosion negatively impacts the interfacial bonding strength of concrete, with a continuous decrease in strength as exposure time increases. Hasan Tawsif Mohammad [13] found in his study on the chloride resistance of ultra-high-performance concrete (UHPC) that UHPC with a water–cement ratio of 0.3 exhibits superior resistance to chloride ion penetration. Similarly, Azar Patrick [14] examined the chloride resistance of mineral admixture concrete and found that metakaolin leads to a highly connected porous network within the concrete, reducing its chloride penetration resistance. In contrast, ground-granulated blast-furnace slag (GGBFS) contributes to lower porosity and higher chloride binding capacity in concrete, thereby enhancing its resistance to chloride ion penetration. Wu Wenjuan et al. [15] conducted a study on the degradation caused by salt fog to concrete structures in a tropical marine environment. They found that the high-temperature and high-humidity salt fog environment caused severe erosion degradation to the surface protective layer of coral aggregate concrete structures on the inner side of the reef harbor pool. This was attributed to the significant loss of calcium ions in the coral aggregate concrete. Under the action of salt fog aggressive ions, the main binding components, such as C-S-H gel and calcium hydroxide, decreased or even disappeared, while magnesium-rich minerals increased. This led to an increase in the porosity of coral aggregate concrete, causing microstructural degradation and shortening its service life. Xu Jin [16] and colleagues studied the degradation characteristics and mechanisms of recycled concrete under salt spray exposure. Their research indicated that the inclusion of an appropriate amount of iron tailings in the concrete mix led to the formation of salt spray corrosion crystals, which slightly improved the mechanical properties and salt spray resistance of the recycled concrete by reducing its porosity. Li Lin [17] and colleagues analyzed the effects of chloride ion corrosion on reinforced concrete components under different environmental conditions. The study revealed that the chloride ion concentration inside the concrete was higher in immersion environments than in salt spray environments. Additionally, the internal chloride ion content increased with prolonged corrosion time, elevated environmental temperature, higher environmental chloride ion concentration and increased stress levels.

Current research on chloride-induced corrosion in concrete primarily focuses on the influence of various mineral admixtures on concrete’s resistance to chloride ion intrusion, and the performance changes of concrete in chloride-laden service environments. However, there have been limited studies published on the effects of different chloride ion forms on the properties of mixed sand concrete under dry–wet cycling conditions. To address this, this paper conducts indoor environmental simulation tests and microstructure tests through a series of experiments, including compression tests and erosion depth experiments. Additionally, field emission scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry (TG), and nuclear magnetic resonance (NMR) tests are employed to investigate the laws and mechanisms of the morphological effects of chloride ions on the properties of mixed sand concrete under dry–wet cycling conditions. Overall, this study provides references for the application of mixed sand concrete in salt lakes, the Hetao irrigation district, and marine construction projects. It also offers insights for achieving green ecological construction and sustainable development in the Hetao irrigation district.

2. Materials and Methods

2.1. Test Materials

(1)

Cement: Ji Dong Cement (P.O 42.5) from the Inner Mongolia region, specific surface area of 324 m²/kg, initial setting time of 180 min, final setting time of 385 min, and satisfactory volume stability.

(2)

Fly ash (FA): Class II fly ash provided by Jin Qiao Power Plant in Hohhot City, with a water requirement ratio of 101%, loss on ignition of 7.9%, and activity of 79%.

(3)

Fine aggregate: Includes aeolian sand (AS) selected from the Kubuqi Desert and basalt sand (BF) produced by mechanical crushing of basalt. The aeolian sand and basalt sand are mixed in a 1:1 ratio to form the mixed sand used as fine aggregate. The laser particle size test revealed that the main particle size distribution range of the aeolian sand is 4.5 μm to 255 μm, with a clay content of 1.39%. The basalt sand has a solidity index of 7% and a clay content of 0.872%. The particle gradation of the fine aggregate, as shown in Figure 1, has a bulk density, apparent density, and fineness modulus of 1578.68 kg/m³, 2764.21 kg/m³, and 1.64, respectively.

(4)

Coarse aggregate: Crushed stone with a particle size of 5.0 to 31.5 mm, with a bulk density of 1550 kg/m³ and an apparent density of 2680 kg/m³.

(5)

Mixing water: Tap water from the Inner Mongolia region, with a pH value of 7.0.

(6)

Admixture: A white powder-type polycarboxylate water reducer with a water reduction rate of 20%.

(7)

Epoxy resin: Bisphenol A type E51 epoxy resin, with a viscosity of 11,000–14,000 mpa·s, transparent color, and a usage temperature range of −50 to 150 °C.

Table 1. Concrete mix ratio and basic mechanical properties.

Mix Ratio/(kg/m3)							Compressive Strength/(MPa)
Water	Cement	Fly Ash	Water-Reducing Agent	Aeolian Sand	Basalt Sand	Stone	7 d	28 d
165.00	290.90	72.70	0.12	356.51	356.51	1163.30	32.26	44.19

shows the mix ratio and fundamental mechanical parameters of mixed sand concrete.

Figure 1. Fine aggregate particle gradation.

Figure 1. Fine aggregate particle gradation.

2.2. Test Method

2.2.1. Macroscopic Performance Test

(1)

Chloride Ion Erosion Test Under Dry–Wet Cycle

The structural degradation of mixed sand concrete subjected to dry–wet cycling conditions was assessed in accordance with the Standard for Test Methods of Long-Term Performance and Durability of Ordinary Concrete (GB/T 50082-2009 [18]). The experimental procedure is shown in Figure 2. The chloride ion erosion was performed for 16 h, followed by drying at 80 ± 5 °C for 6 h, and naturally cooling down to room temperature for 2 h. Each test cycle lasted for 24 h, and a total of 140 test cycles were designed. Before the erosion of the mixed sand concrete, the five surfaces of the concrete were coated with E51 epoxy resin for waterproofing treatment (except for the reserved surface). In the liquid ion erosion condition, a 2 mol/L sodium chloride solution was prepared as the erosion medium for immersion erosion. In the salt spray erosion condition, a 2 mol/L sodium chloride solution was prepared as the erosion medium and introduced into a self-made salt spray erosion chamber for chloride salt spray erosion. To ensure the sodium chloride concentration in the erosion solution and chloride salt fog gas during the test, the sodium chloride solution was regularly replaced.

(2)

Compressive Strength and Penetration Depth Test

According to the Standard Test Methods for Physical Mechanical Properties of Concrete (GB/T 50081-2019 [19]), the compressive strength and penetration depth of the mixed sand concrete specimens were tested at erosion ages of 0 days, 28 days, 56 days, 84 days, 112 days, and 140 days. The loading rate for compressive strength testing was 0.5 MPa/s (using a WHY-3000 universal testing machine, Shenzhen Wance Testing Machine Co., Ltd., Shenzhen, China). To increase the accuracy of the test, three parallel specimens were selected for each group, and the arithmetic mean was calculated. The penetration depth was measured using a 0.1 mol/L silver nitrate solution and a high-precision vernier caliper. Ten different measuring points were selected, and the arithmetic mean was calculated.

2.2.2. Microstructure Test

(1)

Scanning Electron Microscopy (SEM) Test

A sample of the mixed sand concrete, measuring approximately 1 cm³, was taken for microstructure analysis. The analysis was conducted using a US-made FEI Quanta 250 field emission scanning electron microscope (Thermo Fisher Scientific, Waltham, MA, USA). The resolution was 0.8 nm (15 kV) and 1.0 nm (1 kV), with a magnification range of 25–100,000×, an acceleration voltage range of 0.1–30 kV, and a probe current range of 1 pA to 200 nA.

(2)

Nuclear Magnetic Resonance (NMR) Test

A cylindrical specimen with a diameter of 48 mm and a height of 50 mm underwent vacuum saturation treatment for over 24 h within a device maintained at −0.1 MPa to achieve a water-saturated state. This process ensured the thorough saturation of internal pores, minimized residual air bubbles, and was followed by a static period of at least 12 h post-saturation. This allowed the water to reach a dynamic equilibrium within the pores, thereby preventing signal drift due to osmotic gradients. This analysis utilized a MesoMR23-060V-I nuclear magnetic resonance (NMR) apparatus (Meso Instruments, Rockville, MD, USA), in conjunction with a vacuum saturation device, to conduct a detailed examination of the internal pore structure within mixed sand concrete. The NMR instrument was equipped with a permanent magnet, exhibiting a magnetic field uniformity of 20 ppm. The probe coil had a diameter measuring 60 mm, while the radiofrequency (RF) transmission frequency surpassed 300 W. Throughout the testing procedure, the main magnetic field intensity was maintained at 0.55 T, with the H proton resonance frequency set at 23.320 MHz. Additionally, the magnet temperature was precisely controlled at 32.00 ± 0.01 °C, and the magnetic field stability was ensured to be below 300 Hz/h, enhancing the accuracy and reliability of the analysis.

(3)

X-ray Diffraction (XRD) Test

Crushed powder of the mixed sand concrete, sieved through a 200-mesh sieve, is selected for qualitative analysis of the microscopic hydration products using a Rigaku Ultima IV XRD instrument from Rigaku Corporation, Tokyo, Japan. The XRD test covers a diffraction angle (2θ) range of 5° to 90°, with a scanning rate of 2°/min.

(4)

Thermogravimetric (TG) Test

Crushed powder of the mixed sand concrete, sieved through a 200-mesh sieve, is used for quantitative analysis of the microscopic hydration products using a Netzsch STA449F5 Jupiter thermogravimetric analyzer from Netzsch-Gerätebau GmbH, Selb, Germany. The temperature range for the TG test is 20 °C to 1000 °C, with a heating rate of 10 °C/min.

3. Results and Discussion

3.1. Compressive Strength

From Figure 3, it can be observed that with the increase in erosion age, the compressive strength of the mixed sand concrete shows an initial increase followed by a decrease under the action of liquid chloride ion erosion. The rate of compressive strength degradation exhibits a slow increase initially, followed by a rapid increase. Under the erosion of spray-born chloride, the compressive strength of the mixed sand concrete shows a continuous decrease. The rate of compressive strength degradation is relatively slow in the early stage of erosion and follows a similar pattern to that under the action of liquid chloride ion erosion in the later stage of erosion.

From Figure 4, it can be seen that in the uneroded mixed sand concrete, the main cracks and numerous microcracks develop on the surface during compressive failure. Under the action of liquid chloride ion erosion, the surface degradation of the mixed sand concrete during compressive failure is similar to the uneroded concrete, but with fewer microcracks. Under the erosion of spray-born chloride, when subjected to a smaller load, the mixed sand concrete surface only develops a few main cracks, and failure occurs quickly.

Under the action of liquid ion erosion, the structural degradation of the mixed sand concrete mainly manifests as surface erosion, with the overall structure remaining relatively intact. However, under the erosion of spray-born chloride, numerous large cracks develop internally, leading to a more porous structure. The structural bearing capacity decreases sharply, and the structure reaches a failure state in a short period of time under different pressure loads.

3.2. Penetration Depth

Figure 5 demonstrates that the penetration depth of the mixed sand concrete consistently grows as the erosion cycles increase due to the combined effects of wet–dry cycles and various forms of chloride ion erosion. The rate of penetration depth growth exhibits a gradual escalation followed by a sudden surge. Nevertheless, when subjected to erosion by spray-born chloride, the mixed sand concrete experiences a greater penetration depth compared to erosion by liquid chloride ions, despite having the same erosion age. Under wet–dry cycles, spray-born chloride causes more degradation to mixed sand concrete compared to liquid chloride ions.

Figure 6 illustrates that the uneroded mixed sand concrete surface appears to be sleek and devoid of any cracks or pores. The liquid chloride ions have severely deteriorated the surface of the mixed sand concrete, resulting in the significant exposure of a large quantity of fine aggregate to the atmosphere. The unblemished surface devoid of erosion gives rise to numerous minuscule fissures, which then enlarge into substantial pores. The surface of mixed sand concrete becomes exceedingly loose and experiences partial separation, revealing aggregates and cement paste, due to the erosion caused by spray-born chloride. There are several fractures in the mesh on the surface that have not separated. Therefore, the chloride front advances from the exterior to the interior in the case of liquid chloride ions, but the erosion caused by spray-born chloride occurs in both directions.

The diffusion rate of liquid chloride ions in mixed sand concrete is less than that of spray-born chloride. In addition, spray-born chloride diffuses more slowly than liquid chloride ions in mixed sand concrete. As a result, the two distinct forms of chloride ions exhibit different erosion orientations when subjected to wet–dry cycles. The mixed sand concrete is saturated when exposed to liquid chloride ions, but not when exposed to spray-born chloride [20,21]. The erosion of spray-born chloride increases the number of sodium chloride crystals and the rate of crystallization in mixed sand concrete during the same drying period. Mixed sand concrete can be subjected to crystallization pressure in a shorter amount of time. In comparison to the erosion conditions of liquid chloride ions, the crystallization pressure inside the mixed sand concrete may have a similar impact-like effect on the internal structure of the concrete, resulting in more severe structural degradation under spray-born chloride erosion.

3.3. XRD

From Figure 7, it can be observed that in the initial state, the mixed sand concrete exhibits diffraction peaks of silicon dioxide, calcium hydroxide, and calcium carbonate. Under the erosion of liquid chloride ions, diffraction peaks of sodium chloride, calcium chloride, and Friedel’s salt appear, while the diffraction peak of calcium hydroxide decreases significantly. Under the erosion of spray-born chloride, only diffraction peaks of calcium chloride and sodium chloride are observed, and the diffraction peak of calcium hydroxide also decreases significantly. This indicates that under the erosion conditions of liquid chloride ions, the internal structure of the mixed sand concrete is degraded by three mechanisms: (1) crystallization pressure generated by the crystallization of sodium chloride inside the mixed sand concrete due to continuous wet–dry cycles; (2) dissolution of calcium ions due to the combination of chloride ions and calcium ions inside the mixed sand concrete, leading to the leaching of soluble calcium chloride; (3) the chloride ions react with the hydration products within the mixed sand concrete, leading to the creation of Friedel’s salt and the deterioration of the interior structure’s density. However, under the erosion of spray-born chloride, the main mechanisms leading to the degradation and destruction of the mixed sand concrete are the crystallization pressure of sodium chloride and the leaching of calcium ions.

3.4. Thermogravimetric

Due to various factors such as the sample selection, instrument parameters, and instrument models in thermal analysis tests, there can be significant differences in the conclusions obtained from the experiments. Based on the relevant literature and the results of this experiment, the thermal decomposition temperatures of various products are summarized as follows [22,23,24,25]:

(1)

Around 70 °C, the moisture in the sample and the air is thermally decomposed, which is not chemically bound.

(2)

Around 100 °C and 275 °C, a significant amount of initial dehydration of C-S-H gel occurs, as well as a small amount of secondary dehydration.

(3)

Around 330 °C, the eroded product Friedel’s salt undergoes thermal decomposition.

(4)

Around 400 °C, the hydration product calcium hydroxide (Ca(OH)₂) undergoes thermal dehydration decomposition.

(5)

Around 600 °C to 700 °C, the calcium carbonate (CaCO₃) in the aggregate undergoes thermal decomposition.

Under the influence of liquid ionic erosion, the mixed sand concrete undergoes four episodes of mass loss, leading to an augmentation in the mass loss of Friedel’s salt when compared to its pristine state. Refer to Figure 8 for details. Concurrently, there is a notable 56% decrease in the relative concentration of calcium hydroxide. See

Table 2. Relative content of hydration products.

Classify	C-S-H/%	Ca(OH)2/%	Friedel Salt/%	CaCO3/%
Initial	2.301	1.552	0.000	2.076
Liquid	1.591	0.686	1.941	2.959
Gas	0.284	0	0	3.947

for details. Analysis of both gaseous and liquid chloride ion erosion indicates that the cumulative consumption of calcium ions through the reactions involving calcium silicate hydrate and calcium hydroxide surpasses the augmentation of calcium ions from calcium carbonate during the carbonation process. This finding underscores the existence of calcium ion leaching, in addition to carbonation, as a mechanism that contributes to the depletion of calcium ions within the concrete matrix. Under the erosion of spray-born chloride, the mixed sand concrete undergoes two mass losses, namely hydrated calcium silicate gel and calcium carbonate. There is no weight loss interval for calcium hydroxide and Friedel’s salt, indicating that under the erosion of spray-born chloride, a large amount of calcium ions leach out from the mixed sand concrete without the occurrence of chloride ion erosion. This is consistent with the conclusions obtained from the analysis in Section 3.3.

The leaching of calcium ions from the mixed sand concrete is significantly greater under the erosion of spray-born chloride compared to the erosion of liquid chloride ions. Under the erosion of spray-born chloride, the structural degradation to the mixed sand concrete is significantly more severe. The analysis of the reasons is outlined as follows: The rate of diffusion of liquid chloride ions in the mixed sand concrete is slower than that of spray-born chloride within the same concrete. Chloride ions in a spray-born state can rapidly infiltrate the inner structure of the concrete. During the same period of erosion, the presence of spray-born chloride leads to an increased concentration of sodium chloride within the mixed sand concrete. The wet–dry cycles cause the mixed sand concrete to undergo greater crystallization pressure, exceeding its load-bearing limit and generating numerous cracks that provide more convenient pathways for the leaching of calcium ions. This leads to a higher leaching rate and quantity of calcium ions from the mixed sand concrete under the erosion of spray-born chloride.

3.5. NMR

Based on the principles of nuclear magnetic resonance (NMR) and the accompanying formulas [26], the position of the peak in the T2 spectrum of NMR is directly linked to the size of the pores. Similarly, the area under the peak is directly linked to the number of pores that correspond to the respective pore size [27,28,29]. From Figure 9a, it can be observed that under the erosion of different forms of chloride ions, the peak positions corresponding to the most probable pore size in the mixed sand concrete shift to the right, and the peak areas corresponding to the larger pore sizes significantly increase. This indicates that the pore sizes inside the mixed sand concrete are continuously transforming towards larger pore sizes. Moreover, the enlargement of pore diameters is more pronounced when exposed to spray-born chloride compared to the effects of liquid chloride ions.

The internal pores of concrete can be categorized into four groups according to their pore sizes: benign pores (0–0.02 μm), slightly detrimental pores (0.02–0.05 μm), harmful holes (0.05–0.2 μm), and highly detrimental pores (higher than 0.2 μm) [30]. The presence of various chloride ions causes the proportion of non-detrimental and less detrimental pores in the mixed sand concrete to drop, while the proportion of detrimental and more detrimental pores increases dramatically due to erosion. The erosion of spray-born chloride leads to a decrease in the proportion of harmless pores and an increase in the fraction of dangerous and more damaging pores. These findings suggest that the pore structure of the mixed sand concrete is significantly impaired when exposed to the erosion caused by spray-born chloride.

The reason for this is that the mixed sand leaches a large amount of calcium ions under the erosion of different forms of chloride ions [31]. Moreover, the pressure generated by the crystallization of sodium chloride causes the formation of new cracks and pores inside the mixed sand concrete, leading to connectivity between the pores.

As a consequence, there is a rise in the quantity of pores and an enlargement of pore dimensions in the mixed sand concrete. The leaching of calcium ions and the crystallization pressure within the mixed sand concrete are significantly greater when exposed to spray-born chloride as opposed to liquid chloride ions. Consequently, the pore structure of the mixed sand concrete suffers more extensive degradation when exposed to the corrosive effects of spray-born chloride, resulting in a greater loss of strength and a higher rate of deterioration.

3.6. SEM

From Figure 10, it can be observed that in the initial state, the internal structure of the mixed sand concrete exhibits good density and integrity, with the presence of hydrated calcium silicate gel, calcium hydroxide, and ettringite hydrate products [32,33]. Under the erosion of liquid chloride ions, the mixed sand concrete develops numerous microcracks and pores internally. Sodium chloride crystals and Friedel’s salt appear within the structure, resulting in a loose and less dense overall structure. Under the erosion of spray-born chloride, a similar formation of cracks and pores occurs within the mixed sand concrete, leading to a loose structure composed of stacked sodium chloride crystals. This indicates that the compact structure within the mixed sand concrete is severely degraded by the combined effects of wet–dry cycles and the erosion of different forms of chloride ions. Moreover, different forms of chloride ions exhibit some differences in the mode and extent of structural degradation within the mixed sand concrete.

Under the erosion of liquid chloride ions, the continuous wet–dry cycles lead to the formation of fatigue cracks within the mixed sand concrete. The precipitation of sodium chloride crystals generates crystallization pressure, which induces tensile stress and causes degradation to the compact structure of the hydrated products within the concrete. The reaction between chloride ions and the hydrated products in the mixed sand concrete results in the formation of Friedel’s salt and highly soluble calcium chloride. Both Friedel’s salt and calcium chloride contribute to the loosening of the structure of the concrete’s hydrated products. The leaching of calcium chloride increases the volume of voids within the mixed sand concrete. Therefore, under the erosion of liquid ions, the degradation to the internal structure of the mixed sand concrete is characterized by fatigue, crystallization pressure, erosion, and leaching.

Under the erosion of spray-born chloride, no distinct features of calcium hydroxide and Friedel’s salt were observed in the microscanning photos. This indicates that there is no significant erosive degradation within the internal structure of the mixed sand concrete under the erosion of spray-born chloride. Therefore, under the erosion of spray-born chloride, the degradation to the internal structure of the mixed sand concrete is characterized by fatigue, crystallization pressure, and leaching, with crystallization pressure playing a dominant role. This is consistent with the previous conclusions.

4. Conclusions

This study aims to investigate the influence of different forms of chloride ions on the properties of mixed sand concrete under the combined effects of drying–wetting cycles and salt fog erosion, and to elucidate the underlying mechanisms. The performance of mixed sand concrete is affected by the erosion from various forms of chloride ions, which all contribute to the outcomes. Based on our experimental results, the following conclusions are drawn:

(1)

Under dry–wet cycling conditions, mixed sand concrete subjected to spray-born chloride ion erosion exhibits an 11.82% decrease in compressive strength and a 26.13% augmentation in erosion depth, in contrast to its performance under liquid chloride ion erosion. Notably, the degradation incurred under spray-born chloride erosion is more pronounced and progresses at a faster rate. Consequently, under compressive loading, the material demonstrates a failure mode characterized by extensive fragmentation, accompanied by a markedly shorter duration to failure.

(2)

Different forms of chloride ions have different erosion directions on concrete under the conditions of wet–dry cycles. The erosion direction of liquid chloride ions on mixed sand concrete is from the outside to the inside, while under the erosion of spray-born chloride, the chloride ions show bidirectional erosion within the mixed sand concrete.

(3)

Under the condition of wet–dry cycles, compared to the erosion of liquid chloride ions, the erosion of spray-born chloride exerts greater crystallization pressure on the structure of the mixed sand concrete. The micro-pore structure and the compact structure of the hydrated products inside the concrete are severely degraded, resulting in higher overall structural degradation and a faster degradation rate of the mixed sand concrete.

(4)

Under the coupling effect of wet–dry cycles and different forms of chloride ion erosion, the erosion and degradation mechanisms of the mixed sand concrete are different. Under the erosion of liquid chloride ions, the structural degradation of the mixed sand concrete is caused by the combined effects of wet–dry fatigue, crystallization pressure, chloride salt erosion, and leaching of calcium ions. Under the erosion of spray-born chloride, the structural degradation of the mixed sand concrete is caused by wet–dry fatigue, crystallization pressure, and leaching of calcium ions, with crystallization degradation playing a major role.

Future research will focus on the long-term applicability of these findings in the desertification areas and the Hetao Irrigation District of the Inner Mongolia Autonomous Region, China. Specifically, we aim to address the urgent need for engineering materials amidst the shortage of river sand resources. Our research endeavors to provide technical references for the green and ecological long-term application in the Hetao Irrigation District, promoting sustainable development through the localized utilization of desert resources.