Electrochemical Degradation Mechanism of Desert Sand Concrete Under the Combined Action of Electric Field and Sulfate

Abstract

To promote the sustainable utilization of desert sand as a regional resource in the infrastructure construction of saline-alkali areas, this paper proposes an accelerated test method based on the coupling of an external electric field (60 V) and a 2% Na₂SO₄ solution for rapid evaluation of its sulfate erosion resistance. The optimal mix proportion (FA 10%, water-to-binder ratio 0.33, cement-to-sand ratio 1:1.5, SF 10%) was determined through orthogonal experiments. By employing multi-scale analytical techniques including electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermal analysis (TG-DTG), the differentiated deterioration mechanisms driven by the electric field were systematically revealed. The results show that the charge-transfer resistance (Rct) decreased by about 95% within 28 d, demonstrating the characteristic of “micro-scale deterioration preceding macro-scale strength loss.” The anode region was dominated by dissolution of hydration products (porosity 5.1%), while the cathode region, due to enrichment of sulfate ions (S content 3.37 wt.%), generated a large amount of expansive products, leading to more pronounced structural damage (porosity 8.3%) and greater mass loss (cathode 12.56% > anode 9.85%). This study not only elucidates the deterioration mechanisms of desert sand concrete under coupled environmental action, but also provides a mechanism-explicit, rapid and efficient laboratory evaluation method for its sulfate resistance, offering practical guidance for durability design and prevention in engineering structures exposed to saline-alkali conditions.

1. Introduction

With the increasingly widespread application of desert sand concrete in infrastructure construction in the saline-alkali regions of Northwest China, the market demand for fine aggregates continues to grow. River sand, as the most commonly used fine aggregate in concrete, has been over-exploited worldwide, leading to severe environmental and resource issues. Therefore, developing sustainable aggregate systems has become an inevitable direction for the transformation of the building materials industry [1,2]. Desert sand, owing to its abundant reserves, low cost, and local availability, is gradually regarded as an ideal substitute for river sand. Using desert sand to prepare concrete not only helps reduce project costs and promote economic development in desert regions, but also effectively alleviates dependence on natural river sand resources, offering significant environmental and sustainability benefits [3,4]. However, the widely distributed saline soil in Northwest China contains various corrosive soluble salts such as sulfates, chlorides, and magnesium salts [5,6,7], which pose severe challenges to the long-term durability of desert sand concrete. Consequently, its corrosion resistance has attracted widespread attention [8].

Sulfate attack is one of the major deterioration mechanisms affecting concrete durability [9,10]. The process involves the transport of SO₄²⁻ within concrete pores, chemical reactions with hydration products, and the formation of expansive products (such as gypsum and ettringite) [11,12,13], leading to structural cracking [14], strength reduction, and loss of bonding performance [15,16]. To accelerate the evaluation of concrete’s sulfate resistance, researchers have adopted various acceleration methods, including wet-dry cycles [17], multi-factor coupling [18,19,20], and electrochemical approaches [21,22]. Among these, the application of an external electric field can significantly promote ion migration and has been widely used for rapid evaluation and protection against chloride ion penetration [23,24,25]. However, existing studies have primarily focused on the behavior of ordinary concrete or recycled aggregate concrete under single sulfate or chloride attack. There remains a lack of systematic research on the deterioration mechanism of desert sand concrete under the coupled action of electric field and sulfate erosion [26].

Compared to conventional concrete, the differences in particle morphology, mineral composition, and gradation of desert sand may influence its ion transport pathways, distribution of reaction products, and microstructural evolution. Furthermore, the distinct electrochemical processes occurring in the cathodic and anodic regions under an applied electric field may lead to spatial differentiation in the erosion mechanisms a phenomenon that has not been fully elucidated in desert sand concrete. Therefore, this study establishes a coupled “electric field–sulfate” acceleration system and employs multi-scale characterization techniques, including electrochemical impedance spectroscopy, X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetry–differential scanning calorimetry, to systematically investigate the electrochemical response, product evolution, and microstructural damage mechanisms of desert sand concrete during accelerated erosion. This paper aims to: (1) clarify the deterioration patterns of desert sand concrete under the coupled electric field–sulfate environment; (2) reveal the differential corrosion mechanisms in the cathodic and anodic regions; (3) establish correlations between macroscopic performance degradation and microstructural evolution, thereby providing theoretical foundations and methodological support for the durability design and rapid evaluation of desert sand concrete in saline-alkali environments.

2. Experimental Study

2.1. Materials

The cement used in this experiment was 42.5R grade ordinary Portland cement (OPC) produced by Xinjiang Tianshan Cement Co., Ltd. (Urumqi, China); fly ash (FA) was sourced from Grade I fly ash produced in the Ganquanbao Industrial Park, Xinjiang, China; silica fume (SF) was obtained from Xinjiang Jingxin Silicon Industry Co., Ltd. (Urumqi, China); desert sand (DS) was collected from the southern part of the Taklimakan Desert, with a fineness modulus of 0.8858. The chemical compositions and contents of the four raw materials, as determined by XRF, are shown in

Table 1. Chemical composition and content of raw materials (wt.%).

Materials	SiO2	CaO	Al2O3	Fe2O3	MgO	SO3	Cl
OPC	24.51	55.51	5.36	6.11	1.83	4.39	0.14
FA	43.52	11.45	16.28	12.51	3.89	6.59	0.27
SF	95.53	0.49	1.11	0.16	0.59	0.17	0.90
DS	65.15	11.80	12.16	4.29	2.35	-	0.15

. The main chemical components of FA, OPC, SF, and DS are SiO₂, CaO, and Al₂O₃. The SEM and XRD patterns of FA, SF, and DS are shown in Figure 1. FA consists of spherical and blocky particles with a rough surface, and its main phases include SiO₂, Al₂O₃, and albite (Na₂O·Al₂O₃·6SiO₂). SF exhibits a rough surface, and its primary phase is SiO₂. DS displays blocky structures of varying shapes, with main phases consisting of SiO₂ and albite. The water used was distilled water produced in the laboratory. The water-reducing agent was PCA-I series polycarboxylate high-performance water reducer produced by Jiangsu Sobute New Materials Co., Ltd. (Nanjing, China), with a water reduction rate of 25%. Na₂SO₄ was supplied by General-Reagent China (Shanghai, China), with a purity of ≥99.0%.

2.2. Experimental Design and Specimen Preparation

Firstly, the optimal mix proportion of natural desert sand concrete was investigated through orthogonal experiments. The orthogonal design considered four factors: FA content (factor A), water-to-binder ratio (factor B), cement-to-sand ratio (factor C), and SF content (factor D). An orthogonal array L₁₆(4⁴) was adopted, with each factor designed at four levels as shown in

Table 2. Level table of orthogonal experimental factors.

	A	B	C	D
1	0	0.3	1:1.5	0
2	10%	0.33	1:2	5%
3	20%	0.36	1:2.5	10%
4	30%	0.39	1:3	15%

. Subsequently, based on the results of mechanical properties and range analysis, the optimal mix proportion was determined to prepare specimens. The specimens were subjected to degradation tests in an electrochemical device combining external electric field and sulfate coupling, with an applied voltage of 60 V and sulfate concentration of 2%. The EIS, mass, and mechanical properties of the concrete before and after degradation were measured, and microstructural tests including XRD, SEM, FTIR, and TG-DTG were conducted.

The required materials were calculated based on the mix proportion and weighed using an electronic balance. The binder materials (OPC, FA, SF) were mixed uniformly in a tray, after which desert sand and the binder materials were sequentially poured into a mixer and stirred for 2 min. The water-reducing agent was mixed with water, and after thorough stirring, the mixture was added to the mixer and stirred for an additional 3~4 min [27]. After mixing, the fresh concrete was placed into 70.7 mm × 70.7 mm × 70.7 mm molds pre-coated with a release agent, compacted on a vibrating table for 45 s, and smoothed with a trowel. The concrete surface was covered with a plastic film, demolded after 24 h, and placed in a standard curing room for 28 d.

2.3. Test Methods

2.3.1. External Voltage Sulphate Coupling and Immersion Testing Without External Voltage

The electrochemical accelerated dissolution test setup is illustrated in Figure 2a. A 2% Na₂SO₄ solution was selected as the erosion medium based primarily on the following considerations: when the sulfate content in the environment is below 2%, its salt expansion hazard to engineering structures is relatively minor, whereas a concentration above 2% can cause significant engineering damage. Furthermore, this concentration offers good electrical conductivity, minimal chemical interference, operational safety, and better reflects actual saline-alkali conditions, thereby enhancing the reliability of the test results. The applied voltage was set at 60 V, a value determined after comprehensively comparing commonly used acceleration voltages in the literature: although higher voltages can further accelerate ion migration and shorten the test period, excessively high voltages may alter the chemical equilibrium of the pore solution, induce notable Joule heating effects and electrode polarization, thereby affecting the true pore structure and permeability of the concrete. A voltage of 60 V has been verified to achieve a good balance between acceleration efficiency and experimental authenticity.

The test was conducted under constant-voltage mode. The anode was an iridium-tantalum-titanium plate electrode with excellent corrosion resistance, and the cathode was a titanium plate electrode [28], both with a thickness of 1 mm. The concrete specimens, measuring 70.7 mm × 70.7 mm × 70.7 mm, were placed between the two electrodes, with the electrodes parallel to the specimen surfaces and a spacing of approximately 70 mm. Based on measured data, the steady-state current density of the system under this voltage was about 796.2 A/m². The volume of solution in the electrolytic cell was approximately 7.5 × 10⁻⁴ m³. Throughout the test, the temperature was maintained within (20 ± 2) °C by a constant-temperature water bath to eliminate the influence of temperature fluctuations on the electrochemical processes. For comparison, a traditional steep test (ST) without applied voltage was set up in the same solution environment, as shown in Figure 2b.

2.3.2. Compression Testing Method

This test utilized a microcomputer-controlled fully automatic compression testing machine (model WHY-3000, Shanghai, China) to measure the compressive strength of concrete. The molded side surface served as the compression face and was aligned with the center of the testing machine’s platen to eliminate the influence of eccentric loading on the test results. The loading rate was set at 0.5 MPa/s. The failure load was recorded after the specimen was completely destroyed. The compressive strength of concrete was calculated according to Formula (1) (the compressive strength measured in this test is a relative value and has not been converted for size).

$${ƒ}_{\mathrm{cu}}={\displaystyle \frac{F}{A}}$$

(1)

In the formulation:

ƒ_cu—Compressive Strength of Concrete (MPa);

F—Destructive load (N);

A—area under pressure (mm²);

The results are calculated with an accuracy of 0.1 MPa.

The extreme difference analysis is calculated using Formulas (2)–(4):

$$K_i=\sum \mathit{qui}$$

(2)

$$\overline{k_i}={\displaystyle \frac{k_i}{S}}\left(S=4\right)$$

(3)

$$R=\max \{{\overline{K}}_1,{\overline{K}}_2\dots \dots {\overline{K}}_{\mathrm{i}}\}-\min \{{\overline{K}}_1,{\overline{K}}_2\dots \dots {\overline{K}}_{\mathrm{i}}\}$$

(4)

2.3.3. EIS Testing Method

Figure 3 depicts a schematic diagram of the EIS test carried out using an electrochemical workstation (CHI660E, Chenhua). The frequency scan ranges from 0.05 Hz to 100 kHz. Before the EIS test, the sample must be placed in an automatic water saturation device for 3 h of pressure saturation, 1 h of water saturation, and 18 h of immersion. EIS testing is carried out by affixing thin steel plates to both sides of the concrete and attaching them to the electrochemical workstation via alligator clips [29,30].

2.3.4. Microstructural Examination

Take the test block before and after destruction in an external electric field-sulphate coupling environment, and examine it for phase composition, microstructure, and other characteristics. First, soak it in anhydrous ethanol for 24 h, then dry it at 60 °C. The initial and damaged specimens were tested using an X-ray diffractometer (XRD, D8 Advance, Billerica, MA, USA) with the following parameters: scan step width of 0.02°, scan range of 10–80°, and scan speed of 10°/min. The microstructure and morphology of desert sand concrete were examined before and after electrochemical corrosion using a scanning electron microscope (SEM, Hitachi S 4800, Tokyo, Japan). Additionally, the SEM images were binarized and processed using ImageJ software (Version 1.53) to estimate the porosity of the anode and cathode regions after 28 d. The Fourier transform infrared spectrometer (FTIR, Great 10, Tianjin, China) was used to investigate changes in the sample’s functional groups before and after corrosion. The test conditions are: potassium bromide pressing method, material: potassium bromide 1:200, scan 32 times, resolution 4 cm⁻¹, test range 400–4000 cm⁻¹. Thermogravimetric analysis (TG-DSC, DSC200F3, Selb, Germany) was performed to determine the content and temperature of hydration products. Testing conditions: at room temperature, the temperature was raised to 1000 °C at a rate of 10 °C per minute in an air atmosphere [31]. To determine the electrochemical properties of desert sand concrete, test the EIS of specimens before and after corrosion using an electrochemical workstation.

3. Result and Analysis

3.1. Orthogonal Experiments and Range Analysis

The 28 d compressive strength results of the orthogonal experiment are shown in

Table 3. Orthogonal experimental design and results.

Sample No.	A/%	B	C	D/%	28 d Compressive Strength/MPa
1	1 (0)	1 (0.3)	1 (1:1.5)	1 (0)	40.2
2	1 (0)	2 (0.33)	2 (1:2)	2 (5)	30.7
3	1 (0)	3 (0.36)	3 (1:2.5)	3 (10)	23.5
4	1 (0)	4 (0.39)	4 (1:3)	4 (15)	24.0
5	2 (10)	1 (0.3)	2 (1:2)	3 (10)	35.2
6	2 (10)	2 (0.33)	1 (1:1.5)	4 (15)	33.4
7	2 (10)	3 (0.36)	4 (1:3)	1 (0)	27.4
8	2 (10)	4 (0.39)	3 (1:2.5)	2 (5)	27.6
9	3 (20)	1 (0.3)	3 (1:2.5)	4 (15)	23.8
10	3 (20)	2 (0.33)	4 (1:3)	3 (10)	27.4
11	3 (20)	3 (0.36)	1 (1:1.5)	2 (5)	29.3
12	3 (20)	4 (0.39)	2 (1:2)	1 (0)	22.0
13	4 (30)	1 (0.3)	4 (1:3)	2 (5)	10.0
14	4 (30)	2 (0.33)	3 (1:2.5)	1 (0)	18.0
15	4 (30)	3 (0.36)	2 (1:2)	4 (15)	19.8
16	4 (30)	4 (0.39)	1 (1:1.5)	3 (10)	30.0

. The results indicate that when the FA content is 10%, SF content is 10%, cement-to-sand ratio is 1:2, and water-to-binder ratio is 0.3, the concrete exhibits the optimal mechanical properties. However, during the experiment, the workability of this mix proportion did not meet the construction requirements and is therefore not recommended for practical application. Further range analysis of the compressive strength test results is necessary to determine the optimal mix proportion. The range analysis results are presented in

Table 4. Extreme value analysis for an orthogonal experiment.

Experiment	Extremely Poor	Factor
		A	B	C	D
28 d compressive strength	K1	118.4	109.2	132.9	107.6
	K2	123.6	109.5	107.7	97.6
	K3	102.5	100.0	92.9	116.1
	K4	77.8	103.6	88.8	101.0
	K1	29.6	27.3	33.2	26.9
	K2	30.9	27.4	26.9	24.4
	K3	25.6	25.0	23.2	29.0
	K4	19.5	25.9	22.2	25.3
	R	11.4	2.4	11	4.6

.

The magnitude of the R value indicates the degree of influence of each factor on the compressive strength, with a larger R value representing a greater degree of influence. Based on the orthogonal experimental results and range analysis presented in Table 3 and Table 4, the order of the factors affecting compressive strength from greatest to least is: A > C > D > B. The optimal mix proportion is A2B2C1D3, which corresponds to the following optimal dosages: FA 10%, water-to-binder ratio 0.33, cement-to-sand ratio 1:1.5, and SF 10%. The orthogonal experiments described above determined the optimal reference mix proportion based on the 28 d compressive strength as the key indicator. This mix proportion provides a benchmark material with reliable performance for subsequent studies, while ensuring good workability and early stage strength. Specimens prepared with this optimal mix proportion were used in the electric field–sulfate coupled acceleration tests, aiming to investigate the resistance capability and underlying mechanisms of desert sand concrete against electrochemically accelerated erosion under its optimal material performance state.

3.2. Mechanical Properties and Macrographic Analysis

Figure 4 shows the variations in compressive strength and the evolution of apparent morphology of desert sand concrete after 0, 7, 14, 21, and 28 d of deterioration under the coupled action of an external electric field and sulfate. Regarding the evolution of compressive strength, the test results indicate that the compressive strength of the concrete exhibited a monotonic decreasing trend with prolonged corrosion time. At 28 d, the compressive strength decreased from an initial value of 31.8 MPa to 29.9 MPa, representing an absolute loss of 1.9 MPa and a relative loss rate of 5.9%. To quantify this macroscopic strength change in depth and scientifically evaluate the additional damage introduced by erosion, this study introduced the unified predictive model for the mechanical properties of desert sand concrete established by Sadat et al. [32]. According to this model, for the reference mix proportion used in this study (where desert sand completely replaces river sand, replacement ratio η_r = 1), the predicted cube compressive strength can be expressed as f_cu(r) = (1 − 0.12 × 1) f_cu = 0.88 f_cu. If estimated based on the theoretical strength of the reference concrete without desert sand, this model’s predicted value represents the typical performance level of ordinary desert sand concrete. The initial strength measured in this test (31.8 MPa) was significantly higher than the conventional predicted value, confirming that the mix proportion optimized through orthogonal experiments in this study (incorporating 10% silica fume and fly ash) effectively enhanced the compactness and strength of the matrix, making its initial performance superior to the average level of ordinary desert sand concrete. However, after 28 d of electric field–sulfate coupled erosion, the strength dropped to 29.9 MPa, indicating a clear performance degradation. This extent of degradation (5.9%) can be regarded as the additional damage caused by the erosion environment on top of the optimized mix proportion.

In terms of the evolution of apparent morphology (Figure 4), using the 0 d specimen as a reference, the concrete exhibited spatially differentiated failure morphologies at the cathode and anode. In the anode region, significant dissolution characteristics appeared with increasing corrosion time: aggregates gradually became exposed and were accompanied by surface paste spalling. Furthermore, under the combined effect of continuous electrification and the low pH environment in the anode region, the cementitious matrix gradually turned yellowish-brown. In the cathode region, substantial attachment of white corrosion products as well as the formation and propagation of microcracks were observed. Moreover, with prolonged exposure time, both the accumulation of products and the number of cracks increased noticeably.

Taken together, the loss in macroscopic compressive strength (6.0%) is a clear signal of performance degradation. However, electrochemical impedance spectroscopy analysis revealed that the charge transfer resistance (Rct) decreased by approximately 95% over the same period (see Section 3.3.1), indicating that the ion migration barrier was nearly dismantled. Microstructural characterization further confirmed that the porosity in the cathode region increased to 8.3%, with substantial enrichment of sulfate expansion products (see Section 3.3.2 and Section 3.3.3). This collective evidence demonstrates that the transport properties and microstructural integrity of the concrete have undergone extremely severe and early-stage deterioration. The phenomenon of “deterioration in microstructure and transport properties occurring much earlier and more severely than the loss in macroscopic strength” is a typical characteristic of the initial stage of concrete corrosion. The microcracks and interconnected pores formed at the early stage set the stage for accelerated mechanical degradation in later phases. It also highlights the unique value of employing non-destructive testing methods, such as electrochemical impedance spectroscopy, for early-stage durability diagnosis.

3.3. Micro-Testing Analysis

3.3.1. EIS Analysis Before and After Corrosion

Figure 5 shows the Bode diagrams of desert sand concrete after 0, 7, 14, 21, and 28 d of corrosion under the coupled action of external electric field and sulfate environment. From the analysis of the impedance modulus in Figure 5a, the impedance modulus is highest at 0 d, demonstrating the high resistance characteristic of a dense microstructure. In the mid-frequency region, the modulus decreases from 1 × 10⁵ to 1 × 10⁴ Ω with increasing electrolysis time. This indicates that sulfate ions penetrate into the concrete interior, react with hydration products to form expansive compounds, resulting in porosities of 8.3% and 5.1% in the cathode and anode regions, respectively, after 28 d, as well as the initiation and propagation of microcracks, leading to a sharp decline in the overall impedance performance of the material. From the analysis of the phase angle in Figure 5b, the peak of the phase angle shifts toward the high-frequency region, indicating that pores continuously increase, microcracks form and interconnect during the corrosion process, thereby providing faster pathways for ion migration, which is consistent with the impedance modulus results [33]. The EIS findings align with the decline in macroscopic mechanical properties and the deterioration of the microstructure.

Figure 6 shows the fitting results of the equivalent circuit model for desert sand concrete at different corrosion periods. Figure 6f presents the Nyquist diagram. From the diagram, it can be observed that as the corrosion time increases, the radius of the capacitive arc in the electrochemical impedance spectrum gradually decreases, and the point resistance value shifts leftward along the horizontal axis. This trend is fully consistent with the quantitative data obtained from the circuit fitting. The Rct value decreases from approximately 119 kΩ·cm² at 0 d to about 6 kΩ·cm² at 28 d, indicating that as the electrolysis time prolongs, the compactness of the specimen decreases and the porosity increases [34]. The deterioration process reflected in the Nyquist diagram aligns with the microstructural damage observed via SEM and the trend of macroscopic compressive strength decline.

3.3.2. Analysis of the Evolution of Corrosion Products and Microstructure Based on XRD and FTIR

Figure 7, through a comprehensive comparative analysis of the XRD patterns and FTIR spectra of specimens under anodic, cathodic, and standard immersion conditions, reveals the significant influence of different corrosion environments on the deterioration mechanism of desert sand concrete. As corrosion progresses, the relative intensity of the SiO₂ diffraction peak in the XRD patterns of the samples gradually increases, indicating continuous dissolution or spalling of the cement paste, which leads to the gradual exposure of stable desert sand particles. The sample under anodic immersion exhibits the highest relative intensity of SiO₂, suggesting that dissolution dominates in the anode region. In the anode region Figure 7a, the intensities of the characteristic diffraction peaks of Ca(OH)₂ and C-(A)-S-H in the XRD patterns decrease markedly with corrosion time, while the formation peaks of gypsum (CaSO₄) and ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O, abbreviated as AFt) strengthen [35]. The FTIR spectra further confirm this process: the intensity of the Si-O-T (T = Si or Al) stretching vibration peak near 960 cm⁻¹ (attributed to C-S-H/C-A-S-H gel) decreases [36], whereas the absorption of S-O bond vibrations in the range of 1150–1070 cm⁻¹ (particularly near 1080 cm⁻¹ for gypsum and 989 cm⁻¹ for AFt) weakens significantly [37], indicating that the anode region primarily undergoes dissolution of hydration products [38].

In the cathode region Figure 7b, XRD and FTIR reveal a more intense accumulation of sulfate products: the diffraction peak intensities of gypsum and ettringite are higher than those in the anode region, and the S-O vibration peaks in the FTIR spectra within the 1150–1070 cm⁻¹ range (particularly near 1080 cm⁻¹ for gypsum and 989 cm⁻¹ for AFt) are also more pronounced. Meanwhile, the intensity of the Si-O-T peak decreases significantly, indicating that the cathode region, due to electromigration, accumulates higher concentrations of ions, promoting more vigorous expansion reactions and leading to more severe microstructural damage. Specifically, CaSO₄ is formed by the reaction of sulfate ions penetrating the concrete with internal Ca(OH)₂ (see Equation (5)). AFt can be generated from the reaction of unreacted tricalcium aluminate (C₃A) with SO₄²⁻ (see Equation (6)), and can also be produced through reactions of CaSO₄ with unreacted C₃A, tetracalcium aluminate hydrate (C₄AH₁₃), and monosulfoaluminate (C₄AH₁₂) (see Equations (7)–(9). The formation of CaSO₄ and AFt generates expansive stress, leading to the initiation and propagation of microcracks.

$$\mathrm{CH}+{{\mathrm{SO}}_4}^{2^{-}}+2\mathrm{H}\to \mathrm{C}\overline{S}{\mathrm{H}}_2+2{\mathrm{OH}}^{-}$$

(5)

$${\mathrm{C}}_3\mathrm{A}+3{{{\mathrm{SO}}_4}^2}^{-}+3\mathrm{CH}+32\mathrm{H}\to {\mathrm{C}}_6\mathrm{A}{\overline{S}}_3{\mathrm{H}}_{32}+6{\mathrm{OH}}^{-}$$

(6)

$${\mathrm{C}}_3\mathrm{A}+3\mathrm{C}\overline{S}{\mathrm{H}}_2+26\mathrm{H}\to {\mathrm{C}}_6\mathrm{A}{\overline{S}}_3{\mathrm{H}}_{32}$$

(7)

$${\mathrm{C}}_4{\mathrm{AH}}_{13}+3\mathrm{C}\overline{S}{\mathrm{H}}_2+14\mathrm{H}\to \mathrm{C}6\mathrm{A}{\overline{S}}_3{\mathrm{H}}_{32}$$

(8)

$${\mathrm{C}}_4\mathrm{A}\overline{S}{\mathrm{H}}_{12}+2\mathrm{C}\overline{S}{\mathrm{H}}_2+16\mathrm{H}\to {\mathrm{C}}_6\mathrm{A}{\overline{S}}_3{\mathrm{H}}_{32}$$

(9)

Under conventional immersion conditions Figure 7c, however, the XRD and FTIR results exhibit distinctly different trends: the early hydration process continues, with some increase in the diffraction peaks of C-(A)-S-H gel and the intensity of the Si-O-T vibration peak; slight characteristics of sulfate attack only appear at later stages, demonstrating that its reaction rate is much lower than under electric-field acceleration. The above analysis indicates that the external electric field not only greatly accelerates the kinetic process of sulfate attack, but also, by driving directional ion migration, leads to different deterioration characteristics in the cathode and anode regions where the cathode is dominated by a combined physico-chemical expansion damage, while the anode focuses more on the chemical dissolution of hydration products. This provides a key basis for understanding the degradation mechanism of concrete under coupled electric-field–sulfate environments.

3.3.3. SEM Analysis

Figure 8 shows the SEM images of the anode and cathode surfaces of desert sand concrete after 7, 21, and 28 d of electrolysis under the coupled action of external electric field and sulfate environment, along with the corresponding energy-dispersive spectroscopy (EDS) compositional analysis. In Figure 8a (anode surface), it can be observed that after 7 d of electrolysis, flaky Ca(OH)₂ and honeycomb-like C-(A)-S-H gel are present. The enlarged view within the red dashed box shows partial dissolution of Ca(OH)₂. After 21 d of electrolysis, the flaky Ca(OH)₂ suffers severe dissolution, and microcracks appear in the gel structure. After 28 d of electrolysis, the flaky Ca(OH)₂ marked by the red dashed box shows significant dissolution, with cracks appearing at the location indicated by the red arrow, and the gel structure becomes loose. This indicates that the anode reaction driven by the electric field accelerates the dissolution of key hydration products, leading to a reduction in the bonding capacity of the cementitious system. The porosity measured in the anode region after 28 d was 5.1%. EDS analysis performed on this region (see attached figure) indicates that S-element was already detectable in the anode area, with a content of about 1.12 wt.%, confirming that sulfate ions have migrated to the anode under the electric-field driving force and participated in the reactions.

Figure 8b (cathode surface) shows that after 7 d of electrolysis, dissolution traces of CaSO₄ can be observed, with pores and microcracks appearing in the areas marked by dashed boxes and arrows. A small amount of AFt formation is visible around the pores. After 21 d of electrolysis, the deterioration of the gel structure continues to develop, with an increase in pores and cracks, and rod-shaped AFt is observed within the pores. After 28 d of electrolysis, the gel structure becomes further loosened due to the electric field effect, resulting in reduced bonding material, significantly enlarged pores marked by dashed boxes, and a noticeable increase in AFt formation. The porosity in this region increased to 8.3% after 28 d, which is markedly higher than that in the anode region. EDS analysis of the cathode region further reveals that its S-element content is as high as 3.37 wt.%, approximately three times that of the anode region, while the Ca content (16.96 wt.%) is also significantly higher. This directly confirms that the electromigration effect leads to strong enrichment of sulfate and calcium ions in the cathode region, thereby triggering more intense sulfate attack reactions and the formation of expansive products such as AFt.

The significant differences in composition (S and Ca contents) and structure (porosity) between the cathode and anode indicate that the deterioration of concrete under the coupled action of electric field and sulfate exhibits spatially differentiated characteristics. The anode region is dominated by electrochemical dissolution of hydration products, whereas the cathode region is governed by expansion reactions driven by ion enrichment. This differentiated deterioration mechanism corroborates with the decrease in charge transfer resistance observed in electrochemical impedance spectroscopy (EIS) and the decline in macroscopic mechanical properties, collectively revealing the microscopic mechanism of electric-field-accelerated sulfate attack.

Figure 9 shows SEM images of desert sand concrete under conventional immersion at 0, 30, and 60 d. For the specimen immersed for 0 d, a small amount of hydrated gel material is observed. After 30 d of immersion, the micro morphology of the specimen indicates that the cement hydration process continues slowly, with hydration products gradually filling the pores, resulting in a denser structure compared to the reference specimen, and no significant corrosion damage is observed. After 60 d of immersion, slight corrosion characteristics begin to appear in the specimen, manifested as the generation of a small number of microcracks and the presence of more abundant gel-like fillers in the pores, with only local areas of the cementitious structure being damaged. This evolution process indicates that under natural immersion conditions without an external electric field, sulfate ions rely solely on concentration gradient for diffusion, resulting in extremely slow migration rates and reaction kinetics, with hydration enhancement dominating the early stage.

3.3.4. TG-DTG Analyses

Figure 10 shows that the TG-DTG curves indicate, after 28 d of erosion, the total mass-loss rates of the cathode region, immersion group, and anode region decrease in sequence, being 12.56%, approximately 11.9%, and 9.85%, respectively. This quantitatively reflects the characteristics of product accumulation and material loss under different erosion mechanisms. In the low-temperature range of 50–300 °C, a pronounced decomposition peak of sulfate products appears at approximately 120 °C in the cathode region [39], with an intensity much higher than those of the immersion group and the anode region. This directly corresponds to its highest sulfur content (3.37 wt.%), confirming the ion-enrichment effect induced by electromigration. In contrast, the mass loss in the anode region during this stage is relatively weak, indicating limited formation of solid sulfate products [40]. In the medium-temperature range of 400–600 °C, the DTG peak around 450 °C characterizing the dehydroxylation of Ca(OH)₂ in the anode region virtually disappears, providing direct evidence that this region is dominated by electrochemical dissolution of hydration products [41]. This peak is also weakened in the cathode region, indicating that part of the Ca(OH)₂ has been consumed by sulfate reactions. In the high-temperature range of 600–1000 °C, the CaCO₃ decomposition peak around 680 °C is most prominent in the cathode region, suggesting substantial formation of secondary carbonates. The anode region exhibits the lowest mass loss in this stage, further supporting its mechanism dominated by material dissolution. Overall, the thermal analysis results reveal, from the perspective of thermal stability, the differences in erosion pathways among the three types of samples: the cathode region follows a “product-accumulation type” deterioration driven by ion enrichment, the anode region exhibits a “material-loss type” deterioration dominated by electrochemical dissolution, while the immersion group demonstrates a slow erosion process relying on natural diffusion [42].

4. Conclusions and Prospects

This study systematically revealed the deterioration mechanism of desert sand concrete under electrochemical action and clarified the engineering application value of the method by constructing a coupled “electric field–sulfate” accelerated test system and comprehensively employing macroscopic mechanical testing, electrochemical impedance spectroscopy, and multi-scale microscopic characterization. The main conclusions are as follows:

(1) A spatially differentiated deterioration mechanism was revealed. Driven by the external electric field, distinctly different failure modes emerged at the cathode and anode. The cathode region exhibited significant enrichment of sulfate ions (S content reaching 3.37 wt.%) due to electromigration, triggering substantial formation of gypsum and ettringite, resulting in damage dominated by expansion and cracking (porosity increased to 8.3%). In contrast, the anode region was primarily governed by electrochemical dissolution of hydration products (especially Ca(OH)₂, porosity 5.1%), leading to loss of cementitious material. This mechanism was consistently verified by EDS, phase analysis via XRD/FTIR, and thermal analysis via TG-DTG.

(2) The correlation between macroscopic mechanical properties and micro- and electrochemical responses was clarified. In terms of mechanical performance, although the macroscopic compressive strength loss rate after 28 d was 5.9% (from 31.8 MPa to 29.9 MPa), and this loss was verified by the model as “additional damage” on top of the optimized mix proportion, simultaneous electrochemical and microstructural characterization showed that the charge transfer resistance (Rct) decreased by approximately 95% and the cathode porosity increased significantly. This indicates that the deterioration of microstructure and transport properties occurs much earlier and more severely than the loss in macroscopic strength. This “micro-scale precedence” phenomenon highlights the unique value of employing non-destructive techniques such as electrochemical impedance spectroscopy for early-stage durability diagnosis.

(3) A rapid evaluation system based on the electrochemical method was established. The coupled acceleration method proposed in this study can significantly amplify ion migration and reaction processes within a short period (28 d). Its core value lies in: rapidly revealing the intrinsic mechanism of material deterioration, particularly the differential processes caused by ion migration; and providing an efficient tool for relative comparison and ranking of sulfate resistance for desert sand concrete with different mix proportions, suitable for preliminary material screening and optimization design.

(4) Durability design insights for engineering practice were proposed. Based on the above mechanisms, in practical saline-alkali environment engineering, it is recommended to focus on the following: For cathode-dominated areas (e.g., long-term submerged zones, the back side of underground structures), measures such as enhancing concrete compactness, using sulfate-resistant cement, or incorporating mineral admixtures should be adopted to suppress sulfate ion ingress and enrichment, thereby preventing expansion damage. For anode-dominated areas (e.g., locations near stray current sources), maintaining the alkaline buffering capacity of the paste is crucial to resist acidic dissolution. It is recommended to use electrochemical impedance spectroscopy (EIS) monitoring, particularly the change in charge transfer resistance (Rct), as an early and sensitive indicator for assessing microstructural damage and degradation of transport properties within concrete, offering a new tool for engineering durability diagnosis and maintenance.

(5) Prospects and limitations. It should be noted that the accelerated test used in this study is intended for mechanism revelation and relative evaluation. A precise quantitative relationship with natural exposure duration needs to be established by incorporating specific environmental parameters. Future research could focus on the following directions: Developing acceleration–natural exposure correlation models based on ion flux or damage accumulation; Investigating the mechanism of electric field action under multi-factor coupled fields such as wet-dry cycles and coexistence of chloride salts; Extending this method to a wider range of low-carbon cementitious material systems to form standardized guidelines for rapid durability evaluation.

This research not only deepens the scientific understanding of the deterioration mechanism of desert sand concrete in saline-alkali environments but also provides theoretical foundations and methodological support for the durability design, rapid performance evaluation, and engineering application maintenance of such materials.