Deterioration of Concrete Under Simulated Acid Rain Conditions: Microstructure, Appearance, and Compressive Properties

Abstract

The effects of acid rain corrosion on the properties of concrete are broadly understood. This study investigated the impact of varying corrosion conditions on the microstructure and mechanical properties of concrete, which has not received sufficient attention using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and compressive tests. In the laboratory, simulated acid rain solutions with pH levels of 0.0, 1.0, and 2.0 were prepared using sulfuric acid solution. A total of 13 sets of 39 concrete cubes each were immersed in these acid solutions for durations of 7, 14, 21, and 28 days. The findings clearly indicate that simulated acid rain corrosion significantly affects both the microstructure and mechanical properties of concrete. Acid alters the material composition of concrete and simultaneously increases the formation of pores within it. This not only changes the number, area, and perimeter of the pores but also affects their shape parameters, including circularity and fractal box-counting dimension. These pores typically measure less than 0.4 μm and include micro- and medium-sized pores, contributing to the more porous and structurally loose concrete matrix. As the duration of acid exposure and the concentration of the acid solution increase, there is noticeable decrease in compressive strength, accompanied by changes in the concrete structure. The rate of strength reduction varies from 6.05% to 37.90%. The corrosion process of acid solution on concrete is characterized by a gradual advancement of the corrosion front. However, this progression slows over time because as the corrosion depth increases, the penetration of the acid solution into deeper layers becomes limited, thereby reducing the rate of strength deterioration. The deterioration mechanism of concrete can be attributed to dissolution corrosion caused by H⁺ ions and expansion corrosion due to the coupling of SO₄²⁻ ions.

1. Introduction

As is widely known, acid rain is formed when pollutants like sulfur dioxide and nitrogen oxides in the atmosphere react with water vapor, producing acids such as sulfuric acid and nitric acid. These acids lower the pH of precipitation below 5.6, resulting in acid rain that falls to the ground. The primary cause is the combustion of various fuels during industrial production, which releases acidic substances like sulfur dioxide and nitrogen dioxide into the atmosphere [1]. Acid rain was first documented in Western Europe in the 1960s and later observed in North America and Japan, with progressively lower pH levels today [2,3,4]. Acid rain has become a widespread environmental pollution issue globally, with major accumulation areas in Europe, North America, Asia, and China, and signs of gradual worsening. Acid rain negatively impacts both the natural environment and human life. Its detrimental effects on concrete structures have garnered ongoing attention from civil engineering scientists. When concrete structures are corroded by acid rain, not only does the compressive strength of the concrete decrease, but the performance of the reinforcement also deteriorates. Consequently, the bearing capacity, durability, and seismic performance of the structure are compromised [5,6,7,8,9,10].

The effects of acid rain corrosion on the properties of concrete are broadly understood. Scholars have generally proposed the corrosion mechanism and neutralization process of acid rain-corroded concrete by analyzing the composition and structure of corrosion products using various techniques such as SEM, XRD, and TGA. For example, Xie [11] used XRD to analyze the mineralogical composition of concrete specimens and found that corrosion products like CaSO₄·2H₂O, CaAl₂Si₂O₈, and Ca₃Al₆O₁₂·CaSO₄ caused an increase in volume and a decrease in strength. Cui [12] analyzed the diffusion mechanisms of CO₂ gas in concrete through SEM observations and concluded that carbonization can lead to changes in the microstructure of concrete. According to Guo [13], during the corrosion process of acid rain on concrete, two types of corrosion occur: H+ dissolution corrosion and SO₄²⁻ expansive corrosion. These types of corrosion result in the consumption of basic substances in concrete, such as Ca(OH)₂ and C-S-H. Using SEM, XRD, and other methods to study the corrosion mechanism of concrete, the main consensus is that there are three primary corrosion processes: acid rupture of concrete by H⁺ [14], expansion failure due to SO₄²⁻ sulfate action [13], and combined failure from H⁺ and SO₄²⁻ [9,15]. Additionally, the carbonization of concrete by CO₂ in the atmosphere may be considered [16], though this issue should not be included in the study of acid rain corrosion of concrete. While these methods effectively reveal the corrosion mechanism of concrete exposed to acid rain, the quantitative characterization of structural changes in concrete has not received sufficient attention.

On the other hand, considerable studies have been conducted on various corroded and deteriorated concrete material components. Yin et al. [17] designed fiber-reinforced polymer (CFRP)-concrete single-shear specimens to investigate the effects of a hygrothermal acid rain environment. Xu et al. [18] tested prestressed concrete beams subjected to simulated acid rain corrosion under low-cyclic loading tests and examined numerous parameters, including failure mode, hysteretic behavior, backbone curve, and ductility. Hua et al. [19] and Zhang et al. [20] conducted experimental studies on concrete-filled steel tubular (CFST) beam–columns or columns with square sections under combined acid rain corrosion conditions. Numerous scholars have investigated the mechanical properties of acid rain-corroded concrete using indoor experiments on components made of different materials and other analytical techniques. However, from the perspective of understanding the internal structural changes in acid rain-corroded concrete, immersion-accelerated tests of concrete cubes can significantly enhance the convenience and efficiency of such studies [8,11,21,22].

Amid the growing global issue of acid rain, understanding the corrosion mechanism of acid rain on concrete and its effect on strength is crucial for evaluating the lifespan and safety of reinforced concrete structures. While previous research has primarily concentrated on the degradation of macroscopic mechanical properties or qualitative analysis of corrosion mechanisms, this study adopts a more comprehensive approach by conducting simulation experiments in an acid rain simulation laboratory to investigate the deterioration of mechanical properties and the changes in the internal pore structure of concrete cubes. Additionally, the corrosion mechanism of acid rain was investigated using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques. Ultimately, the study reveals the relationship between macroscopic behavior and mechanical properties of concrete following acid rain corrosion.

2. Experimental Program

Two commonly used methods for simulating acid rain corrosion of concrete worldwide are outdoor long-term exposure tests and indoor accelerated corrosion tests. The outdoor long-term exposure method involves placing experimental components directly in natural environmental conditions to simulate real corrosion processes in harsh environments. Although this method requires a longer test duration, it closely mimics actual environmental corrosion conditions. In contrast, the laboratory accelerated corrosion method offers shorter test cycles, lower costs, and easier analysis of results due to controlled test conditions. This method, compared to outdoor exposure, allows for quicker assessments under simulated conditions [21,23,24,25]. Based on the research experience of Gu et al. [26], this study adopted accelerated indoor immersion tests for evaluating acid rain corrosion effects on concrete.

2.1. Materials and Specimen Preparation

The concrete mix design in this study adhered to the Chinese national standard “Standard for test methods of concrete physical and mechanical properties” (GB/T 50081-2019) and was optimized through multiple trials, resulting in a cement:sand:aggregate:water:water reducer ratio of 1:1.9:3.7:0.5:0.001, with a target slump of 100 mm as the control parameter [27]. The cement used is P·O 42.5 grade ordinary Portland cement produced by Huaxin Cement Factory, a renowned Chinese brand, procured locally for the experiment. The main chemical components of the cement are specified in

Table 1. Chemical composition of cement/%.

Chemical Composition	Cao	SiO2	Al2O3	Fe2O3	MgO	SO3	Others
Percentage	60.3	20.68	6.06	5.46	1.32	2.01	4.17

. The coarse aggregate used is 2~25 mm stone with a density of 2450 kg/m³ and water absorption ≤0.75%. The fine aggregate consists of mountain sand and middle sand from the second zone; both have good grading and a particle size range of 2.0~3.0 mm. To enhance the workability and density of the concrete blocks and facilitate the pouring of test blocks, the water reducer provides a water reduction rate of over 25%, featuring a solid content of 39% and a pH value of 5.2. Tap water was utilized to mix the concrete.

According to the specified mix ratios, the quantities of sand, stone, cement, water, and water-reducing agent were weighed and combined in a wet iron tank for mixing. Water was added slowly in two stages to ensure thorough mixing. The resulting mixture was poured into standard molds (100 mm × 100 mm × 100 mm) that had been coated with a release agent. After filling the molds, the concrete specimens were left undisturbed in a natural environment for 24 h. Subsequently, an air gun was used to pump air underneath the concrete mold, allowing for the removal of the mold while taking care to avoid any impact that could compromise the integrity of the concrete edges and corners. Following molding, the specimens were transferred into a standard curing box maintained at a temperature of 20 ± 2 °C and a relative humidity of ≥95% for a period of 28 days. After the curing process, each concrete test block was individually weighed to ensure its density fell within the specified range of 2.450 kg/m³ ± 5%. Any blocks failing to meet this criterion were excluded from further testing. Ultimately, 39 standard concrete cubes measuring 100 mm × 100 mm × 100 mm, meeting the specified criteria, were selected for experimentation. Additionally, there were 20 spare concrete cubes measuring 70 mm × 70 mm × 70 mm for contingency purposes.

In corrosion-resistant plastic tubs, concrete cubes were placed and divided into three groups labeled pH0, pH1, and pH2. After various durations (7 days, 14 days, 21 days, and 28 days) of exposure to corrosion, the test blocks were removed and air-dried in an indoor environment (Figure 1). Details of the concrete specimens tested in this study are summarized in

Table 2. Details of the concrete specimens tested in this study.

Specimen ID	D × L × H (mm × mm × mm)	Corrosion Time (d)	pH Value	Count of Specimens
cs	100 × 100 × 100	28	Only pure water	3
pH07d	100 × 100 × 100	7	0	3
pH014d	100 × 100 × 100	14	0	3
pH021d	100 × 100 × 100	21	0	3
pH028d	100 × 100 × 100	28	0	3
pH17d	100 × 100 × 100	7	1	3
pH114d	100 × 100 × 100	14	1	3
pH121d	100 × 100 × 100	21	1	3
pH128d	100 × 100 × 100	28	1	3
pH27d	100 × 100 × 100	7	2	3
pH214d	100 × 100 × 100	14	2	3
pH221d	100 × 100 × 100	21	2	3
pH228d	100 × 100 × 100	28	2	3

.

2.2. Techniques and Procedures

2.2.1. Simulated Acid Rain Environment

In laboratory simulations, acid rain corrosion of concrete was accelerated using acid solutions prepared by mixing pure water with sulfuric acid. The pure water used was produced using reverse osmosis technology. Various studies, summarized in

Table 3. The pH values used in studies.

Sequence	Study Objects	pH Value(s)	Author(s) of the Literature
1	100 × 100 × 400 mm concrete beams	1.5, 2.5, 3.5	Wang et al. [28]
2	Three-point bending (TPB) concrete specimens	1.5, 2.5, 3.5	Zhou et al. [9]
3	Mode II fracture toughness, compressive strength, and elastic modulus of concrete	1, 2, 3, 7	Zhou et al. [10]
4	Glass fiber-reinforced polymer (GFRP) concrete	2, 3, 4	Zhou et al. [29]
5	Seismic behaviors of RC frame beam–column joints	3	Guan et al. [30]
6	Carbon fiber-reinforced high performance Concrete containing limestone powder	0.5	Li et al. [31]
7	Deterioration of cement concrete	1, 3.5, 5.6	Xie et al. [11]
8	CFRP-reinforced concrete beams	1.5	Luan et al. [32]
9	700 × 100 × 700 mm squat reinforced concrete walls	2, 3, 4	Zhou et al. [33]

, commonly use acid solutions with a pH ranging from 0 to 4. For convenience and to meet study requirements, the acid concentrations were categorized into three groups: pH0 (hydrogen ion concentration of 1 mol/L), pH1 (hydrogen ion concentration of 0.1 mol/L), and pH2 (hydrogen ion concentration of 0.01 mol/L). The concentrations were calibrated using acid–base neutralization titration methods.

2.2.2. Test Procedure

After reaching the predetermined corrosion duration as per the experimental plan, the concrete cubes were taken out, and free water on the surface was initially eliminated using a low-heat hair dryer, followed by drying in an oven at 60 °C for 8 h. Subsequently, the specimens were subjected to axial compressive strength tests, conducted in strict accordance with the Chinese national standard “Standard for test methods of concrete physical and mechanical properties” (GB/T 50081-2019). The load was applied at a rate of 0.5 MPa/s. Following the experiment, the failure patterns and stress–strain curves of the concrete cubes were recorded and analyzed, and the compressive strength of the concrete was determined by identifying the peak value on the curves. According to the Chinese national standard “Standard for test methods of concrete physical and mechanical properties” (GB/T 50081-2019), the strength value determination follows these provisions: (1) The arithmetic average of the measured values from three specimens is taken as the strength value of the specimens, accurate to 0.1 MPa. (2) According to the Chinese national standard, if the difference between either the maximum or minimum value among the three measured values exceeds 15% of the median value, then the two values are discarded. Instead, the middle value is selected as the compressive strength value for the specimens. (3) If the difference between either the maximum or minimum value among the three measured values exceeds 15% of the median value, the test results for the specimens are considered invalid.

After completing the strength tests, small pieces from the surfaces of the concrete cubes were observed using SEM to examine the microstructure of both corroded and uncorroded sections and conduct a comparative analysis, while EDS elemental analysis was performed on the areas of interest. The SEM equipment used for observation is the GEMINI series of field emission scanning electron microscopes produced by the German company ZEISS. This equipment offers excellent detection efficiency and can achieve sub-nano resolution imaging, making it suitable for high-resolution imaging and analysis of concrete materials.

3. Results and Discussion

3.1. Visual Inspection of Corroded Concrete

After preparing the corroded concrete specimens, visual inspections were conducted to assess their surface states, as depicted in Figure 2. These observations align with findings from the existing literature [8,25,34]. Significant differences in surface color were observed among concrete specimens subjected to varying concentrations and durations of corrosion (Figure 2a). These differences were most pronounced between different concentration levels. Notably, it was observed that immersion in pure water had no discernible effect on the concrete initially. However, as the exposure time increased, the surface color gradually changed from dark (pH 2) to dark gray (pH 1) and eventually to white (pH 0). After drying, all specimens exhibited a certain degree of whiteness, with the pH0 specimens showing the most pronounced effect. This visual assessment suggests a clear correlation between the severity of corrosion (indicated by pH levels) and the observable changes in concrete surface color, reflecting the progressive impact of acid solution exposure on the material. At low concentrations and short durations, there were no significant changes observed in the concrete surface, apart from color alterations. This process primarily affects the surface through neutralization, where the water released promotes further cement hydration. Additionally, crystallization of CaSO₄·2H₂O occurs on the surface and within the material, blocking pore channels and reducing pore size. This phenomenon inhibits the formation of significant holes or loose material production on the concrete surface under these conditions. According to Xie [35], the strength of concrete is not significantly affected until a considerable amount of lime is lost from the material. During prolonged exposure to low-concentration corrosion or under high-concentration corrosion, the concrete surface exhibits noticeable honeycomb voids. Initially, this leads to a loss of quality in the cubes. As the corrosion progresses, the concrete surface develops varying shades of white, influenced by newly generated materials. Over time, this white substance accumulates further on the concrete surface, forming a muddy mixture of gypsum, silica gel, and aluminum hydroxide. These substances result from chemical reactions between cement hydration products in concrete and H⁺ ions present in acid rain solutions [25]. At this stage, substance losses escalate, accompanied by a significant passivation phenomenon observed particularly at the prism corners of the specimens. The surface of the specimens becomes loose and powdery, and coarse aggregates are exposed. This deterioration is more pronounced in specimens exposed to solutions with higher acidity and longer corrosion times.

3.2. Microscopic Feature

In acid rain corrosion, hydrogen ions play a primary role in damaging concrete. The corrosion typically involves the dissolution of calcium hydroxide and hydrated calcium silicate. This dissolution occurs due to the stronger solubility of neutralization reaction products of acid rain and calcium hydroxide in acid environments. Consequently, the alkalinity of concrete diminishes significantly, leading to the gradual transformation of high-calcium hydration products into low-calcium forms. Since concrete is naturally alkaline, increased acidity causes it to transition toward neutrality over time. In this process, carbon dioxide from the atmosphere gradually infiltrates the interior of concrete and reacts with internal alkaline substances [1,36]. Additionally, hydrogen ions from acid gases or acid rain penetrate the internal pores of concrete. This initiates a series of chemical reactions as follows [15,37]:

Ca(OH)₂ + H⁺→Ca²⁺ + 2H₂O

nCaO·mSiO₂ + 2n H⁺→nCa²⁺ mSiO₂ + n H₂O

nCaO·mAl₂O₃ + 2n H⁺→nCa²⁺ mAl₂O₃ + n H₂O

In the reactions, n and m are stoichiometric weighting coefficients. On both the fresh concrete surface and internally, calcium hydroxide, calcium silicate (C-S-H gel), and calcium aluminate are widely distributed (Figure 3). Typically, these components contribute to a relatively dense structure with few pores. Once concrete is exposed to an acid solution, the acidic solution present in its pores undergoes neutralization reactions, leading to a decrease in the concrete’s alkalinity [38]. This process results in a gradual loss of calcium hydroxide and an increase in pore volume, making the concrete more susceptible to infiltration by acidic media from the surface to the interior. This damages the internal structure of concrete, leading to significant deterioration in its physical properties, mechanical properties, and durability. As depicted in Figure 3, the concrete of pH028d exhibits serious internal cavities, forming a honeycomb structure. This phenomenon primarily occurs because soluble substances are carried away through dissolution by hydrogen ions. The substance observed, mainly composed of a flocculent structure, as shown in Figure 3d, underscores the role of hydrogen ions (H⁺) in acid solutions. These ions primarily act by dissolving, decomposing, and transforming substances such as Ca(OH)₂, calcium silicate hydrate (C-S-H gelatum), calcium aluminate, and others. This process leads to structural corrosion and consequent reduction in concrete strength [39].

Subsequently, in reinforced concrete structures, as the acid solution continues to infiltrate and the corrosion reaches the protective layer and affects the steel bars, it induces corrosion of the steel. This corrosion causes the steel bars to expand, leading to cracking or even spalling of the concrete protective layer. Ultimately, this results in a loss of bond strength between the steel bars and the concrete, leading to structural failure of the concrete [40]. This process highlights the critical importance of protecting reinforced concrete from acid corrosion to maintain structural integrity.

In addition to hydrogen ions, acid rain also contains sulfate radicals, which are a predominant component and a crucial factor in concrete corrosion. The sulfate radicals in acid rain infiltrate concrete, leading to three stages of corrosion: ettringite corrosion, ettringite and gypsum co-corrosion, and gypsum corrosion [39]. The main reaction equations are as follows:

4CaO·Al₂O₃·19H₂O + 3CaSO₄ + 14 H₂O→3CaO·Al₂O₃·CaSO₄·32 H₂O + Ca(OH)₂

Ca(OH)₂ + Na₂SO₄·10 H₂O→CaSO₄·2H₂O + 2NaOH + 8 H₂O

As the most intense and harmful type of corrosion, the generation of ettringite can lead to a significant increase in volume while also improving the early-stage density of concrete and filling its internal pores. However, when the sulfate concentration exceeds 1000 mg/L, it can lead to simultaneous corrosion involving ettringite and gypsum. Furthermore, the formation of gypsum has been observed to cause expansion in concrete [41]. However, under high sulfate concentrations, gypsum-type corrosion becomes predominant. During this sulfate attack process, sulfates primarily produce significant amounts of calcium sulfate and ettringite (tricalcium aluminate sulfate). Unstable ettringite may transform into monosulfate hydrate while also generating small amounts of iron ions that can replace aluminum ions in the lattice structure. Consequently, after sulfate corrosion, the interior of the concrete primarily consists of sheet-like and laminated sulfatealuminate salts (Figure 3e,f). Acid corrosion of concrete is a gradual process characterized by the formation of a corrosion front within the concrete (Figure 4). As acid continues to infiltrate, the corrosion front advances and deepens into the concrete. This ongoing process induces changes in the concrete’s composition. Elemental analysis using energy dispersive spectroscopy (EDS) reveals significant differences in the elements and structure between corroded and uncorroded substances on both sides of the corrosion front. Whether analyzed at specific points or in sections, these differences are clearly evident. The structure of the non-corroded concrete area is dense, with calcium and oxygen constituting more than 90% of the elements present, while sulfur is almost absent (Figure 4b,d). In contrast, within the corroded area, the materials appear flaky or needle-like, with a loose structure. Here, the content of calcium and oxygen is significantly reduced, and sulfur content reaches about 20%. This emphasizes the critical impact of sulfate corrosion on concrete, highlighting its importance in concrete degradation processes.

Due to variations in mineral composition, water–cement ratio, sand content, porosity, curing duration, and admixtures in concrete mixes, the impact of environmental factors such as ion concentration, corrosion duration, and corrosion methods in acid rain is substantial [42]. Concrete also exhibits non-uniform porous structures and anisotropic characteristics, contributing to the complexity of its corrosion mechanisms and processes under changing environmental conditions. Xie [43] conducted laboratory tests to simulate the impact of acid rain on concrete, revealing that the loss rates of calcium oxide and strength were negatively correlated with sulfate levels. The damage inflicted by acid rain on concrete primarily stems from corrosion due to hydrogen ion dissolution and sulfate-induced expansion. Current studies on ion diffusion, changes in corrosion composition, internal stress damage effects, and structural deterioration under the combined influence of acid and sulfate corrosion are still developing. However, it is evident that the degradation mechanism of concrete in acid rain environments can be attributed to dissolution corrosion by H⁺ ions and expansion corrosion due to SO₄²⁻.

3.3. Quantitative Analysis of Pore Structure in Corroded Concrete

To quantitatively analyze the impact of simulated acid rain corrosion on the concrete microstructure, the areas of interest in Figure 4 representing corroded and uncorroded regions were extracted, as shown in Figure 5a,b, respectively. These images are then subjected to threshold processing to create binary images where concrete aggregates and mortar appear black, and pores appear white, as depicted in Figure 5c,d. This step enhances the contrast and prepares the images for further quantitative analysis. The aggregate and mortar appear in black, while the pores appear in white in the threshold images. This adjustment enhances image contrast, ensuring that even small pores are not overlooked during the binarization process. Subsequently, the binarized images were imported into ImageJ software (IJ 1.46r), where the particle analysis function was employed to calculate the number of pores in the selected area, their respective areas, and the roundness of their perimeters.

The count, area, and perimeter of pores only reflect the two-dimensional morphology of pores. The external morphology and the degree of roundness can also be quantitatively evaluated by the pore roundness rate, as follows [44]:

C = 4πA/P²

where A is the pore area, P is the pore perimeter, and C is the roundness rate of the pore, which is between 0 and 1. The larger the value of C is, the more circular the pore shape is, and C = 1 means the pore is circular.

In addition to the parameters mentioned earlier, the intricate pore structures can be elucidated using fractal theory, which suggests that most natural object surfaces exhibit fractal properties in space. This theory provides a foundational framework for applying fractal models in the field of image analysis. The box-counting dimension is extensively applied in materials science. It characterizes the heterogeneity, complexity, and irregularity of pores, and in this study, the area distribution fractal dimension [45], specifically the fractal box-counting dimension, is employed. The calculation principle is described as follows [46]:

Let A be any nonempty subset of Rⁿ space. For any r > 0, let Nr(A) represent the minimum number of nnn-dimensional cubes (or boxes) with side length r needed to cover A. If there exists a number D such that as r → 0:

$${\mathrm{N}}_{\mathrm{r}}\left(\mathrm{A}\right)\propto 1/{\mathrm{r}}^{\mathrm{D}}$$

Thus, D is the fractal box-counting dimension of A. Note that the box-counting dimension is D only if there exists a positive number k such that:

$$\underset{\mathrm{r}\to 0}{\lim }\left({\displaystyle \frac{{\mathrm{N}}_{\mathrm{r}}\left(\mathrm{A}\right)}{1/{\mathrm{r}}^{\mathrm{D}}}}\right)=k.$$

To calculate the box dimension D, square grids of various sizes with side length rrr are used as overlaying boxes after converting the original image into a binary format. The number of pore pixels Nr within each grid is determined. By varying r, different values of Nr are obtained. The natural logarithms of r and Nr are then plotted, and the least squares method is used to fit these values to a straight line. The slope K of this line is calculated, and the pore fractal dimension D is given as D = −K, the negative inverse of the slope.

3.3.1. Parameter Quantization

It is clear that the structures on both sides are markedly different. To analyze the similarities and differences in pore structure between corroded and uncorroded areas, the corroded and uncorroded areas were individually extracted (Figure 5a,b), processed for noise reduction and binarization, and then subjected to pore analysis. The pore sizes were statistically analyzed based on their dimensions: pores smaller than 0.1 μm were categorized as micropores. Pores ranging from 0.1 to 0.6 μm were classified as mediumpores, with subdivisions of 0.1–0.2 μm, 0.2–0.4 μm, and 0.4–0.6 μm. Macropores were defined as pores ranging from 0.6 to 1 μm, while pores larger than 1 μm were categorized as large pores. Given that the identified pores ranged from 0.062 μm to 4.229 μm, micropores were further subdivided into 0.06–0.08 μm and 0.08–0.1 μm categories. Therefore, the pores that can be identified are divided into eight aperture intervals. The number of pores, pore area, pore circumference, and porosity roundness within a certain range of pore sizes are shown in Figure 6, and the overall statistical results are in

Table 4. Statistical results of pore parameters in corroded and uncorroded areas.

Analysis Area	Pore Count	Total Area (μm2)	Average Pore Size (μm)	Area Ratio (%)	Average Roundness
Corroded area	3430	193.758	0.056	17.281	0.609
Uncorroded area	469	135.782	0.29	12.642	0.537

.

The corroded area contains 3430 pores, whereas the uncorroded area has only 469 pores (Table 4). The number of pores in the corroded area is significantly greater than in the uncorroded area across all pore size categories. Particularly in the micro and medium pore sizes, the corroded area shows a substantial increase in pore count. This suggests the formation of numerous small-sized pores during the corrosion process, which aids in the deterioration of concrete structures and provides an accessible pathway for subsequent acid penetration. This phenomenon is also evident in the cumulative percentage curve of pore counts, which steeply rises in the range of small pores but becomes gentler for larger pores (Figure 6). As pore size increases, the total pore area also increases. Specifically, the total area in the corroded and uncorroded areas is 193.758 μm² and 135.782 μm², respectively, with a larger proportion of pores distributed above 1 μm in size (Figure 6, Table 4). Compared to pore area, pore circumference follows a similar growth pattern. This suggests that slightly larger pores, especially those larger than 1 μm, although fewer in number, occupy a significant area, facilitating air removal and the passage of acid solutions. Regarding pore roundness, more than half of the pores exhibit larger roundness proportions. The roundness in the corroded area is 0.609, whereas in the uncorroded area, it is only 0.537. The uncorroded area mainly consists of matrix pores between minerals (Figure 5b,d). Therefore, the analysis reveals that the internal pores of uncorroded concrete are primarily mineral bonding cracks formed between mineral particles. These pores have relatively low roundness and are sparse in number. After corrosion, numerous tiny pores are formed inside the concrete, particularly those less than 0.4 μm in size. These pores appear as thin strips, needles, and nearly round shapes, significantly increasing the internal porosity and creating a looser structure within the concrete.

3.3.2. Fractal Pore Structure

Indicators such as pore count, area, perimeter, proportions, and roundness rate can quantitatively characterize concrete structures in different areas. The significant differences indicate that corroded concrete exhibits strong heterogeneity, complexity, and irregularity, which can be analyzed using fractal theory. The fractal box-counting dimension reflects the self-similarity characteristics of objects. A higher box-counting dimension suggests greater complexity, whereas a lower dimension indicates more focused and simpler fractal characteristics. Here, n-dimensional cubes (boxes) of side length r were required to cover the corroded and uncorroded areas, and then the minimum number of cubes (boxes) lnN_r were obtained (Figure 7). By varying r, different values of N_r are obtained. Subsequently, the least squares method is applied to fit the lnr and ln N_r values to a straight line, with the slope of the line providing the box-counting dimension. The box-counting dimension, which is 1.686 for the corroded area and 1.520 for the uncorroded area, highlights the greater complexity of the corroded region. Specifically, the formation of micropores during corrosion contributes significantly to this complexity, influencing the overall complexity of the pore structure. Although the area occupied by micropores in the corroded area is small, their sheer number contributes to a more complex fractal structure. Conversely, slightly larger pores exhibit a simpler structure, reducing the heterogeneity and overall complexity of the pore network, reflected in a lower box-counting dimension. As corrosion advances and internal porosity increases, the pore structures become progressively more intricate. Overall, the fractal dimension, together with metrics like pore area and quantity, captures the distribution, shape, and complexity of the pore structure. Under the influence of acid rain corrosion, changes in the pore structure are reflected by variations in the fractal dimension, which have a significant impact on the mechanical properties and durability of concrete. A higher fractal dimension generally indicates increased pore complexity, facilitating greater acid penetration and resulting in a decline in concrete strength.

3.4. Effect of Corrosion on Concrete Strength

3.4.1. Failure Modes from Visual Perspective

Based on SEM and EDS analysis results, significant changes are observed in the internal pores and substances of the concrete cubes corroded by acid solution, which undoubtedly affect their compressive failure characteristics (Figure 8). In contrast, the failure pattern of the three concrete cubes soaked in pure water (Figure 8a) indicates minimal cracking, with only 2–3 vertical main cracks and scattered minor fissures accompanied by slight concrete crushing at the base. As time and concentration increase, the reaction intensifies, resulting in noticeable changes in the concrete surface color, shifting from yellow-brown to white. The cementitious material on the surface continues to corrode, exposing more coarse aggregate over time. Particularly under severe conditions of high-concentration acid corrosion, the acidic environment dissolves the surface hardening of non-metallic materials such as concrete and masonry. This initial dissolution leads to the formation of cracks and voids on the surface, reducing structural compactness, weakening strength, and potentially leading to structural failure. When concrete structures are exposed to acidic environments over extended periods, alkaline compounds within the concrete react with simulated acid rain. This reaction leads to increased visibility of exposed coarse aggregate, gradual formation of holes and defects, and widening of cracks following pressure damage (Figure 8b,c). These effects contribute to a progressive loss of concrete strength, eventually leading to structural damage. Moreover, during compressive failure, surface shedding predominantly exposes materials formed after corrosion, revealing fresh failure surfaces (Figure 8d). These surfaces show more severe vertical cracks and scattered minor fissures compared to others, accompanied by significant crushing. This evidence underscores that the porous and loose structure formed after corrosion significantly impacts the strength of the concrete cube. During compression, the corroded concrete portion suffers initial damage, leading to reduced stress-bearing capacity, a sudden decline in the stress–strain curve, and ultimately structural failure of the entire concrete. Consequently, the compressive strength is governed by the corroded segment of the concrete block. Hence, structural defects in the corroded portion profoundly affect the overall strength of the concrete.

3.4.2. Compressive Strength Test Results

The compression-testing machine measures the compressive strength of the specimens, generating the stress–strain curves (Figure 9), from which the compressive strength of the concrete is determined by identifying the peak value. The tests are conducted using 100 mm × 100 mm × 100 mm specimens. For non-standard specimen sizes, the measured strength values must be adjusted using a size conversion factor of 0.95 to accurately determine the concrete strength. This adjustment is necessary to standardize the results obtained from specimens that may differ slightly from the standard dimensions. Therefore, after multiplying the values obtained from the tests by 0.95 with the standard size requirement, the values can be confidently interpreted as indicative of the concrete strength under consideration. By measuring the original strength of the concrete specimens and that after corrosion, the reduction rate of strength can be calculated and obtained as follows [11]:

Rc (%) = (Rco − Rce)/Rco × 100%

where Rc stands for the reduction rate of the compressive strength of the cement concrete specimen, Rco is the compressive strength value after being cured in a standard curing room for 28 days, namely the initial strength value, and Rce is the compressive strength value after exposure to simulated acid rain. In particular, the Rc value of concrete soaked in pure water is 0.

It is evident from Figure 9 that the stress–strain curve of the concrete cubes provides the compressive strength values for each group of specimens, as listed in Figure 10. Across all corrosion concentrations, the compressive strength decreases with longer corrosion times. However, the extent of this reduction varies, with the decrease gradually diminishing as the concentration decreases. Concrete cubes soaked in pure water exhibit the highest compressive strength at 24.8 MPa. In contrast, specimens exposed to the highest concentration (pH0) for the longest duration (28 days) show a significantly reduced strength of only 15.4 MPa, indicating a reduction rate of 37.90% (Figure 11). This trend underscores how corrosion, especially under severe conditions, markedly weakens the structural integrity of concrete over time. The failure modes observed reveal distinct differences between specimens with lower and higher reductions in compressive strength. Specimens with minor reductions exhibit superficial damage characterized by limited cracking and a largely intact core. Conversely, specimens with significant reductions show extensive cracking, spalling, and brittle failure, resulting from deeper corrosion penetration and severe microstructural degradation, such as increased porosity and the formation of expansive products like gypsum and ettringite. Thus, it is crucial to ensure proper compaction of the concrete to reduce porosity and enhance impermeability. Additionally, using cement with lower tricalcium aluminate content can minimize the formation of ettringite and suppress sulfate-induced expansion. Furthermore, implementing protective measures to prevent the formation and growth of surface corrosion layers is essential for preserving the integrity and mechanical properties of the concrete.

As corrosion progresses, the corrosion front moves inward, deepening the corrosion depth and further weakening concrete strength. The thickness of the corrosion layer directly impacts compressive strength, with a larger corroded area leading to greater reductions. Notably, as corrosion time extends, compressive strength progressively decreases, with the reduction rate peaking under pH0 acid corrosion after 28 days. However, the magnitude of reduction diminishes gradually over time. This occurs because the advancing corrosion front encounters a barrier formed by the corrosion layer, which slows acid ingress. Additionally, sulfate-induced expansion reduces porosity, further impeding acid penetration. Over time, the advancing velocity of the corrosion front decreases, leading to a gradual thickening of the corrosion layer and a slower rate of compressive strength reduction. Future studies will incorporate absorption tests to gain deeper insights into acid ingress and its effects on concrete permeability and durability [47]. These findings emphasize the critical role of corrosion depth, microstructural changes, and time in determining concrete’s structural performance.

4. Conclusions

SEM and EDS methods were used to explore the deterioration process of simulated acid rain corrosion of concrete, especially the quantitative comparison and analysis of the impact on the internal structure of concrete. Meanwhile, compression tests on concrete cubes were conducted, revealing the failure modes and compressive strengths of the concrete under various corrosion conditions, along with a detailed comparative analysis of the results. The main findings of this study are listed as follows:

(1)

Simulated acid rain corrosion of concrete progresses gradually, creating a corrosion front within the material. As acid permeates deeper, it significantly alters the composition and structure of concrete. This advancing corrosion front deepens over time, driven by dissolution due to H⁺ ions and expansion due to SO₄²⁻ ions, contributing to the concrete’s deterioration mechanism.

(2)

In the analysis of pore characteristic parameters, the corroded concrete structure exhibits distinct characteristics. The corrosion process generates numerous pores inside the concrete, particularly micro and medium pores smaller than 0.4 μm, which appear as thin strips, needles, and nearly round shapes. This substantially increases the internal porosity and looseness of the concrete structure, leading to a reduction in its strength.

(3)

The compressive strength of concrete decreases noticeably due to acid corrosion, with reductions increasing as corrosion time and acid concentration increase. However, as the corrosion front progresses, the penetration of acid into deeper areas becomes limited, resulting in a slowing down of the rate at which compressive strength decreases.

(4)

To enhance corrosion resistance, ensuring proper compaction reduces porosity and improves impermeability. Using low-tricalcium aluminate cement minimizes ettringite formation and sulfate expansion. Protective measures against surface corrosion layers are vital for maintaining concrete integrity.