Mechanical Properties and Chloride Salt Corrosion Resistance of All-Lightweight Shale Ceramsite Concrete

Abstract

In investigating the mechanical properties and chloride corrosion resistance of all-light shale ceramsite concrete (ALSCC), samples of ALSCC20–ALSCC45 with a compressive strength of C20–C45 were individually prepared. The compressive strength, split tensile strength, and elastic modulus of ALSCC were analyzed. Additionally, the chloride salt corrosion resistance of ALSCC was assessed by examining chloride ion penetration depth, steel corrosion rate, and compressive strength after ALSCC30 corrosion. Furthermore, the microstructure of ALSCC hydration products was observed using scanning electron microscopy (SEM). Results indicate that as the strength grade increases, the water–cement ratio decreases, and the internal structure becomes denser, thereby improving the mechanical and corrosion resistance properties of ALSCC. Notably, the chloride corrosion resistance of ALSCC surpasses that of ordinary concrete. SEM images reveal that the hydration of ceramsite with Ca(OH)₂ in concrete generates Ca[Al(OH)₄]₂ precipitation and C–S–H gel, which enhances internal filling and improves chloride corrosion resistance. Furthermore, based on the results of the ALSCC30 corrosion test, a calculation model for predicting compressive strength in a chloride environment was proposed. This model effectively predicts the compressive strength of ALSCC under chloride exposure conditions.

1. Introduction

The development of lightweight concrete has seen rapid growth in recent decades, mainly due to its low density and high thermal insulation properties [1,2]. Compared to traditional lightweight concrete, all-light shale ceramisite concrete (ALSCC) is composed of high-temperature roasted ceramisite slate. What sets it apart is that shale ceramsite shares similar properties with shale pottery sand, including comparable density and good compatibility. This unique composition not only maintains the high strength and durability of the concrete but also reduces its self-weight by 35% to 40%. Although ALSCC offers numerous significant advantages, further research is needed to explore its mechanical properties and durability in practical applications. Therefore, to ensure sufficient durability in chloride-rich environments such as marine immersion and salt spray areas, it is crucial to understand the mechanical properties and corrosion resistance of the material to evaluate its long-term effectiveness [3,4,5].

In a chloride-containing environment, chloride ions permeate the concrete through diffusion, penetration, and other pathways, leading to the corrosion of steel reinforcement [6,7]. This significantly affects the durability and mechanical properties of concrete structures. Therefore, improving the resistance of concrete to chloride salt corrosion has become a major focus of research [8]. Zuo et al. [9] analyzed the impact of fly ash on the corrosion process of reinforcing steel in concrete under chloride salt environments. They found that incorporating an appropriate amount of fly ash into concrete can enhance the cement paste’s ability to immobilize chloride ions, slow the transport of chloride ions within the concrete, and improve the durability of concrete structures in chloride salt environments. Sun et al. [10] studied the effect of limestone powder on the chloride ion penetration resistance of concrete and found that when the replacement rate of limestone powder was 24%, the resistance of concrete to chloride ion penetration decreased. Zhao et al. [11] provided new insights into the method of adding steel fibers to enhance the chloride ion corrosion resistance of concrete. They found that zinc phosphate-modified steel fiber-reinforced concrete not only maintains microstructural stability under chloride ion corrosion but also exhibits good load-bearing capacity. Sun et al. [12] investigated the effect of silica fume on the durability of concrete under chloride ion erosion and observed that incorporating silica fume reduced the rate of chloride ion penetration. Yi et al. [13] studied the effects of seawater erosion on concrete and found that adding materials such as fly ash, silica fume, and metakaolin to ordinary Portland cement enhances the cement’s resistance to chloride ion penetration and maintains its mechanical properties after exposure to seawater. Existing research has elucidated the mechanisms by which various additives influence the chloride ion resistance of concrete, providing a theoretical foundation for optimizing concrete mix proportions. However, most current studies focus on ordinary concrete, with relatively little research on the chloride ion permeability of ALSCC. This paper aims to enhance the chloride salt corrosion resistance of ALSCC by optimizing its mix proportion and conducting in-depth studies on its corrosion resistance mechanisms in chloride salt environments.

To comprehensively evaluate the performance of concrete in chloride-containing environments, establishing a concrete compressive strength calculation model for predicting and analyzing strength is of great significance [14,15]. Wei et al. [16] established a multi-factor calculation model for the compressive strength of biomass ash concrete based on single-factor analysis and least squares nonlinear regression analysis. Ge et al. [17] developed a compressive strength calculation model for hybrid recycled aggregate concrete by modifying the Bolomey compressive strength model. Song et al. [18] proposed a high-accuracy strength calculation model for silica fume concrete through theoretical analysis of research results on the strength of concrete mixed with silica fume. Chen et al. [19] investigated the strength loss behavior of concrete under the combined effects of freeze-thaw cycles and prestressing and established a compressive strength loss calculation model considering these factors based on experimental results. Researchers have proposed various strength calculation models for different types of concrete using different analysis methods and research subjects. However, there is currently no compressive strength calculation model for ALSCC in chloride environments. This paper compares ALSCC30 samples before and after soaking and, based on the corrosion test results of ALSCC30, proposes a compressive strength calculation model for ALSCC under chloride corrosion. This model effectively predicts the compressive strength of ALSCC in chloride environments.

In this study, four types of ALSCC with varying strength grades were designed to systematically analyze the mechanical properties of ceramsite concrete. The analysis included compressive strength, split tensile strength, and elastic modulus. Additionally, a detailed investigation of the chloride corrosion resistance of ALSCC was conducted, focusing on chloride ion penetration depth, steel corrosion rate, and the compressive strength of ALSCC30 after corrosion. Furthermore, the microstructure of the hydration products of ALSCC was analyzed using scanning electron microscopy (SEM). The research findings in this paper are highly significant for optimizing the performance and engineering application of concrete materials. They also provide strong support for constructing safer and more durable infrastructure.

2. Materials and Methods

2.1. Raw Materials and Mix Proportions

In this experiment, shale ceramsite and shale pottery sand were used as coarse and fine aggregates for concrete. Images of shale ceramsite and shale pottery sand are shown in Figure 1, and the particle-size distribution of shale ceramsite and shale pottery sand is shown in

Table 1. Grain-size distribution of shale ceramsite and sand.

Shale ceramsite	Particle size	<2.5 mm	2.5 mm~4.5 mm	4.5 mm~9.5 mm	>9.5 mm
	Proportion	1.08%	7.45%	74.61%	16.86%
Shale pottery sand	Particle size	<0.63 mm	0.63 mm~1.25 mm	1.25 mm~2.5 mm	>2.5 mm
	Proportion	52.75%	16.15%	23.34%	3.76%

. The experiment used P.C42.5 ordinary Portland cement and P.O52.5 ordinary Portland cement. Its performance indicators are detailed in

Table 2. Performance index of ordinary Portland cement [20].

Cement Type	Fineness (%)	Initial Setting Time (min)	Final Setting Time (min)	SO3 (%)	MgO (%)	Ignition Loss(%)	Target fcu(28) (MPa)
P.C42.5	1.2	130	172	2.25	1.7	1.02	47.3
P.O52.5	3.6	100	153	2.49	1.93	2.07	58.0

, and the concrete mixes are shown in

Table 3. Mix ratios of concrete.

Strength Grade	Cement (kg/m3)	Shale Ceramsite (kg/m3)	Shale Pottery Sand (kg/m3)	Ball Milled Powder (kg/m3)	Admixture (kg/m3)	Water (kg/m3)	Unit Weight (kN/m3)
ALSCC20	420	290	410	90	10.2	250	14.65
ALSCC30	460	280	430	90	11.0	245	15.10
ALSCC40	490	320	500	110	15.0	220	15.90
ALSCC45	520	310	540	110	15.8	220	16.46

.

2.2. Sample Preparation

To ensure the reliability of the test data, this article conducted 180 samples according to the Chinese standard “Standard for Test Method of Mechanical Properties on Ordinary Concrete” (GB/T 50081-2002) [21]. The specific parameters of the samples are detailed in

Table 4. Parameters of ALSCC samples for mechanical property test.

Type of Test	Sample Nomenclature	Sample Size (mm)	Number of Samples	Test Age (days)
Cube compressive strength	ALSCC20	150 × 150 × 150	12	3, 7, 14, 28
	ALSCC30		12
	ALSCC40		12
	ALSCC45		12
Axial compressive strength	ALSCC20	150 × 150 × 300	12
	ALSCC30		12
	ALSCC40		12
	ALSCC45		12
Split tensile strength	ALSCC20	150 × 150 × 150	12
	ALSCC30		12
	ALSCC40		12
	ALSCC45		12
Elastic modulus	ALSCC20	150 × 150 × 300	9	7, 14, 28
	ALSCC30		9
	ALSCC40		9
	ALSCC45		9

. Additionally, 36 samples were prepared according to the Chinese standard “Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete” (GBT 50082-2009) [22] and the “Test Code for Hydraulic Concrete” (SL/T 352-2020) [23]. The specific parameters of these samples are provided in

Table 5. Parameters of ALSCC samples for chlorine salt corrosion resistance test.

Type of Test	Sample Nomenclature	Sample Size (mm)	Number of Samples	Steel Bar Diameter (mm)	Groups
Chloride ion permeability	ALSCC20	Ø100 × 50	3	-	-
	ALSCC30		3	-	-
	ALSCC40		3	-	-
	ALSCC45		3	-	-
Electrochemical steel bar corrosion	ALSCC20	100 × 100 × 400	2	12	-
			2	20	-
	ALSCC30		2	12	-
			2	20	-
	ALSCC40		2	12	-
			2	20	-
	ALSCC45		2	12	-
			2	20	-
ALSCC30 corrosion	ALSCC30	150 × 150 × 150	4	-	Soaking group
	ALSCC30		4	-	Control group

. Furthermore, Figure 2 illustrates the design of the electrochemical steel bar corrosion test samples.

ALSCC samples were prepared according to the Chinese standard “Standard for Testing the Performance of Ordinary Fresh Concrete” (GB/T 50080-2016) [24]. The specific production process is illustrated in Figure 3. The steps for producing electrochemical steel bar corrosion samples are as follows: First, the concrete pad is installed and numbered in the self-made mold (Figure 4). Next, the mixed ceramsite concrete mortar is poured into the mold, and the steel bar is secured with a self-made steel fork. After pouring, the steel fork is placed on a shaking table and shaken for one minute to fully compact the concrete in the mold. Finally, after the vibration stops, the steel fork is removed.

2.3. Mechanical Properties

Compressive strength test, split tensile strength test, and modulus of elasticity test according to the Chinese standard “Standard for Test Method of Mechanical Properties on Ordinary Concrete “ (GB/T 50081-2002) [21].

2.3.1. Compressive Strength

(1)

After reaching the test age, the sample was transferred from the standard curing room to the test chamber. The surface of the sample was dried and placed at the center of the press.

(2)

A specific loading speed was maintained during the test (6.75 kN/s for ALSCC20 and ALSCC30; 11.75 kN/s for ALSCC40 and ALSCC45). The load value was recorded at the point of sample failure. The cube compressive strength test setup is shown in Figure 5, and the axial compressive test setup is shown in Figure 6.

(3)

Compressive strength was calculated using Equation (1).

$$p=\frac{F}{A},$$

(1)

where p is the compressive strength (in MPa) of ALSCC; F is the ultimate load (in N) from the test; A is the pressure-bearing area (in mm²) of ALSCC.

2.3.2. Split Tensile Strength

(1)

After reaching the test age, the samples were removed from the curing chamber. The centerline was then drawn, and the position of the mat strips was determined. The mat strips were positioned in the center of the positioning frame, and the sample was fixed. Finally, the mat strips were placed on top, and the steel plate was pressed (Figure 7).

(2)

The positioning frame was placed in the center of the lower pressure plate of the testing machine. The testing machine was then started, allowing the upper pressure plate to slowly descend and ensure uniform contact with the steel mat plate. Consequently, the split pressure surface was perpendicular to the top surface of the sample.

(3)

The fueling valve was loaded uniformly during the test, with a loading rate of 0.02 MPa/s for ALSCC20 and ALSCC30 samples and 0.08 MPa/s for ALSCC40 and ALSCC45 samples.

(4)

Split tensile strength was calculated using Equation (2).

$$f_{\mathrm{ts}}=\frac{2F}{\pi A}=0.637\frac{F}{A},$$

(2)

where f_ts is the split tensile strength (in MPa) of ALSCC; F is the damage load (in N) from the test; A is the split surface area (in mm²) of ALSCC.

2.3.3. Modulus of Elasticity

(1)

When the sample reached the corresponding test age, it was removed from the standard curing room, and a center line was drawn on both sides of the sample using a pencil. Then, strain gauges were attached along the center line of the sample.

(2)

The sample was placed in the center of the press, and the strain gauges were connected. The upper bearing plate was then slowly lowered until it came into contact with the top surface of the sample.

(3)

The sample was pre-compressed three times to a load of 0.5 MPa (noted as F₀) and held steady for 60 s. The deformation readings ${\epsilon }_0$ at the measuring points on both sides of the sample were recorded every 30 s. Then, the load was uniformly increased to one-third of the axial compressive strength f_cp (noted as F_a) and held steady for 60 s, with deformation readings ${\epsilon }_{\mathrm{a}}$ recorded every 30 s. The test procedure ensured that the load was increased continuously and uniformly, with a loading rate of 0.3 MPa/s for ALSCC20 and ALSCC30 samples, and 0.5 MPa/s for ALSCC40 and ALSCC45 samples. The elastic modulus test procedure is shown in Figure 8.

(4)

Elastic modulus was calculated using Equations (3) and (4).

$$\Delta n={\epsilon }_{\mathrm{a}}-{\epsilon }_0,$$

(3)

$$E_{\mathrm{c}}=\frac{F_{\mathrm{a}}-F_0}{A}\times \frac{L}{\Delta n},$$

(4)

where $\Delta n$ is the average value (in mm) of deformation on both sides of the sample when loaded from F₀ to F_a in the formal test; ${\epsilon }_{\mathrm{a}}$ is the average value (in mm) of deformation on both sides of the sample when loaded to F_a; ${\epsilon }_0$ is the average value (in mm) of deformation on both sides of the sample when loaded to F₀; E_c is the elastic modulus (in MPa) of concrete under hydrostatic stress; F_a is the load value (in N) for one-third axial compressive strength f_cp; F₀ is the initial load (in N) at a stress of 0.5 MPa; A is the pressure-bearing area (in mm²) of sample; L is the measuring distance (in mm).

2.4. Chlorine Salt Corrosion Resistance Test Methods

2.4.1. Chloride Permeation Resistance

(1)

According to the RCM method (Figure 9) of the Chinese standard “Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete” (GBT 50082-2009) [22], a test voltage of 30 V was used after the samples were saturated with water. In addition, the anode was injected with a 0.3 mol/L NaOH solution, and the cathode was injected with a 10% NaCl solution by mass concentration.

(2)

After the sample was split along the central axis, a 0.1 mol/L AgNO₃ solution was dropped onto its surface. When the fracture surface of the sample showed a color change, it was divided into 10 equal portions along the diameter. The depth of effective chloride ion penetration was then measured and substituted into Equation (5) to calculate the chloride ion migration coefficient.

$$D_{\mathrm{RCM}}=\frac{0.0239(273+T)L}{(U-2)t}\left(X_{\mathrm{d}}-0.0238\sqrt{\frac{(273+T)L{X}_{\mathrm{d}}}{U-2}}\right),$$

(5)

where D_RCM is the unsteady-state migration coefficient (in m²/s); T is the average of the initial and ending temperatures of the anode solution (in K); L is the thickness of the sample (in m); X_d is the average value of chloride ion penetration depth (in m); U is the absolute value of the voltage used (in V); t is the testing time (in s).

2.4.2. Electrochemical Steel Bar Corrosion Test

(1)

The sample was placed into a 5% NaCl solution corrosion chamber, maintaining the solution level flush with the sample. One end of the sample was connected to the positive pole of the DC power supply, while the other end of a stainless-steel rod was immersed in the solution, forming a current circuit corrosion sample.

(2)

According to Faraday’s law [25], its rusting time t can be calculated by using Equation (6).

$$t=\frac{\Delta m\cdot Z\cdot F}{M_{\mathrm{Fe}}\cdot I},$$

(6)

where t is the energization time (in s); $\Delta m$ is the quality loss of steel reinforcement (in g); Z is the reaction electrode chemical valence (+2); F is the Faraday’s constant (96,500 C/mol); M_Fe is the atomic weight of iron (56 g/mol); I is the current intensity (in A).

The theoretical mass loss rates of steel bar corrosion were set at 10% and 20%, and the energized corrosion time was calculated using Equation (6) (

Table 6. Reinforcement parameters and corrosion time.

No.	Sample Nomenclature	Steel Bar Diameter (mm)	Steel Bar Weight (g)	Corrosion Current (A)	Corrosion Time (days)
A1	ALSCC20	12	321.83	0.08	15.10
A2		12	315.66	0.16	15.73
A3		20	911.16	0.20	18.17
A4		20	914.23	0.40	18.17
B1	ALSCC30	12	320.32	0.08	15.03
B2		12	319.64	0.16	15.93
B3		20	915.96	0.20	18.27
B4		20	927.33	0.40	18.49
C1	ALSCC40	12	320.67	0.08	15.98
C2		12	322.36	0.16	16.07
C3		20	922.86	0.20	18.41
C4		20	917.06	0.40	18.29
D1	ALSCC45	12	319.25	0.08	15.92
D2		12	320.63	0.16	15.99
D3		20	915.92	0.20	18.27
D4		20	912.32	0.40	18.20

).

(3)

After the sample reached the corrosion period and showed signs of degradation, the steel bar was removed and descaled with 10% hydrochloric acid. It was then neutralized with lime water and washed with water. Afterward, the steel bar was placed in a drying box for 4 h, and finally, it was weighed.

(4)

The corrosion rate of steel bars was calculated using Equation (7).

$$\rho =\frac{m_0}{m_{\mathrm{t}}},$$

(7)

where $\rho$ is the corrosion rate of steel bars; m₀ is the quality of steel bars before corrosion (in g); m_t is the quality of corroded steel bars (in g).

2.4.3. ALSCC30 Corrosion Test

(1)

The ALSCC30 samples were divided into a soaking group and a control group. The samples in the soaking group were immersed in a 5% NaCl solution for corrosion after being cured for 7 days.

(2)

After the corrosion time reached 3, 7, 14, and 28 days, the ALSCC30 samples were cleaned and dried for the compressive strength test.

3. Mechanical Properties Analysis

3.1. Compressive Strength

The compressive strength of each ALSCC grade meets the corresponding strength requirements and increases with the grade, as illustrated in Figure 10 and Figure 11. This is because as the strength grade increases, the water–cement ratio decreases, and the hydration reaction becomes more adequate, resulting in fewer internal pores within the concrete and improved compressive strength. The linear regression analysis of ALSCC axial compressive strength f_c,28d and cubic compressive strength f_cu,28d revealed the relationship between f_c,28d and f_cu,28d (Equation (8)), with the corresponding curves shown in Figure 12. The f_c,28d of ALSCC is slightly higher than f_cu,28d, consistent with the typical pattern of normal concrete compressive strength. The ratio of f_c,28d to f_cu,28d ranges from 0.94 to 0.99 for ALSCC, while it is approximately 0.80 for ordinary concrete [26]. Analysis: When the sample is subjected to compressive failure, longitudinal and transverse strains are generated. Frictional forces occur at the contact surface between the sample and the testing machine, causing the sample to expand laterally to counteract this constraint force. Based on the split tensile test data (Figure 13), the splitting tensile strength of ALSCC is significantly higher than that of ordinary concrete. Therefore, the sample can withstand greater force during lateral expansion in the middle section. Consequently, f_c,28d of ALSCC is higher, approaching or even equaling f_cu,28d.

$$f_{\mathrm{cu},28\mathrm{d}}=1.05f_{\mathrm{c},28\mathrm{d}}-0.46,$$

(8)

3.2. Split Tensile Strength

The split tensile strength of the low-strength-grade samples did not change significantly with age, whereas that of the high-strength-grade samples increased considerably with age (Figure 13). The split tensile strengths of ALSCC20–ALSCC45 samples increased by 40%, 18.18%, 65.4%, and 65.5% from 3 to 28 days, respectively. Additionally, the split tensile strength of the ALSCC45 sample was 2.29 times higher than that of the ALSCC20 sample. Based on material proportioning analysis, increasing the amount of cement and reducing the amount of water are more effective in enhancing the split tensile strength of ALSCC. A lower water–cement ratio makes the internal structure of concrete denser, reducing the formation of pores and microcracks, thereby increasing the split tensile strength of concrete. Additionally, increasing the amount of cement enhances the cementitious system of concrete and improves its mechanical properties.

3.3. Elastic Modulus

The higher the strength grade, the smaller the change in the elastic modulus of ALSCC with age, as shown in Figure 14. From day 7 to day 28, the elastic modulus of ALSCC20 to ALSCC45 increased by 6.6%, 5.3%, 5.3%, and 4.9%, respectively. This is because the hydration reaction of high-strength concrete is more sufficient in the early stage, leading to most cement particles being hydrated in a relatively short time and forming a stable hydration product. Consequently, there are fewer further hydration reactions over time, resulting in little change in the elastic modulus. In contrast, due to the incomplete initial hydration of low-strength concrete, more hydration reactions continue to occur with age, leading to greater changes in elastic modulus. The elastic modulus of ALSCC is relatively low, approximately 50% of that of ordinary concrete [27]. This is due to the fact that the stiffness, bulk density, and porosity of aggregate all impact the elastic modulus of concrete. Among these factors, porosity has the greatest influence on the elastic modulus. However, it should be noted that ALSCC aggregates consist of shale ceramsite and sand, which have more internal pores and a lighter texture. As a result, their bulk density is about 65% that of ordinary concrete.

4. Analysis of the Chlorine Salt Corrosion Resistance Test

4.1. Resistance to Chloride Ion Penetration

The depth of chloride penetration in ALSCC is inversely proportional to the concrete strength (

Table 7. Depth of chloride ion penetration in samples of different strength grades.

Strength Grade	ALSCC20	ALSCC30	ALSCC40	ALSCC45
Average depth of penetration of chloride ions (mm)	31.62	25.96	16.03	9.98
	29.72	23.81	16.02	11.36
	30.12	25.47	16.04	11.17

). The maximum depth of chloride ion penetration in ALSCC20 and ALSCC45 samples reached 31.62 mm and 11.36 mm, respectively. The depth of chloride ion penetration in the ALSCC45 sample was only 35.9% of that in the ALSCC20 sample. This is probably because concrete with lower strength may contain more pore space, making it easier for chloride ions to penetrate. In contrast, the internal structure of ALSCC45 is compact, and its low porosity helps to reduce the depth of chloride ion penetration. Sections of ALSCC samples after splitting for each strength grade are shown in Figure 15.

The rapid chloride migration (RCM) values of ALSCC samples for each strength grade are presented in

Table 8. RCM test values of samples.

Strength Grade	RCM Value/(10−12 m2/s)
ALSCC20	17.44
ALSCC30	14.10
ALSCC40	7.20
ALSCC45	4.69

. The test results of chloride ion penetration resistance were used to compare and analyze the RCM values of C40 and C50 ordinary concrete [28] with those of ALSCC. It was observed that the RCM values of ALSCC are lower than those of ordinary concrete, with the RCM values of ALSCC40 and ALSCC45 samples being 15% and 31% lower than those of C40 and C50 samples, respectively (as shown in Figure 16). This is due to the ability of ceramsite to absorb some water, reducing the water content in the capillary pores of the concrete, thereby decreasing the ability of chloride ions to penetrate through capillary action. Additionally, ceramsite (primarily composed of Al₂O₃ and SiO_2, accounting for three-quarters of its components) hydrates with Ca(OH)₂ in the concrete to form Ca[Al(OH)₄]₂ precipitates and C–S–H gel, which fill the spaces between the cement hydration products. This increases the internal filling of the concrete and enhances its resistance to chloride ions. The mechanism for resisting chloride ion penetration is illustrated in Figure 17, and the scanning electron microscope (SEM) results are depicted in Figure 18.

4.2. Electrochemical Corrosion of Steel Bars

In the mildly corroded samples with an expected corrosion rate of 10%, the corrosion rate decreases with the increase in ALSCC strength grade, while the effect of rebar diameter on the corrosion rate increases (Figure 19a). The corrosion rates of 20 mm diameter bars in ALSCC20–ALSCC45 samples were 9.11%, 8.56%, 8.2%, and 7.72%, respectively, which were 0.23%, 0.31%, 0.36%, and 0.44% higher than those of 12 mm diameter bars. This was because the low-strength samples had poorer resistance to deformation and were prone to cracking, allowing the salt solution to penetrate the concrete samples. Therefore, the influence of the diameter of the reinforcement on the corrosion rate is relatively small. In contrast, high-strength samples are less prone to cracking, but larger-diameter bars can cause cracks due to the rust expansion effect. Consequently, the effect of rebar diameter on the corrosion rate is relatively significant.

In the severely corroded samples with an expected corrosion rate of 20%, the corrosion rate decreases insignificantly with increasing ALSCC strength grade, and the effect of rebar diameter on the corrosion rate is minimal (Figure 19b). The corrosion rates of 20 mm diameter bars in ALSCC20–ALSCC45 samples were 18.78%, 18.52%, 17.60%, and 15.92%, respectively, which were 0.07%, 0.09%, 0.33%, and 0.51% higher than those of 12 mm diameter bars. This was attributed to the fact that more microcracks or pores occurred in samples with high corrosion rates. Consequently, the salt solution can easily penetrate into the concrete interior and corrode the reinforcement, making the effect of reinforcement diameter on the corrosion rate relatively small.

4.3. Calculation Model of the Compressive Strength of ALSCC Corroded by Chloride Salt

To investigate the impact of chloride corrosion on the compressive strength of ALSCC, the compressive strength loss rate was used to express the change pattern after different immersion times. The calculation formula is shown in Equation (9). The calculation formula is as follows (9):

$$K=\left(\frac{f_{\mathrm{t}}^{\prime }-f_{\mathrm{t}}}{f_{\mathrm{t}}}\right)\times 100\%,$$

(9)

where K is the rate of loss of compressive strength of concrete cubes; $f_{\mathrm{t}}^{\prime }$ is the cube compressive strength (in MPa) of ALSCC samples soaked in chloride environment for t days; $f_{\mathrm{t}}$ is the cube compressive strength (in MPa) of the uncorroded sample.

The compressive strength of ALSCC30 cubes corroded by chloride salt exhibited a growing trend (Figure 20), with the growth rate showing an exponential relationship with immersion time. Based on the data obtained from the test, the model for calculating K (Equation (10)) can be derived as follows:

$$K=26.37t^{\mathrm{a}},$$

(10)

where t is the immersion time of the corroded concrete samples; a is the model parameter, a = −0.29.

By combining Equations (9) and (10), the compressive strength calculation model (Equation (11)) for chloride salt-corroded ALSCC cubes was derived as follows:

$$f_t^{\prime }=f_t\times \left(26.37t^{\mathrm{a}}+b\right)({\mathrm{R}}^2=0.972),$$

(11)

where b is the model parameter, b = 1.

Comparing the theoretical and measured compressive strength values of the corroded ALSCC cubes showed good overall agreement and high model prediction accuracy (Figure 21).

5. Conclusions

In this study, a novel lightweight aggregate concrete was prepared using shale ceramsite and shale pottery sand, characterized by its light weight, high strength, and resistance to chloride salt corrosion. Tests were conducted on its compressive strength, axial compressive strength, split tensile strength, elastic modulus, resistance to chloride ion penetration, electrochemical steel bar corrosion, and ALSCC30 corrosion. The following main conclusions were drawn from the experimental results:

(1)

ALSCC exhibits good mechanical properties, which are positively correlated with strength grade. The cube compressive strength, axial compressive strength, split tensile strength, and elastic modulus of ALSCC45 are 3.92, 2.16, 2.29, and 1.42 times higher than those of ALSCC20 at 28 days, respectively. As the water–cement ratio decreases, the compactness of the concrete improves; pores and microcracks are reduced, and the amount of cement is increased, enhancing the cementitious system and improving its mechanical properties.

(2)

The chloride corrosion resistance of ALSCC is superior to that of ordinary concrete. When compared with ordinary concrete of the same strength grade, the RCM values of ALSCC40 and ALSCC45 specimens are 15% and 31% lower than those of C40 and C50 samples, respectively. This is due to the ability of ceramsite particles to absorb water and reduce the water content in the pores. This reduction in water content leads to decreased permeability to chloride ions. Additionally, hydration of ceramsite and Ca(OH)₂ in the concrete results in the precipitation of Ca[Al(OH)₄]₂ and the formation of C–S–H gel. These products fill the spaces between cement hydration products, thereby increasing the internal filling within the concrete and enhancing its resistance to chloride ions.

(3)

The depth of chloride ion penetration in ALSCC is inversely proportional to its strength grade. The penetration depth of chloride ions in the ALSCC45 sample is only 35.9% of that in the ALSCC20 sample. As the strength grade increases, the porosity of the concrete decreases, reducing the penetration rate and depth of chloride ions.

(4)

A calculation model for the compressive strength of ALSCC under chloride salt corrosion was proposed based on the ALSCC30 corrosion test results. The theoretical values of the model showed good agreement with the measured values, demonstrating high prediction accuracy with an R² value of 0.972.