Design and Preparation of a Novel Double-Modified Cement-Based Protective Coating Material and Its Improved Protection Performance Against Chloride Corrosion

Abstract

The service of reinforced concrete structures (RCSs) in harsh coastal environments is often threatened by chloride corrosion. The penetration of chloride ions through concrete pores into the steel/concrete interface will cause the depassivation and corrosion of steel rebars, which will lead to the deterioration and failure of RCSs durability. It is important to repair and protect the corrosion damage of existing concrete structures and ensure their high durability, and the high performance of repairing and protecting materials is crucial. In this paper, a novel cement-based protective coating material with low porosity, high impermeability and chloride-corrosion resistance was designed and prepared by introducing polypropylene fiber and high-performance cement into commercial cement-based protective materials through the double modification strategy of fiber-toughening and substrate-enhancing, in order to provide a reliable corrosion protection solution for the high durability and long life of RCSs under chloride erosion environment. Based on this, the microstructure and pore structure of the double-modified coating material was systematically analyzed by SEM, XRD, X-CT and other characterization methods. The impermeability and chloride corrosion resistance of this material were scientifically evaluated, and the protection mechanism was systematically discussed. The results show that the impermeability of the double-modified coating material is about 2.8 times higher than that of the untreated mortar. At the same time, the corrosion current density was significantly reduced to 8.60 × 10⁻⁷ A·cm⁻², which was about 86% lower than that of the untreated sample (6.11 × 10⁻⁶ A·cm⁻²). The new cement-based coating material optimized by double-modification effectively inhibits the formation and propagation of microcracks in the protective coating through the bridging effect of fibers. At the same time, the regulation of cement hydration products and the densification of pore structure are realized by adjusting the composition of cement matrix. Based on the above two aspects of microstructure improvement, the chloride-corrosion protection performance of the novel cement-based protective coating material has been greatly improved.

1. Introduction

In the field of modern construction engineering, reinforced concrete structures (RCSs) are widely used in all kinds of infrastructure construction for its good mechanical properties, relatively low cost and easy forming [1,2]. However, with the passage of time and the increasingly harsh service environment, the durability of RCSs has become increasingly prominent, becoming the main source of risk that threatens the long-term safe and stable operation of engineering structures, which has aroused great concern in the academic and engineering circles. Many studies focus on this and strive to explore effective coping strategies [3,4,5,6].

The durability of RCSs involves many complex factors, among which the decrease in durability induced by chloride corrosion is one of the most prominent and destructive factors [7,8,9,10]. When properly designed, for example, C30/37 concrete, can have sufficient resistance to chloride ion penetration. However, in practical engineering, the concrete used does not strictly follow the design standards, such as using unreasonable water–cement ratios or not using admixtures. For this unreasonably designed concrete, or concrete with degraded impermeability performance under long-term service, the erosion of chloride ions is an urgent issue that needs attention. Chloride ions can invade the interior of RCS in many ways due to its special chemical activity. From a microscopic point of view, the extremely small ionic radius of chloride ions makes it possess a strong penetration ability. It can gradually penetrate through fine channels such as pores and microcracks inside the concrete. When chloride ions reach the surface of the steel rebar, the pH value will be rapidly reduced, and the rebar passive film formed in the alkaline environment of the concrete will be destroyed, which will lead to the corrosion of the rebar [11,12]. Further, the corrosion products of steel rebars have a larger volume than its matrix, which will generate expansion stress around the rebar, resulting in a decrease in the bonding force between the rebar and the concrete. The concrete begins to peel off, and the cracks expand further, eventually leading to a decrease in the durability of the RCS [13,14]. Therefore, it is difficult to achieve the expected goal by relying solely on the rebar passive film wrapped in concrete to protect the rebar from corrosion. In order to fundamentally solve the problem of structural deterioration caused by rebar corrosion, it is necessary to prevent chloride ions from penetrating into the surface of the rebar during the service period of the structure.

The concrete protective layer plays an important role in the durability guarantee system of RCSs. It effectively blocks the direct contact between the external chloride salt, water and other corrosive media and the rebar, greatly delays the transmission process of chloride ions to the surface of the rebar, and provides a strong guarantee for the rebar to maintain a stable passivation state [15,16,17,18]. On the other hand, the concrete protective layer also has a certain alkaline reserve, which helps to maintain the high alkalinity of the environment in which the rebar is located, strengthen the stability of the rebar passive film, and strengthen the protective effect from the chemical level [19,20]. The common concrete protective layer is Portland cement repair mortar, which is mainly composed of Portland minerals. Its main hydration product is hydrated calcium silicate (C-S-H) gel. The compact accumulation of C-S-H gel is the guarantee of high strength of Portland cement materials. This repair material has low cost and high cost-effectiveness, and most of the materials used in the construction of concrete structures are Portland cement. Therefore, the compatibility between Portland repair mortar and matrix can be fully guaranteed. However, the drying shrinkage of Portland cement repair mortar itself is relatively large, and there is a problem of poor volume compatibility with the matrix, which can easily cause cracking of the bonding surface. At the same time, Portland cement repair mortar has large structural pores, and the transmission of chloride ions in the concrete protective layer repair mortar is difficult to completely eliminate under practical working conditions. However, it is difficult to completely eliminate the transmission of chloride ions in the concrete protective layer under actual working conditions. Chloride ions will continue to approach the area where the rebars are located through the pore structure of the concrete protective layer itself, the microcracks generated during the construction process, and the cracks that gradually sprout under long-term load, following the physical transmission mechanisms such as diffusion and penetration. Once the corrosion of rebar occurs and the concrete protective layer cracks, the durability protection of the structure will fail, and the chloride erosion rate will increase exponentially, which will bring a fatal blow to the life of the structure [21,22].

In view of the key position of concrete protective layer in structural durability and the severe challenge of chloride corrosion, it is urgent to explore effective ways to improve the impermeability and resistance to chloride ion erosion of protective layer. The traditional ways to improve the impermeability and resistance to chloride ion erosion of concrete protective layers are generally divided into two aspects. On the one hand, from the perspective of material optimization, the selection of high-quality and low-permeability cement varieties, through the rational allocation of mineral admixtures, such as fly ash [23,24], slag powder [25], can significantly refine the internal pore structure of concrete protective layers, reduce pore connectivity, and improve the impermeability of protective layers. On the other hand, at the level of mix design, the water–binder ratio should be accurately controlled to ensure that the water–binder ratio value should be reduced as much as possible under the premise of meeting the requirements of construction work, so as to reduce the free water content inside the concrete protective layers, reduce the porosity, and enhance the chloride corrosion resistance of the protective layers from the source [26,27]. However, for existing RCSs, due to the influence of multiple factors such as environmental erosion, load action and material aging for a long time, it is inevitable that different degrees of durability damage will occur. Repairing and improving its durability has become an important task in the engineering field. Among many repair methods, the application of repair measures combined with protective integrated cement-based materials shows unique advantages. Such cement-based materials cover a variety of types, such as polymer-modified cement-based materials. By introducing a polymer emulsion, an organic–inorganic composite network structure is formed during the hydration and hardening process of cement, which not only retains the good compatibility between cement-based materials and concrete matrix, but also endows materials with excellent toughness and crack resistance. It can effectively fill existing structural cracks and prevent further penetration of corrosive media [28,29,30]. Yang et al. [31] studied the chloride ion penetration resistance and microstructure of styrene butadiene rubber (SBR) lotion modified silicate repair mortar, and found that SBR improved the ion penetration resistance and ion transport resistance of repair mortar, and reduced the capacitance of mortar. By observing the microstructure of the mortar, it was found that with the increase in SBR content, the internal pore structure of the mortar was improved, and some connected pores were closed, effectively enhancing the material’s impermeability. Fiber-reinforced cement-based materials, for example, mixed with steel fiber, polypropylene fiber and other different types of fibers, with the help of fiber bridging and crack resistance, can significantly improve the toughness of the material, inhibit the generation and expansion of cracks, and strengthen the impermeability of the structure [32,33]. Feng et al. [34] prepared a carbon fiber modified repair mortar (CFRMS) and studied its strength and bonding properties. It was found that carbon fiber can enhance the mechanical strength of mortar, and when added together with steel fiber (SF) to repair mortar, it can simultaneously improve the tensile and compressive strength of composite mortar. However, carbon fiber itself has an agglomeration effect and poor dispersibility in mixing water, which further increase the difficulty of preparing carbon fiber modified repair mortar. Compared to high cost carbon fiber, polypropylene (PP) fiber has higher strength, elastic modulus, stable chemical properties, and extremely strong acid and alkali resistance. It also has excellent self-dispersion and is more suitable as a modified material for repairing mortar. From the analysis of the mechanism of action, these repair and protection integrated materials can quickly form a protective layer with high impermeability, crack resistance and high durability at the repair site while repairing the surface damage and filling cracks of the existing structure. With its own excellent early performance, it can reach a certain strength in a short time and minimize the risk of continuous damage to the structure during the repair period.

This study focuses on the design, preparation, service performance evaluation and mechanism exploration of chloride-corrosion protection and repair-protection integrated in cement-based protective coating materials, for the protection of concrete and rebar in environment of long term immersion in seawater, with exposure classes of XS (corrosion environment caused by chloride in seawater), and subdivided exposure classes of XS2 (long term immersion in seawater) according to the EN206 standard. Due to the insufficient impermeability and protective performance of commercial coating materials, this paper uses fiber and 52.5R cement to modify and enhance commercial coating materials, to prepare a new type of cement-based protective coating material, which is coated on the prepared mortar substrate.

On the basis of commercial coating material, fiber and 52.5R cement with higher strength, high impermeability, and superior early performance were used for double-modification of protective materials by fiber toughening and substrate enhancing. The mortar matrix is prepared with 42.5 cement, without the addition of admixtures, to simulate the existing structures of C30/37 with substandard performance or insufficient long-term durability. The microstructure characteristics, pore structure and self-permeability of the novel double-modified cement-based protective coating materials were investigated by means of SEM, XRD, X-CT and impermeability test. The protective effect of the coating materials on the chloride corrosion of reinforced mortar samples was evaluated by electrochemical corrosion test, so as to reveal the protective mechanism of the double-modified cement-based protective coating materials. The modified coating material can be applied for different degrees of corrosion damage and repair requirements of reinforced concrete structures, such as mild corrosion, where the concrete protective layer remains intact without obvious cracking or detachment, and severe corrosion, where the reinforced concrete structure has suffered severe corrosion, resulting in cracking or even partial detachment of the concrete protective layer. In mild corrosion, a protective coating material with higher density and impermeability can suppress the permeation and transmission of chloride ions, thus achieving the effect of structural corrosion protection, while in severe corrosion, coatings can be used as structural repair materials with their outstanding early service performance and good density and impermeability, to suppresses the erosion of chloride ions.

2. Materials and Methods

2.1. Coating Method and Preparation of Samples

2.1.1. Raw Materials

In this paper, commercial coating material is used as the basic material, and the chemical composition of the coating material is retested by XRF technology (SHIMADZU XRF-1800, Kyoto, Japan). The chemical composition is shown in

Table 1. Chemical composition of commercial coating material.

Chemical Composition	CaO	SiO2	Al2O3	CO2	Fe2O3	SO3	Na2O	K2O
Content (wt.%)	52.79	19.34	10.06	5.08	4.77	3.86	2.29	1.01

.

Firstly, polypropylene short fiber was used to toughen commercial coating material, and its macro and micro morphology was shown in Figure 1. On the basis of fiber toughening, P·II 52.5R Portland cement was further used to enhance and modify the substrate of cement-based protective coating material. The apparent density of P II·52.5R Portland cement is 3.52 m²/g, and the fineness is 8.7. The chemical composition was retested by XRF technology, as shown in

Table 2. Chemical composition of P·II 52.5R Portland cement.

Chemical Composition	CaO	SiO2	Al2O3	CO2	Fe2O3	SO3	K2O
Content (wt.%)	59.90	16.47	5.21	6.68	5.06	4.57	1.01

.

By adding polypropylene short fibers in the coating, bridging role can be obtained to prevent crack propagation, and fiber toughening effect can be achieved. The fibers can improve the toughness of protective materials, reduce the possibility of cracking, and to some extent enhance their impermeability, increase the resistance to chloride ion diffusion, enhancing their corrosion protection performance.

2.1.2. Mix Proportion Design and Preparation of Double-Modified Cement-Based Protective Coating Materials

The proportion of cement-based anticorrosive coating materials is shown in

Table 3. Proportion of cement-based anti-corrosion coating material (1 kg coating material).

	Water–Cement Ratio	Water/kg	Commercial Coating Material/kg	P·II 52.5R Portland Cement/kg	Polypropylene Fiber/kg
Commercial coating material	0.25	0.2	0.8	0	0
Fiber-toughened coating material	0.25	0.2	0.8	0	0.003
Double-modified coating material	0.25	0.2	0.56	0.24	0.003

. In the preparation process, the dispersion of polypropylene fiber in cement paste is a problem worthy of discussion. It is generally believed that the dispersion of cement paste fiber obtained by mixing polypropylene fiber into dry material first and then stirring with water is better than that of mixing polypropylene fiber after adding water. Therefore, in this paper, the fiber-toughening modification of cement-based protective coating materials was prepared by pre-doping method. It has to be noted that a nontraditional water–cement ratio of 0.25 was used due to the considerations of flowability as a protective coating material. Traditional water–cement ratio of 0.45 would lead the flowability of the freshly mixed slurry to be too high to achieve uniform coating on the surface of existing concrete structures. After multiple experiments, we found that reducing the water–cement ratio to 0.25 can meet the flowability requirements of protective materials. However, uniform mixing of 52.5R cement at low water–cement ratios (0.25) is challenging due to its high fineness. At low water–cement ratios, cement particles are difficult to fully disperse and can easily form clumps, affecting uniformity. Generally, efficient additives and optimized stirring processes can be used to improve fluidity. In this paper, we employed methods such as extending the mixing time to ensure sufficient dispersion of cement particles and avoid agglomeration formation, and using high-speed mixing to increase shear force and promote uniform mixing.

First of all, the commercial coating material powder, polypropylene fiber, P·II 52.5R Portland cement powder and mixing water were weighed by using the balance according to the ratio in Table 3. The two powder materials and polypropylene fibers were placed in a stirring pot for low-speed stirring for 30 s. After the dry material is stirred, half of the mixing water is added, and the other half of the water is added to continue stirring for 1 min after stirring at a low speed for 1 min. Finally, after 3 min of high-speed stirring, the required cement-based protective coating material slurry is obtained. The newly prepared slurry was poured into the mold prepared in advance and placed on the vibration table for 1 min. Then, cover the surface with cling film to prevent the loss of moisture during the solidification and hardening process. After molding, the test piece is left to stand and cured at room temperature for 1 day before demolding. And it was placed in a standard curing room (temperature of 20 ± 2 °C, relative humidity of more than 95%) for 7 days. Because the concrete structure repair protection project requires the protective coating material to have sufficient service performance as soon as possible after construction, the early performance of the cement-based protective coating material is particularly important. Therefore, this research will focus on the microstructure characteristics and service performance of cement-based protective coating materials after 7 days of curing.

By the calculation based on the market price of the raw materials, the costs of commercial coating material, fiber-toughened coating material, and double-modified coating material are about 8.00, 8.03, and 5.73 yuan/kg, respectively. Due to the lower price of the modified P·II 52.5R cement compared to the commercial coating, and the use of low cost polypropylene fibers, the cost of the modified commercial coating has been reduced by approximately 28% compared to the commercial coating.

2.1.3. Preparation of Electrochemical Corrosion Test Sample

HRB400 carbon rebars with a diameter of 11.2 mm were selected as rebar and embedded in cement mortar prepared according to the mixing ratio in

Table 4. Mix ratio of mortar.

Raw Materials	P·O 42.5 Cement	Standard Sand	Water
Content (kg/m3)	700	1259	280

. It has to be noted that P·II 52.5R Portland cement was used in the above section for the preparation of modified coating material, while P·O 42.5 cement was used for the cement mortar matrix sample here. After molding, standard curing was carried out, and finally an electrochemical test sample with a size of Φ50 mm × 50 mm was prepared. In order to meet the requirements of electrochemical test, one end of the rebar is connected with a copper wire, and both ends of the rebar-mortar sample are packaged with epoxy resin, while the cylindrical surface of the mortar is retained as the exposed surface of the electrochemical corrosion test.

2.1.4. Preparation of Electrochemical Corrosion Test Sample with Cement-Based Protective Coating

The proportion of cement-based anticorrosive coating material is strictly in accordance with Table 3. After the mixing process is completed, the rebar mortar sample is put into the coating material slurry, so that the protective coating material completely wraps the cylindrical surface of the rebar mortar sample without touching the top epoxy resin surface. After standing for 1 h, it was taken out and placed in the maintenance room for standard maintenance. After 7 days, it was taken out for electrochemical corrosion performance test.

2.2. Test Performed on the Samples

2.2.1. SEM Analysis

SEM observation is mainly used from the perspective of morphology to analyze the fiber distribution characteristics in coating materials, as well as the morphology of cement hydration products. Based on the morphology analysis, the cement hydration products in the coating materials are preliminarily judged, and their potential performance is judged based on the cement hydration products. A sample of about 0.5 cm³ was cut from the sample of cement-based protective coating material to be tested using a cutting machine and placed in a vacuum drying oven at a temperature of 45 °C for 24 h. Subsequently, the morphology of mortar samples was analyzed by ZEISS Sigma 360 scanning electron microscope. Before observation, the sample is sprayed with gold to enhance its electrical conductivity, which facilitates scanning electron microscope observation.

2.2.2. XRD Analysis

XRD analysis can identify the phases in coating materials, and combines SEM observation to further clarify the substances in the coating materials, especially the substances of cement hydration products. XRD phase analysis was performed using a Bruker (Berlin, Germany) D2 Phaser X-ray diffractometer. The X-ray spectrum of the instrument is generated by a Cu target. The working voltage and current are 350 kV and 10 mA, respectively. When collecting data, the step size is 0.02°, the scanning rate is 2°/min, and the scanning angle range is 5~90°.

2.2.3. X-CT Analysis

X-CT can be used to observe the bonding quality of coating/mortar and analyze its interface characteristics. More importantly, it can analyze the pore structure characteristics of the mortar matrix and coating materials, and quantitatively study their porosity, providing data support for the analysis of impermeability and resistance to chloride ion transport performance. X-CT microscopic imaging analysis of the samples was performed using Xradia 515 Versa 3D X-ray microscope (Zeiss, Oberkochen, Germany) and NanoVoxel-3000 3D X-ray microscope (Sanying, Suzhou, China). The NanoVoxel-3000 3D X-CT equipment is mainly used to observe the microstructure characteristics such as pore structure and fiber distribution of cement-based protective coating materials. The acceleration voltage is set to 150 kV, the current is 200 μA, and the spatial resolution is 5.6 μm. Xradia 515 Versa 3D X-CT was used to observe the reinforced mortar samples covered with cement-based protective coatings, focusing on the morphology of the coatings and the characteristics of the bonding interface with the mortar layer. The accelerating voltage is set to 80 kV and the power is 7 W.

2.2.4. Impermeability Test of Cement-Based Protective Coating Material

The test of impermeability of concrete is based on the “Chinese Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete” (GB/T 50082-2009) [35]. The specimen is a cylindrical specimen of Φ175 mm × Φ185 mm × 150 mm. The seepage height of the specimen should be calculated following equations:

$${\mathrm{h}}_{\mathrm{i}}={\displaystyle \frac{1}{10}}\sum _{\mathrm{j}=1}^{10}{\mathrm{h}}_{\mathrm{j}}$$

(1)

where h_j is the water penetration height at the j-th measuring point of the i-th sample, and h_i is the average water penetration height for the i-th sample. For a group of six specimens, the average penetration height ($\overline{\mathrm{h}}$) was calculated as:

$$\overline{\mathrm{h}}={\displaystyle \frac{1}{6}}\sum _{\mathrm{i}=1}^6{\mathrm{h}}_{\mathrm{i}}$$

(2)

The arithmetic mean value of the seepage height of a group of 6 specimens should be used as the measured value of the seepage height of the group of specimens.

2.2.5. Electrochemical Test

The 3.5% sodium chloride solution was selected as the corrosion medium, and the CHI 660E electrochemical workstation (Chenhua, Shanghai, China) and three-electrode test system (including reference electrode (SCE), stainless steel mesh auxiliary electrode and working electrode (WE) were used to test the mortar-rebar electrochemical samples. The test items include open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning polarization test (PDP).

The OCP test is divided into two stages: in the initial stage of corrosion, the open circuit potential changes within 10 h are continuously tested to obtain a continuous OCP evolution curve; in the subsequent long-term test, a phased test was performed to record its OCP value. The EIS test was carried out in stages during the whole process of immersion corrosion. The open circuit potential was used as the initial potential, a disturbance voltage of ±10 mV was applied, and the test frequency range was set to 0.01~10,000 Hz. Potentiodynamic polarization scanning (PDP) was carried out only after the long-period corrosion test. The initial voltage was set to 0.25 V negative shift in the open circuit potential value, the end voltage was 1 V, and the test rate was controlled at 1 mV/s.

3. Results and Discussion

3.1. Analysis of SEM Microstructure Characteristics of Cement-Based Protective Coating Materials

Figure 2a–f show the SEM microstructure characteristics of different types of protective coating materials, including commercial coating materials (Figure 2a,b), fiber-toughened coating material (Figure 2c,d), and double-modified coating material (Figure 2e,f). It can be seen from the high-rate SEM images that there are fibrous C-S-H gel hydration products in these coating materials. As can be seen from Figure 2a,b, the surface of the commercial coating material is relatively uniform, but there are certain pores and small cracks, which may be due to the volume shrinkage or external stress during the hydration process, affecting the compactness and protective performance of the coating. In Figure 2c,d, the coating material with fibers showed a good combination of fibers and C-S-H products, and the uniform distribution of fibers in the material effectively improved the pore structure. It is generally believed that fibers can reflect the bridging effect in cement-based materials, effectively prevent crack propagation, and improve the durability and impermeability of the coating. The morphology in Figure 2e,f further proves the synergistic effect of double-modified coating material with fibers and high-standard cement. The C-S-H products in the composite system not only cover the surface of the coating, but also are closely combined with the fibers, showing a denser microstructure and significantly reduced porosity. Overall, the double modification of fiber-toughening and substrate-enhancing not only optimizes the morphology and distribution of C-S-H hydration products of cement-based protective materials, but also significantly improves the microstructure integrity of concrete.

3.2. XRD Phase Analysis of Cement-Based Protective Coating Materials

Figure 3 shows the XRD patterns of commercial coating materials and double-modified coating materials. There are significant differences in the type, quantity and crystallinity of hydration products between the two. By observing the XRD patterns, it can be found that the hydration products of commercial coating materials mainly include Ca(OH)₂, C-S-H, a small amount of 3CaO·SiO₂ and ettringite (AFt). The diffraction peak of Ca(OH)₂ is more obvious, especially at the position of 2θ about 18° and 34.1°, indicating that the hydration reaction of the coating material has not been fully carried out, and Ca(OH)₂ occupies a large proportion in the material. Different from commercial coating materials, double-modified coating materials showed more C-S-H and less Ca(OH)₂ in XRD patterns. In addition, the diffraction peak of ettringite also has a certain enhancement.

3.3. Impermeability Test of Cement-Based Protective Coating Materials

In order to evaluate the early impermeability of cement-based protective coating materials, the impermeability of three protective coating materials after 7 days of curing was tested, respectively, and compared with the impermeability of 28-day standard curing mortar samples without protective treatment. The results are shown in Figure 4. It can be seen from the figure that the impermeability of the untreated mortar sample is poor, which is mainly attributed to the high connectivity porosity inside the mortar, which makes it easy for water to penetrate into the mortar. Compared with standard mortar samples, the early impermeability of commercial coating materials is slightly better than that of standard mortar samples, but the enhancement effect of impermeability is not significant. After adding a certain amount of polypropylene fiber to the commercial cement-based protective material, the anti-permeability effect of the sample was improved. It can be deduced that the addition of fibers can effectively toughen the coating, inhibit the occurrence and propagation of cracks in the coating, and improve the density and impermeability of the coating. In addition, on the basis of fiber toughening, P·II 52.5R Portland cement was used to further enhance and modify the coating substrate, and the density of the coating was improved. Under the synergistic effect of fiber-toughening and high standard cement densification, the impermeability of the double-modified coating material is further improved, which is about 2.8 times higher than that of the untreated mortar sample, indicating that fiber toughening and substrate enhancing are effective methods to enhance the impermeability of cement-based protective coating materials.

3.4. Evaluation of Corrosion Protection Performance of Cement-Based Coating Materials Based on Electrochemical Corrosion Test

The overall corrosion resistance of mortar rebar samples treated with different protective coatings was studied by electrochemical corrosion test technology, and the chloride ion corrosion resistance of cement-based protective coating materials in chloride environment was systematically evaluated. Firstly, X-CT technology was used to observe the mortar rebar samples treated with fiber toughening and high standard cement reinforced double-modified coating, and the image was shown in Figure 5. It can be seen from the figure that the mortar matrix contains more pore structure. The protective coating is evenly distributed with a thickness of about 2 mm. At the same observation rate, the coating has no obvious pores, which is significantly better than the compactness of the mortar matrix. It has to be noted that, due to the thin thickness of the coating, the interface bonding strength between coating and mortar has not been tested yet. From X-CT observation, the coating is tightly coated on the surface of the mortar matrix and there is no cracking and peeling can be observed at the coating/matrix interface, indicating a good bonding of the coating is obtained.

It has to be noted that, due to the different shrinkage rates and elastic moduli of the two types of cement in the coating and matrix, shear stress may occur at the interface after hardening, leading to potential problems such as delamination or microcracks. In the future study, we will take the quantitative analysis of the interface bonding between coatings/matrix as an important research object.

Firstly, the open circuit potential (OCP) was used to preliminarily judge the corrosion tendency of mortar reinforcement samples under the action of cement-based protective materials. In general, when the OCP value of rebar drops below −350 mV, the corrosion probability will rise sharply. Figure 6 shows the open circuit potential curves of the original and treated reinforced mortar samples in 3.5% NaCl solution during the short-period (10 h) and long-period (10 d) corrosion test, respectively. Among them, the OCP value of the original mortar rebar sample dropped sharply to below −200 mV in a short time, indicating that the original mortar rebar sample without any coating protection had poor resistance to chloride ion penetration. The OCP values of mortar rebar samples treated with commercial coating material, fiber-toughened coating material, and double-modified coating material were maintained at a high level (above −150 mV) in a short period of time. After fiber-toughened coating treatment, the OCP value of mortar rebar samples was higher than that of commercial coating coated samples. The OCP value of the mortar rebar sample treated with the double-modified coating was further improved, and it was above −100 mV in the 10 h immersion cycle. After one day of immersion corrosion, the OCP value of the original mortar rebar sample decreased to below −600 mV, indicating that the rebar had been corroded. The OCP values of samples treated with different coatings decreased to a certain extent with the prolongation of immersion time. Among them, the OCP value of samples treated with commercial coatings decreased to below −350 mV after 4 days. During the whole test cycle, the OCP values of samples treated with different coatings were in the order of commercial coating ≤ fiber-toughened coating ≤ double-modified coating. The OCP values of fiber-toughened coating coated sample and double-modified coating coated sample remained above −300 mV after 10 days.

The electrochemical impedance spectra of mortar rebar samples treated with different protective coatings after soaking in 3.5% NaCl solution for 10 h and 10 day are shown in Figure 7. After a short time of immersion (10 h) (Figure 7a), the untreated mortar rebar sample has the smallest capacitive arc radius. After the coating treatment by the cement-based protective coating material, the capacitive arc radius of the mortar rebar sample is significantly increased compared with the untreated sample, but the contrast difference between the three coating treated samples is not obvious. The Bode phase angle diagram of the mortar rebar sample after soaking for 10 h is shown in Figure 7b, and the phase angle has only one peak height in the middle and low frequency range. In the low frequency region, the untreated original mortar rebar sample has the smallest phase angle peak. The phase angle peak of the mortar rebar samples treated with different treatments is consistent with the radius of the capacitive arc in the Nyquist diagram, and the phase angle peak of the three coating treated samples is not obvious. With the extension of soaking time (10 days), the capacitive arc radius and phase angle peak of the impedance spectrum of the mortar rebar samples treated with three different coatings decreased, and the capacitive arc radius and phase angle peak of the double-modified coating coated samples remained at a high level, indicating that the cement-based protective coating modified by fiber toughening and high-standard cement reinforcement can provide a longer period of stable corrosion protection.

The capacitance arc in EIS mainly involves the capacitance arc of mortar and protective coating, and rebar, reflecting the transmission characteristics of electrolytes in mortar and the charge transfer process on the surface of rebar. After the coating material is applied to the mortar substrate, due to the high density of the coating material, the electrolyte transport process in the mortar is suppressed, thus improving the overall capacitance arc of the coating material and the mortar. At the same time, due to the dense coating effectively suppressing the transmission of corrosive media such as chloride ions, the chloride ion concentration at the interface of rebar/mortar is greatly reduced, and the passivation performance of rebar is effectively guaranteed, thereby enabling the rebar to maintain a large capacitance arc. Thus, larger capacitance arcs indicates that the mortar substrate protected by the coating has better corrosion medium transmission inhibition characteristics, and better passivation layer performance was maintained on the rebar surface, thereby providing longer-term corrosion protection.

The R(Q(R(QR)))W equivalent circuit is used to fit the impedance spectra of the samples soaked for different times. The resistance after fitting is shown in Figure 8. The R in the circuit is solution resistance, mortar resistance (R_motar) and rebar resistance (R_rebar) from front to back. It can be seen from the figure that the R_motar and R_rebar of all samples decreased with the extension of soaking time. The order of R_motar and R_rebar of samples treated with different coatings was as follows: commercial coating ≤ fiber- toughened coating ≤ double-modified coating. The R_motar and R_rebar of samples treated with double-modified coating had the minimum decrease rate with the extension of soaking time, indicating that the fiber toughened and high standard cement modified coating effectively improved the resistance to chloride ion intrusion of mortar rebar samples. It can provide better corrosion protection for mortar rebar samples.

The electrochemical polarization curves of different samples immersed in 3.5% NaCl for 10 days are shown in Figure 9. The relevant Tafel polarization parameters, including corrosion potential (E_corr), corrosion current density (i_corr), etc., were derived by the Tafel extrapolation method and summarized in

Table 5. Electrochemical data obtained from the PDP curves of the mortar-rebar samples.

Samples	Ecorr (VSCE)	icorr (A·cm−2)	Epit (VSCE)
as-received	−0.620	6.11 × 10−6	/
Commercial coating	−0.382	4.06 × 10−6	0.35
Fiber-toughened coating	−0.313	1.32 × 10−6	0.81
Double-modified coating	−0.229	8.60 × 10−7	0.82

. After the coating protection treatment, the E_corr of the mortar rebar sample was improved, and the reinforced mortar rebar sample after the double-modified coating treatment had the largest self-corrosion potential (from −620 mV to −229 mV), indicating that the sample had the smallest corrosion tendency. After coating protection treatment, the corrosion current density of the sample also decreased. Compared with the untreated sample, the i_corr of the sample treated by double-modified coating decreased by an order of magnitude (from 6.11 × 10⁻⁶ A·cm⁻² to 8.60 × 10⁻⁷ A·cm⁻²). In the anodic polarization region, the passivation zone appeared in the coated samples, indicating that the coating protection improved the resistance to chloride ion penetration of mortar rebar samples.

3.5. Discussion on Chlorine Salt Corrosion Protection Mechanism of Double-Modified Cement-Based Protective Coating Materials

In order to explore the anti-chlorine salt corrosion protection mechanism of different coating materials on mortar matrix, the pore structure characteristics of mortar matrix and three protective coating materials were studied by X-CT technology. Firstly, the pore structure of mortar matrix was investigated. The results of X-CT tomography analysis are shown in Figure 10, where Figure 10a is its three-dimensional overall morphology, and Figure 10b is the three-dimensional distribution of porosity. It can be seen from the figure that a large number of pores of different sizes are distributed in the mortar matrix, and the overall porosity is about 12.14%. In addition, cracks of a certain size were also observed inside the mortar. A large number of pore structures and some cracks in the mortar provide expansion channels for the penetration of corrosive media such as water and chloride ions into the mortar/rebar interface, which leads to the corrosion of rebar.

The results of X-CT tomography analysis of commercial coating materials are shown in Figure 11. Figure 11a is its three-dimensional overall morphology, Figure 11b is the three-dimensional distribution of porosity, and Figure 11c is its representative cross-section morphology. It can be seen from the figure that there are also many pores in the commercial coating, but its overall porosity is lower than that of the mortar matrix, which is about 10.196%. This is also consistent with the test results of the impermeability of the sample, which successfully explains the impermeability of the commercial coating material. It can be seen from Figure 11c that in addition to the matrix composed of cement hydration products in commercial coating materials, there are also a certain amount of quartz sand with different shapes, and pore structures with different sizes are also distributed.

The X-CT tomography analysis results of coating materials modified by fiber toughening and fiber/high standard cement double modification are shown in Figure 12. It can be clearly seen from Figure 12(a3,b3) that the fibers are evenly distributed in the coating material, and the proportion is about 0.13%–0.15%. The fibers are relatively complete and can play a bridging role in the coating to prevent crack propagation. It can be seen from Figure 12(a2) that the porosity of the coating after fiber toughening is further reduced compared with the untoughened coating, reaching 9.889%. After further modification by high standard cement, the porosity in the coating is significantly reduced (Figure 12(b2)), which is greatly reduced from 10.196% of the commercial coating material to 2.123%. In addition, as shown in Figure 12(b4), with the addition of high-standard cement, the cross-sectional morphology of the double-modified coating material appears denser, and the pore structure is significantly reduced. In addition, the proportion of matrix composed of cement hydration products increased significantly, while the proportion of quartz sand decreased. It can be seen that the synergistic effect of fiber toughening and high standard cement modification is an effective method to reduce the porosity of the coating and improve the density and impermeability of the coating [36]. The X-CT scan results are consistent with the results of the sample impermeability test and the electrochemical corrosion performance test, and can successfully explain the enhanced resistance to chloride ion corrosion protection of the double-modified coating.

Figure 13 shows the X-CT scanning results of pore structure and distribution of different sizes in different coating samples, and

Table 6. Ratio of pore volume of different coatings obtained from the X-CT scanning results.

	<100 μm	100 μm~200 μm	200 μm~1 mm	>1 mm
Commercial coating material (%)	0.602	3.066	6.393	0.135
Fiber-toughened coating material (%)	0.626	2.938	6.217	0.108
Double-modified coating material (%)	0.282	0.536	1.243	0.0621

lists the ratio of pore volume of different coatings obtained from the X-CT scanning. It can be seen from the figure and table that in the commercial coating (Figure 13(a1–a4)), the pores with sizes smaller than 100 μm, between 100 μm and 200 μm, between 200 μm and 1 mm, and larger than 1 mm account for about 0.602%, 3.066%, 6.393% and 0.135%, respectively. After fiber toughening (Figure 13(b1–b4)), the porosity of pores with different sizes is 0.626%, 2.938%, 6.217% and 0.108%, respectively, and the overall porosity is slightly lower than that of the commercial coating. After the double modification by fiber toughening and high standard cement (Figure 13(c1–c4)), the porosity of pores with different sizes decreased significantly, which were 0.282%, 0.536%, 1.243% and 0.0621%, respectively, which were 53%, 82%, 80% and 54% lower than those of commercial coatings.

Based on the analysis of microstructure characteristics and the performance test of comprehensive protective coating materials, the corrosion protection mechanism of different cement-based protective coatings against chloride ion intrusion corrosion is further discussed. The schematic diagram of the corrosion protection mechanism is shown in Figure 14. As shown in Figure 14a, there is a significant pore structure in the matrix composed of cement hydration products without any treatment. These pores vary in size and coexist in isolation and penetration. In addition, some cracks are inevitably distributed in the mortar. Erosion media such as water and chloride ions can penetrate and migrate from the surface to the inside through the pore structure of the mortar matrix, and can diffuse rapidly with the help of some cracks, and finally reach the rebar/mortar interface at a faster rate. After the chloride ion is adsorbed on the surface of the rebar and accumulates to a certain amount, it causes the rebar to depassivate and cause corrosion. The expansion stress caused by the volume expansion of the corrosion products of rebar will lead to the cracking of the mortar protective layer and the formation of penetrating cracks, which provides a direct fast channel for the expansion of the corrosive medium, and then accelerates the corrosion [37,38].

After the commercial coating material protection, the resistance to chloride ion penetration of mortar rebar samples in a short period of time is significantly improved. However, due to the fact that the pore structure of the commercial coating material has not been significantly improved, its impermeability has not been significantly improved compared with the mortar matrix, and the diffusion channels of corrosive media such as water and chloride ions have not been effectively blocked, which cannot provide long-term stable chloride ion erosion protection (as shown in Figure 14b).

After fiber toughening, fiber can greatly prevent the formation and development of microcracks in cement-based protective coating materials, and greatly reduce the number and size of cracks in the coating. At the same time, the fiber can enhance the bonding between the protective coating material and the existing mortar matrix to a certain extent, form a denser coating/mortar interface, and improve the anti-chloride ion corrosion protection ability of the coating material (as shown in Figure 14c).

Fiber toughening cannot efficiently solve the large number of pore structures brought into the commercial cement-based protective coating materials during the hydration process. Based on fiber toughening, fiber-coating is further enhanced by high-standard cement to strengthen cement hydration products and greatly inhibit the generation of micro-scale pores during hydration [39,40]. Under the synergistic effect of fiber toughening inhibiting crack formation and high-standard cement strengthening inhibiting the formation of micro-scale pore structure, it can greatly reduce the defects such as pores and cracks in protective materials, improve the density of protective materials, efficiently block the diffusion and invasion of corrosive media such as water and chloride ions, greatly improve its protective performance, and provide long-term and stable corrosion protection for mortar rebar samples.

4. Conclusions

In this study, a novel double-modified cement-based protective coating material based on fiber toughening and substrate-enhancing strategies was designed and prepared for the threat of chloride corrosion of RCSs in harsh coastal environments and the actual needs of corrosion damage repair and protection of existing concrete structures. The microstructure, impermeability and protection mechanism were studied, and the conclusions are as follows:

(1)

The X-CT scanning analysis shows that the microstructure of the double-modified cement-based protective coating material is denser and the porosity is lower. The porosity is greatly reduced from 10.196% of the commercial cement-based protective coating material to 2.123%. The reduction in porosity can effectively improve the impermeability of the protective coating material, making its impermeability about 2.8 times higher than that of the untreated mortar.

(2)

The corrosion current density of the mortar rebar sample treated with the double modified coating is 8.60 × 10⁻⁷ A·cm⁻², which is about 86% lower than that of the untreated sample (6.11 × 10⁻⁶ A·cm⁻²), which significantly reduces the corrosion rate. This indicates that the double modified cement-based protective coating can more effectively prevent chloride ion penetration, thereby delaying the occurrence of rebar corrosion.

(3)

The introduction of fiber effectively inhibits the formation and propagation of microcracks in cement-based protective coating materials through bridging effect, and significantly improves the toughness of coating materials. At the same time, the enhancement of the substrate realizes the optimal regulation of the cement hydration products of the cement-based protective coating material, which makes the pore structure of the coating material more density. Therefore, the double-modified cement-based protective coating material has excellent resistance to chloride ion penetration, which can provide long-term protection for RCSs in chloride erosion environment.

(4)

Regarding the large-scale application of the new coating materials, especially in some special application scenarios, where coating materials face external loads, even impact or erosion effects, the strength and adhesion of coatings are key properties. The potential lack of coating strength and adhesion can lead to the failure of protection, and further design and regulation are needed for optimization.