# Experimental Study on the Influence of a Cementitious Permeable Crystallization Admixture (CPCA) in Improving Concrete Durability

Authors: Yu-chuan Yang; Hai-bo Li; Xing-guo Yang; Shi-quan Chen; Jia-wen Zhou
Journal: Advances in Civil Engineering
 (2022)

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## Abstract

The corrosion of steel reinforcement in reinforced concrete is a serious problem affecting the durability of concrete structures. And the steel corrosion caused by chloride ion erosion is the most serious damage to reinforced concrete structures, which affects the safety and service life of the structure. In order to prevent steel corrosion caused by chloride, it is necessary to pay more attention to chloride ion penetration damage. At present, the most commonly used method is to use additives in concrete structures, so as to improve the durability of concrete by reducing its permeability coefficient. Cement-based permeable crystalline admixtures can effectively improve the impermeability and mechanical properties of concrete by complex crystallization reactions between reactive chemicals and cement gels. In the paper, the Penetron admix 803 was selected as a typical CPCA in the experimental study, and the rapid chloride migration (RCM) experiments and mechanics experiments were carried out for four concrete structures with different dosages of Penetron admix 803 (PA8). Compared with plain concrete, the concrete structure containing PA8 makes it difficult for water and other liquids to enter by filling micropores and shrinkage cracks by scanning electron microscope test, so its impermeability is greatly improved. And concrete structure containing permeable crystalline admixture can reduce chloride ion permeability effectively. The experimental results showed that with the increase of PA8 content, the diffusion coefficient of the concrete structure decreased rapidly from 45.8 × 10−12 m2/s to 17.7 × 10−12 m2/s, while the service life of concrete structures increased continuously. And the mechanism of the chemical reaction between PA8 and the internal components of concrete structure was revealed, which was mainly due to the catalytic reaction between the reactive chemicals of PA8 and the cement hydrates of concrete structure forming insoluble crystals filled with concrete micropores. Experimental and theoretical results indicate that the permeable crystalline admixture can greatly improve the chloride-related durability and mechanical properties of the concrete structure.


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## Body

## 1. Introduction

With the increasing demand for a more convenient living and working environment, infrastructure and buildings are being built on a large scale all over the world, especially in developing countries. As one of the most popular building materials, reinforced concrete has been widely used, and the performance requirements of concrete are also getting higher and higher [1, 2]. The durability of concrete structures has been studied for a long time. Relevant literature show that the durability of reinforced concrete mainly depends on the surrounding environment and exposure conditions, such as concrete carbonization, steel corrosion, and freeze-thaw damage [3–5]. In particular, it should be pointed out that the main reason for the shortened service life of reinforced concrete structures is due to reinforcement corrosion, and reinforcement corrosion will occur in many reinforced concrete projects such as underground structures, marine structures, highway elevated bridges, and so on, which exposed to chloride ions [6, 7].Previous studies on the mechanism of chloride ion penetration into concrete and the causes of reinforcement corrosion have been carried out, mainly including laboratory experiment, theoretical analysis, and numerical simulation [8–13]. Figure 1 shows the failure mechanism of chloride ion on reinforced concrete structure. The main source of chloride ions causing steel corrosion is the concrete mixing material itself and the external environment infiltration. The chloride ion generated by the concrete itself is mainly caused by the internal composition of cement, aggregate, admixture, and water containing chloride ion. On the other hand, the marine environment and deicing salt can bring external chloride ions into the concrete through its micropores [14]. Relevant research results show that the microstructure of concrete, the concentration of chloride ions in the concrete and the external environment have a great influence on the durability of the concrete structure [9, 11, 15]. For those reasons, an efficient way to eliminate or mitigate the adverse effects of steel corrosion is to use concrete with high impermeability to limit chloride penetration from surrounding environment, including waterproof coating and adding admixtures (e.g., mineral admixtures or cementitious permeable crystallization admixtures) [16–20].Figure 1
The failure mechanism of chloride ion on reinforced concrete structure.Admixtures used in concrete have many purposes, including improving their mechanical properties, impermeability, freeze-thaw durability, and corrosion resistance [16–21]. Mineral admixtures (e.g., coal fly ash, slag powder, and silica fume) are effective to improve the impermeability of concrete, mainly due to the chemical reaction between its active particles and calcium hydroxide in concrete, resulting in hydration products, thereby reducing the porosity of concrete structure. Chen et al. [19] studied the influence of slag as cement replacement in various types of fiber-reinforced concrete on the chloride penetration resistance of the concrete and found that slag had a positive effect on the chloride penetration resistance of the concrete. And Sasanipour et al. [20] revealed nanosilica suspension modification on recycled aggregates can improve the corrosion resistance of recycled aggregate concrete greatly. In addition, Zeng et al. [21] investigated the effects of the surface pretreatment method by soaking recycled concrete aggregates in silica fume slurry on the mechanical and durability properties of recycled aggregate concrete. The results showed that using pretreated recycled aggregates significantly improved the durability properties of mixes, especially chloride ion penetration and electrical resistivity. However, the mineral composition of these materials is difficult to obtain, and the cost of using them in concrete structures is relatively high, which restricts their promotion and application in some areas [22].Compared with mineral admixtures, similar effects can be achieved with small amounts of CPCA, which can effectively control costs. In terms of mechanism, CPCA can be used as an admixture to reduce the permeability of the concrete structure. In particular, CPCA is hydrophilic, which can easily react with water during concrete mixing, compared with many mineral admixtures. When such admixtures are added to the concrete mixing process, a large number of needle-like crystals will be generated inside the concrete structure in the moist curing environment so that the physical and mechanical properties of the concrete will be improved [23]. Some previous studies have found that concrete specimens containing CPCA have reliable performance in forming insoluble substances that can block the microscopic pore/crack inside the concrete so that the concrete will be denser and its internal cracks have self-healing ability [24–27]. Thus, the impermeability of concrete will be effectively improved. In terms of chloride ion corrosion resistance, CPCA can effectively improve the corrosion resistance of reinforced concrete [28]. However, most of the previous studies ignored the influence of CPCA content change on chloride ions diffusion coefficient and lacked direct numerical values to predict the potential service life and durability of concrete structures.The direct method to reveal concrete durability is to calculate the potential service life by simple and clear methods. Based on Fick’s second law, a method was proposed to estimate the service life of concrete considering the chloride binding capacity, time dependence of chloride diffusion coefficient, and environmental conditions [29]. Some two-dimensional (2D) and three-dimensional (3D) models were also developed to simulate chloride diffusion [30–33]. However, with more and more parameters involved in the calculation process, it is obvious that some comprehensive difference equations must be solved by the numerical analysis method [34]. Previous experiments conducted by many scholars mainly focused on revealing the role of CPCA and predicting the service life of concrete structures, rather than simulating the diffusion mechanism of chloride ions [28–33]. Therefore, the one-dimensional (1D) diffusion method can meet the above requirements.Considering the important influence of CPCA on the durability of the concrete structure, it is necessary to carry out targeted experimental research. At present, Penetron 401 (P4), Penetron Plus 501 (PP5), and Penetron admix 803 (PA8) are cementitious permeable crystalline admixtures. In the paper, Penetron admix 803 (PA8) was selected as a typical CPCA in the experimental study. A large number of engineering projects have shown that adding PA8 with a content of at least about 1% of cement consumption in concrete structures can meet its safety requirements, and the cost increase is relatively low at this time [12–18]. Therefore, four kinds of concrete structures with 0% (plain concrete), 0.8%, 1.0%, and 1.2% PA8 were selected to analyze the influence of its content on the chloride diffusion capacity and durability of concrete. During the experiment, all concrete specimens were cured in the standard curing room for 28 d, and then rapid chloride migration (RCM) test was carried out to obtain the basic parameters and determine the diffusion coefficient. Finally, a classical and simple method for estimating the service life was introduced to analyze the improvement effect of the admixture on the durability of concrete.


## 2. Materials and Method

### 2.1. Experimental Materials

Concrete is generally composed of cement, sand, aggregates, water, and some admixtures. Figure2 shows the cement material used for making concrete specimens, which is PO 42.5R silicate cement produced by China United Cement Corporation, with sufficient strength and moderate setting time. Considering its good workability, cement is used in many concrete projects. The cementitious permeable crystallization admixture (CPCA) used in the paper is Penetron admix 803 (PA8), which is a powder material composed of Portland cement, quartz sand, and a variety of active chemical components. And it can be easily incorporated into concrete in the mixing process.Figure 2
The cement material used for making concrete specimens: (a) cement and (b) the cementitious permeable crystallization admixture (CPCA).
(a)(b)In the experiment, continuous graded river sand (particle size of about 0–5 mm) and fine-grained gravel were used to make concrete. In particular, the particle size of gravel can be divided into two groups (5–20 mm group and 20–40 mm group), and each group accounts for 31.5% in concrete. In addition, some calcium hydroxide solution (Ca(OH)2), sodium hydroxide solution (NaOH), and sodium chloride solution (NaCl) were prepared to ensure the rapid chloride migration (RCM) test.


### 2.2. Sample Preparation

The concrete samples used in this paper were composed of typical C30 strength grade concrete and PA8, and reliable performance can be obtained by adjusting the mix proportion of each material. Plain concrete was prepared with ordinary Portland cement with a strength grade of 42.5 MPa, and concrete structures with PA8 contents of 0.8%, 1.0%, and 1.2% were prepared by adding different amounts of PA8.As shown in Table1, cement content of 348 kg/m3 was used for plain concrete, and special concrete samples with 2.784 kg/m3 (0.8%), 3.48 kg/m3 (1.0%), and 4.176 kg/m3 (1.2%) PA8 content were prepared. Water-cement ratio (W/C) was the same in all concrete samples, and the amount of 1 m3 concrete materials was summarized in Table 1.Table 1
Material compositions of the concrete (unit: kg/m3).
MixtureW/CCementSandAggregateAdmixtures5–20 mm20–40 mmPlain0.4434871060560500.8% PA80.443487106056052.7841.0% PA80.443487106056053.4801.2% PA80.443487106056054.176Note. W/C is the ratio between water and cement.Initially, all test concrete materials were made into cuboids with the size of 100 mm × 400 mm × 400 mm. After about 24 hours, they were demoulded. Then, according to the standard test method for mechanical properties of concrete, they were cured under the standard condition of 95% relative humanity and 20±2°C temperature. After 21 days of curing, four cylindrical structures (Φ100  mm × 100  mm) were made for each concrete material, and 50 mm thick samples were cut from each cylinder. These standard concrete samples were marked with the type and continued to be cured in the standard curing room for 28 days (as shown in Figure3).Figure 3
Standard concrete samples with different PA8 contents: (a) plain concrete samples, (b) optimized concrete samples with 0.8% PA8 content, (c) optimized concrete samples with 1.0% PA8 content, and (d) optimized concrete samples with 1.2% PA8 content.
(a)(b)(c)(d)


### 2.3. Experimental Method

For each mix proportion, three good cylindrical samples (diameter Φ = 100 mm, heightH = 50 mm) were chosen for the laboratory test. After samples were removed from the curing room and relative parameters were measured, a vacuum pressure of −0.097 MPa condition was imposed on the dry samples and maintained for 3 h. Then, a period of 1 h vacuum saturation treatment for the sample after adding calcium hydroxide (Ca(OH)2) solution, and a further soaking period of 17 h to making all pores in concrete kept saturated condition. Sodium hydroxide solution (NaOH) with the concentration of 0.3 mol/L and sodium chloride solution (10% NaCl by mass) were poured into the chambers on both sides of concrete samples, where the electrode in the NaOH chamber works as the anode and the electrode in the NaCl chamber acts the cathode. During the RCM test, a continually external voltage and electric current were applied through the concrete samples to simulate chloride ions diffusion from the concrete surface to its internal structures, and the experimental parameters were synchronously recorded by the test machine. The rapid chloride migration test setup was shown in Figure 4. After a specified test time (24 h), concrete samples were taken away from the rubber sleeve and split into two halves. Some silver nitrate (AgNO3) solution with the concentration of 0.1 mol/L was sprayed on the two halves. If some chloride ions penetrate into concrete samples, their profile will appear obvious white, and the chloride ion penetration depth Xd can be measured (as shown in Figure 5). According to the standard test method for the long-term performance and durability of ordinary concrete, the chloride diffusion coefficient can be determined by the following equation [35]:(1)D=0.0239×273+TLU−2tXd−0.0238273+TLXdU−2,where D represents the chloride ion diffusion coefficient, m2/s; U means the absolute value of voltage used, V; T is the average value of initial and final environment temperature of the anode solution, C°; L is the thickness of concrete samples, mm; Xd is the average chloride penetration depth, mm; and t presents the test duration, h. Here, the parameters L and Xd should be accurate to 0.1 mm, while the accuracy of D should be 0.1 × 10−12 m2/s.Figure 4
Setup for rapid chloride migration test (RCM): (a) schematic diagram, (b) apparatus for the test samples, and (c) data acquisition system.
(a)(b)(c)Figure 5
The chloride ion penetration results of the rapid chloride migration (RCM) test: (a) concrete samples with two halves, (b) silver nitrate (AgNO3) solution, (c) result of color appearance for plain concrete samples, (d) result of color appearance for special concrete samples with 0.8% PA8 content, (e) result of color appearance for special concrete samples with 1.0% PA8 content, and (f) result of color appearance for special concrete samples with 1.2% PA8 content.In addition, by means of a scanning electron microscope (SEM), the microstructure and internal pore distribution of concrete samples can be analyzed. Comparing the microstructure of different concrete samples, the difference in internal pore structure can be studied. As soon as the RCM test finished, some concrete samples were prepared for the SEM test to compare the difference in microstructure under different PA8 content.


## 2.1. Experimental Materials

Concrete is generally composed of cement, sand, aggregates, water, and some admixtures. Figure2 shows the cement material used for making concrete specimens, which is PO 42.5R silicate cement produced by China United Cement Corporation, with sufficient strength and moderate setting time. Considering its good workability, cement is used in many concrete projects. The cementitious permeable crystallization admixture (CPCA) used in the paper is Penetron admix 803 (PA8), which is a powder material composed of Portland cement, quartz sand, and a variety of active chemical components. And it can be easily incorporated into concrete in the mixing process.Figure 2
The cement material used for making concrete specimens: (a) cement and (b) the cementitious permeable crystallization admixture (CPCA).
(a)(b)In the experiment, continuous graded river sand (particle size of about 0–5 mm) and fine-grained gravel were used to make concrete. In particular, the particle size of gravel can be divided into two groups (5–20 mm group and 20–40 mm group), and each group accounts for 31.5% in concrete. In addition, some calcium hydroxide solution (Ca(OH)2), sodium hydroxide solution (NaOH), and sodium chloride solution (NaCl) were prepared to ensure the rapid chloride migration (RCM) test.


## 2.2. Sample Preparation

The concrete samples used in this paper were composed of typical C30 strength grade concrete and PA8, and reliable performance can be obtained by adjusting the mix proportion of each material. Plain concrete was prepared with ordinary Portland cement with a strength grade of 42.5 MPa, and concrete structures with PA8 contents of 0.8%, 1.0%, and 1.2% were prepared by adding different amounts of PA8.As shown in Table1, cement content of 348 kg/m3 was used for plain concrete, and special concrete samples with 2.784 kg/m3 (0.8%), 3.48 kg/m3 (1.0%), and 4.176 kg/m3 (1.2%) PA8 content were prepared. Water-cement ratio (W/C) was the same in all concrete samples, and the amount of 1 m3 concrete materials was summarized in Table 1.Table 1
Material compositions of the concrete (unit: kg/m3).
MixtureW/CCementSandAggregateAdmixtures5–20 mm20–40 mmPlain0.4434871060560500.8% PA80.443487106056052.7841.0% PA80.443487106056053.4801.2% PA80.443487106056054.176Note. W/C is the ratio between water and cement.Initially, all test concrete materials were made into cuboids with the size of 100 mm × 400 mm × 400 mm. After about 24 hours, they were demoulded. Then, according to the standard test method for mechanical properties of concrete, they were cured under the standard condition of 95% relative humanity and 20±2°C temperature. After 21 days of curing, four cylindrical structures (Φ100  mm × 100  mm) were made for each concrete material, and 50 mm thick samples were cut from each cylinder. These standard concrete samples were marked with the type and continued to be cured in the standard curing room for 28 days (as shown in Figure3).Figure 3
Standard concrete samples with different PA8 contents: (a) plain concrete samples, (b) optimized concrete samples with 0.8% PA8 content, (c) optimized concrete samples with 1.0% PA8 content, and (d) optimized concrete samples with 1.2% PA8 content.
(a)(b)(c)(d)


## 2.3. Experimental Method

For each mix proportion, three good cylindrical samples (diameter Φ = 100 mm, heightH = 50 mm) were chosen for the laboratory test. After samples were removed from the curing room and relative parameters were measured, a vacuum pressure of −0.097 MPa condition was imposed on the dry samples and maintained for 3 h. Then, a period of 1 h vacuum saturation treatment for the sample after adding calcium hydroxide (Ca(OH)2) solution, and a further soaking period of 17 h to making all pores in concrete kept saturated condition. Sodium hydroxide solution (NaOH) with the concentration of 0.3 mol/L and sodium chloride solution (10% NaCl by mass) were poured into the chambers on both sides of concrete samples, where the electrode in the NaOH chamber works as the anode and the electrode in the NaCl chamber acts the cathode. During the RCM test, a continually external voltage and electric current were applied through the concrete samples to simulate chloride ions diffusion from the concrete surface to its internal structures, and the experimental parameters were synchronously recorded by the test machine. The rapid chloride migration test setup was shown in Figure 4. After a specified test time (24 h), concrete samples were taken away from the rubber sleeve and split into two halves. Some silver nitrate (AgNO3) solution with the concentration of 0.1 mol/L was sprayed on the two halves. If some chloride ions penetrate into concrete samples, their profile will appear obvious white, and the chloride ion penetration depth Xd can be measured (as shown in Figure 5). According to the standard test method for the long-term performance and durability of ordinary concrete, the chloride diffusion coefficient can be determined by the following equation [35]:(1)D=0.0239×273+TLU−2tXd−0.0238273+TLXdU−2,where D represents the chloride ion diffusion coefficient, m2/s; U means the absolute value of voltage used, V; T is the average value of initial and final environment temperature of the anode solution, C°; L is the thickness of concrete samples, mm; Xd is the average chloride penetration depth, mm; and t presents the test duration, h. Here, the parameters L and Xd should be accurate to 0.1 mm, while the accuracy of D should be 0.1 × 10−12 m2/s.Figure 4
Setup for rapid chloride migration test (RCM): (a) schematic diagram, (b) apparatus for the test samples, and (c) data acquisition system.
(a)(b)(c)Figure 5
The chloride ion penetration results of the rapid chloride migration (RCM) test: (a) concrete samples with two halves, (b) silver nitrate (AgNO3) solution, (c) result of color appearance for plain concrete samples, (d) result of color appearance for special concrete samples with 0.8% PA8 content, (e) result of color appearance for special concrete samples with 1.0% PA8 content, and (f) result of color appearance for special concrete samples with 1.2% PA8 content.In addition, by means of a scanning electron microscope (SEM), the microstructure and internal pore distribution of concrete samples can be analyzed. Comparing the microstructure of different concrete samples, the difference in internal pore structure can be studied. As soon as the RCM test finished, some concrete samples were prepared for the SEM test to compare the difference in microstructure under different PA8 content.


## 3. Experimental Results

### 3.1. Chloride Ion Diffusion in Concrete

Chloride ions diffusion coefficient in concrete directly reflects the ability of concrete to resist chloride penetration from external salty environment to internal structure. The larger coefficient value means that concrete may suffer more severe chloride corrosion, and the reinforcement bar will generally expand out of control. On the other hand, the white precipitations on the surface of concrete sprayed with silver nitrate (AgNO3) solution can also indicate the difference between concrete samples. As shown in Figure 5, compared with the concrete samples without silver nitrate solution (AgNO3), almost all plain concrete sections produced white precipitates (AgCl), while others produced white precipitates (AgCl) in some areas. The distribution area of these white precipitates (AgCl) can reflect the permeability of concrete, and the bigger area means lower chloride ions resistance. Combined with (1), the chloride ion diffusion coefficient D can be calculated from the chloride ion penetration depth Xd and other experimental parameters (as shown in Table 2). The experimental results indicate that the dosage of PA8 has a great influence on the chloride diffusion coefficient of concrete structure, especially in high dosages. This can be explained as that when the cementitious permeable crystallization admixture was added, more hydrated crystals were generated, which can better fill the pores in the concrete. In addition, with the increase of the amount of admixture, the resistance to chloride ion sensitivity gradually decreases, which indicates the admixtures can effectively extend the durability of concrete structure so that the steel bars in concrete structures can maintain the initial state as long as possible.Table 2
Experimental results of chloride diffusion coefficient.
GroupNo.L (mm)Xd (mm)T (ºC)U (V)t (h)D (×10−12 m2/s)AverageD (×10−12 m2/s)Plain concrete149.537.817.9515.002436.345.8250.546.717.9515.002446.4349.646.317.9515.002445.20.8% PA8151.530.117.0020.022421.822.5250.530.117.0020.022421.3352.533.017.0020.022424.51.0% PA8152.626.717.4520.072419.220.5252.627.517.4520.072420.1351.930.217.4520.072422.01.2% PA8152.023.317.3518.572418.017.7250.019.117.3518.572414.0350.323.317.3518.572417.4If a mathematic equation was introduced describing the effect of the admixture within a certain range, the exponential functionD = 4.5 × 10−12×e−0.8y (D represents the chloride ion diffusion coefficient and y is the addition ratio of PA8) may be reasonable due to its correlation coefficient R2 = 0.992, as shown in Figure 6. The experimental results reveal that the decrease ratio of D was the largest when 0.8% PA8 was added to concrete samples. According to the calculation results of Table 2, with the increase of PA8 content, the increase of chloride penetration resistance may have a maximum limit, and it is impossible to completely prevent chloride ion penetration. When the addition ration of PA8 reaches a certain degree, the above-proposed equation (D = 4.5 × 10−12 × e−0.8y) cannot represent the added effect.Figure 6
Relationship between the chloride diffusion coefficientD and the addition ratio of PA8.


### 3.2. Microstructure Analysis

With the help of a scanning electron microscope (SEM), the sections of these concrete samples were amplified and analyzed, and the distribution of internal cracks and microscopic pores in the section of concrete samples was observed. As shown in Figure7, the microstructure characteristics of concrete samples with different PA8 content can be clearly compared by scanning electron microscope images under different magnifications. Previous researchers have done some data analysis and pointed out that the chloride diffusion coefficient in large pores could be three times larger than that in small pores, which indicates that the decreases in microscopic pores can effectively improve their ability to resist chloride erosion damage [36–38]. The comparative analysis of Figures 7(a) and 7(d) showed that with the increase of PA8 content in concrete samples, the surface of optimized concrete structure was smoother than plain concrete, and obvious large pores became small pores. Meanwhile, by comparing and analyzing the microscopic images amplified by 20,000 times in Figures 7(b) and 7(c), some fine needle-like crystals were formed in the concrete samples containing PA8, and dendritic crystals were formed in the pores of concrete due to the addition of PA8. The generated crystals can reduce porosity by sealing pores, which will effectively improve the durability of concrete and prevent the invasion of water and chemicals to avoid damaging the concrete structure. These crystals were likely to be due to the reaction between the admixture and cement hydration products, thus forming substances insoluble in water.Figure 7
Microstructural characteristics of these concrete samples with different addition ration of PA8 (SEM images under different magnifications): (a) plain concrete without PA8, (b) concrete with 0.8% PA8 content, (c) concrete with 1.0% PA8 content, and (d) concrete with 1.2% PA8 content.
(a)(b)(c)(d)In fact, for plain concrete, in the process of mixing cement and concrete aggregate, as the aggregate particles exert shear stress on the cement paste [39], the interfacial transition zone always occurs around the aggregate, which will accelerate the migration of chloride ions, resulting in a large migration rate [40]. According to Figure 7, it can be seen that the acicular crystals generated in the concrete by adding PA8 will fill the internal cracks and micropores in the cement paste and the interfacial transition zone so that the concrete structure has high compactness and uniformity. Therefore, the above analysis shows that the use of CPCA in the concrete mixing process can effectively reduce the porosity of the concrete structure. There are some capillary pores in the concrete structure, which improves the resistance of concrete to chloride ion penetration, and these properties form a favorable internal environment for reinforcement.


## 3.1. Chloride Ion Diffusion in Concrete

Chloride ions diffusion coefficient in concrete directly reflects the ability of concrete to resist chloride penetration from external salty environment to internal structure. The larger coefficient value means that concrete may suffer more severe chloride corrosion, and the reinforcement bar will generally expand out of control. On the other hand, the white precipitations on the surface of concrete sprayed with silver nitrate (AgNO3) solution can also indicate the difference between concrete samples. As shown in Figure 5, compared with the concrete samples without silver nitrate solution (AgNO3), almost all plain concrete sections produced white precipitates (AgCl), while others produced white precipitates (AgCl) in some areas. The distribution area of these white precipitates (AgCl) can reflect the permeability of concrete, and the bigger area means lower chloride ions resistance. Combined with (1), the chloride ion diffusion coefficient D can be calculated from the chloride ion penetration depth Xd and other experimental parameters (as shown in Table 2). The experimental results indicate that the dosage of PA8 has a great influence on the chloride diffusion coefficient of concrete structure, especially in high dosages. This can be explained as that when the cementitious permeable crystallization admixture was added, more hydrated crystals were generated, which can better fill the pores in the concrete. In addition, with the increase of the amount of admixture, the resistance to chloride ion sensitivity gradually decreases, which indicates the admixtures can effectively extend the durability of concrete structure so that the steel bars in concrete structures can maintain the initial state as long as possible.Table 2
Experimental results of chloride diffusion coefficient.
GroupNo.L (mm)Xd (mm)T (ºC)U (V)t (h)D (×10−12 m2/s)AverageD (×10−12 m2/s)Plain concrete149.537.817.9515.002436.345.8250.546.717.9515.002446.4349.646.317.9515.002445.20.8% PA8151.530.117.0020.022421.822.5250.530.117.0020.022421.3352.533.017.0020.022424.51.0% PA8152.626.717.4520.072419.220.5252.627.517.4520.072420.1351.930.217.4520.072422.01.2% PA8152.023.317.3518.572418.017.7250.019.117.3518.572414.0350.323.317.3518.572417.4If a mathematic equation was introduced describing the effect of the admixture within a certain range, the exponential functionD = 4.5 × 10−12×e−0.8y (D represents the chloride ion diffusion coefficient and y is the addition ratio of PA8) may be reasonable due to its correlation coefficient R2 = 0.992, as shown in Figure 6. The experimental results reveal that the decrease ratio of D was the largest when 0.8% PA8 was added to concrete samples. According to the calculation results of Table 2, with the increase of PA8 content, the increase of chloride penetration resistance may have a maximum limit, and it is impossible to completely prevent chloride ion penetration. When the addition ration of PA8 reaches a certain degree, the above-proposed equation (D = 4.5 × 10−12 × e−0.8y) cannot represent the added effect.Figure 6
Relationship between the chloride diffusion coefficientD and the addition ratio of PA8.


## 3.2. Microstructure Analysis

With the help of a scanning electron microscope (SEM), the sections of these concrete samples were amplified and analyzed, and the distribution of internal cracks and microscopic pores in the section of concrete samples was observed. As shown in Figure7, the microstructure characteristics of concrete samples with different PA8 content can be clearly compared by scanning electron microscope images under different magnifications. Previous researchers have done some data analysis and pointed out that the chloride diffusion coefficient in large pores could be three times larger than that in small pores, which indicates that the decreases in microscopic pores can effectively improve their ability to resist chloride erosion damage [36–38]. The comparative analysis of Figures 7(a) and 7(d) showed that with the increase of PA8 content in concrete samples, the surface of optimized concrete structure was smoother than plain concrete, and obvious large pores became small pores. Meanwhile, by comparing and analyzing the microscopic images amplified by 20,000 times in Figures 7(b) and 7(c), some fine needle-like crystals were formed in the concrete samples containing PA8, and dendritic crystals were formed in the pores of concrete due to the addition of PA8. The generated crystals can reduce porosity by sealing pores, which will effectively improve the durability of concrete and prevent the invasion of water and chemicals to avoid damaging the concrete structure. These crystals were likely to be due to the reaction between the admixture and cement hydration products, thus forming substances insoluble in water.Figure 7
Microstructural characteristics of these concrete samples with different addition ration of PA8 (SEM images under different magnifications): (a) plain concrete without PA8, (b) concrete with 0.8% PA8 content, (c) concrete with 1.0% PA8 content, and (d) concrete with 1.2% PA8 content.
(a)(b)(c)(d)In fact, for plain concrete, in the process of mixing cement and concrete aggregate, as the aggregate particles exert shear stress on the cement paste [39], the interfacial transition zone always occurs around the aggregate, which will accelerate the migration of chloride ions, resulting in a large migration rate [40]. According to Figure 7, it can be seen that the acicular crystals generated in the concrete by adding PA8 will fill the internal cracks and micropores in the cement paste and the interfacial transition zone so that the concrete structure has high compactness and uniformity. Therefore, the above analysis shows that the use of CPCA in the concrete mixing process can effectively reduce the porosity of the concrete structure. There are some capillary pores in the concrete structure, which improves the resistance of concrete to chloride ion penetration, and these properties form a favorable internal environment for reinforcement.


## 4. Durability Evaluation and Discussion

### 4.1. Durability Assessment

The corrosion of concrete structures is generally divided into three stages: induction period, progressive stage, and expiration period [40]. In fact, once chloride ion penetrates into concrete structures until the content reaches the critical concentration, the steel reinforcement begins to rust, resulting in a sharp expansion, and the concrete structure cracking will develop rapidly. Therefore, it is appropriate to use the duration of the above first stage to represent the service life of reinforced concrete structures [35]; thus, the service life time t can be described as follows:(2)t=TCx,t≤Ccr,where x is the concrete cover depth and Ccr is the critical value of chloride ion concentration at the beginning of reinforcement corrosion.According to the above RCM test and SEM image results, the cementitious permeable crystallization admixture (CPCA) plays an important role in reducing the corrosion risk of concrete structures, but the direct value of their service life is not given. Considering Fick’s second diffusion law and the boundary condition of constant chloride content, the variation of chloride concentrationC(x,t) with the chloride penetration depth x and time t can be determined as follows [29]:(3)Cx,t=C0+Cs−C01−erfx2Dt,where Cs represents the chloride concentration in an exposed environment of the concrete surface, C0 represents the initial chloride concentration in concrete, D is the chloride diffusion coefficient of concrete, and erf is the error function. In addition, Cs and D are assumed to be constants. However, the traditional Fick’s second diffusion law ignores the influence of concrete on chloride binding capacity and concrete structure defects, which may make calculated results deviate from the actual condition. Combining the pre-existing theories, one-dimensional diffusion equations can be derived, which can be used to calculate the service life directly [29, 35] as follows:(4)C=C0+Cs−C01−erfx2KDt0m/1+R1−m×t1−m,(5)t=1+R1−mx24KDt0merf−1Cs−Ccr/Cs−C021/1−m,where D is the chloride diffusion coefficient of concrete at hydration time t0, m is an experimental constant, t0 refers to the hydration time, K is a deterioration effect coefficient of chloride diffusion performance of concrete, and R is a coefficient that represents the chloride ion binding capacity of concrete.In practical engineering, the initial chloride contentC0 can be limited to 0.0125% for reinforcement concrete members in a marine environment [35]. When the critical chloride content Ccr and surface chloride concentration Cs are reached, the value of Ccr can be 0.05%, and the value of m is 0.64 [40–42], [43]. According to (5), combined with the commonly used coefficient R and K, the service life of the above four groups of concrete samples can be revealed. Finally, the influence of PA8 content (which reflects in the diffusion coefficient D) and the concrete cover depth x on the durability of concrete was obtained.According to experience, for plain concrete, the values of coefficientsR and K are 7 and 1, respectively; for the concrete with PA8, the values of R and K are 10 and 2, respectively. Therefore, combined with the values of all parameters, the durability of the above four groups of concrete samples can be calculated according to (5). Table 3 shows the calculation results of service life with different CA contents for concrete samples. It can be seen from Table 3 that the service life of plain concrete is 51.29 years, but the service life of concrete with 0.8%, 1.0%, and 1.2% PA8 content increases to about 130 years, 169 years, and 254 years, respectively. Therefore, PA8 can significantly improve the durability of concrete, and its service life increases with the increase of PA8 content. Since the design service life of concrete buildings is generally about 100 years, if PA8 is used to improve the durability of concrete, it is only necessary to add an appropriate amount, and there is no need to increase the service life of concrete too much. With the increase of PA8 content, the cost of concrete engineering will be partially increased. According to engineering experience, the recommended mixing ratio of PA8 is 1%.Table 3
Calculation results of service life with different PA8 contents for concrete samples.
PA8 (%)Ccr (%)mRC0 (%)Cs (%)DRCMKt0 (year)x (cm)Service life0.00.050.6470.01250.5014.4410.0767551.290.80.050.64100.01250.507.1020.07675130.151.00.050.64100.01250.506.4620.07675169.201.20.050.64100.01250.505.5820.07675254.13Figure8 shows the calculation results of the service life of concrete structures with different PA8 contents and Cs values. As shown in Figure 8, the service life of the concrete structure decreases with the increase of Cs value, which means that the increase of chloride ion concentration will reduce the service life of concrete structures. Therefore, it can be seen that the concrete structure in the marine environment is more vulnerable to seawater erosion, thus reducing the service life of the concrete structure, but adding an appropriate amount of PA8 can effectively enhance the durability of the concrete structure.Figure 8
Calculation results for the service life of concrete structures with different PA8 andCs contents.


### 4.2. Discussion

In fact, the corrosive ions in the external environment of concrete structures are not only chloride ions, but other destructive substances (such as sulfate) can also shorten the service life of concrete structures [29, 35, 41, 42], which means that the above analysis results only simulate the impact of chloride ions on the service life of concrete structures. However, with the change of PA8 content, the chloride diffusion coefficient D and service life of concrete samples are in line with the actual situation, that is, the overall trend is accurate.According to the above analysis of SEM images, the addition of PA8 can effectively improve the compactness of concrete internal structure, so it is reasonable to believe that the material may improve the mechanical properties of concrete partially. Therefore, the paper selected plain concrete and concrete samples with recommended dosage (1.0%) for uniaxial tensile strength and uniaxial compressive strength tests (as shown in Figure9).Figure 9
Mechanical strength test of plain concrete and optimized concrete samples: (a) electrohydraulic mechanical press, (b) electrohydraulic servo universal testing machine, and (c) mechanical strength test results (note: the optimized concrete represents 1.0% PA8 concrete structure).
(a)(b)(c)As shown in Figure9, standard cube specimens (150  mm × 150  mm × 150 mm) were used in the uniaxial compressive and tensile strength tests. The loading device used in the compressive strength test is an electrohydraulic mechanical press (Figure 9(a)), which continuously increases the pressure on the upper and lower surfaces of the concrete specimen until the specimen is damaged, so as to obtain its peak load and failure mode. However, the loading device used for the tensile strength test was an electrohydraulic servo universal testing machine (Figure 9(b)), and the tensile strength of concrete was measured by the splitting method. The tensile strength test is mainly based on the characteristics of low tensile strength and large elastic modulus of concrete. The ultimate tensile strain is very small when the specimen is subjected to splitting tensile failure. Therefore, the stress control method was used for loading in the test process. Based on the test results in Figure 9(c), it can be seen that the mechanical properties of concrete samples with the recommended addition amount have been significantly improved. The uniaxial tensile strength increased from 1.40 MPa to 1.70 MPa, which increased by about 21%; the uniaxial compressive strength increased from 28.70 MPa to 30.30 MPa, about 6%.In addition, deriving from the above experimental observations and analyses, the resistance to chloride penetration of concrete with the cementitious permeable crystallization admixture is greatly improved when compared with plain concrete. The main reason for this phenomenon is that there are a large number of active chemical particles in the admixture, which can react with water and cement hydration by-products in concrete. Combined with the analysis in Figure7, this chemical reaction can generate an insoluble crystal in the pores and capillary channels of concrete structure, which will permanently seal microcracks, pores, and capillary, and prevent water or liquid from penetrating in any direction. In short, the potential paths of chloride ion diffusion and water penetration can be blocked, so as to protect concrete from erosion, even in harsh environmental conditions. The chemical reaction can be described as follows:(6)3CaO−SiO2+MxRx+H2O⟶CaSixOxR−H2Ox+MxCaRx−H2Ox.The above complex physical and chemical reaction cycle can be summarized in Figure10. As shown in Figure 10, when the concrete structure is in a dry environment, due to the lack of necessary solution, the admixture will be in a dormant state. If the concrete is exposed to a wet environment or immersed in water, water will activate the admixture to react with calcium silicate and water to generate calcium silicate hydrate insoluble in water, thereby blocking the pores inside the concrete structure.Figure 10
Mechanism of circulation crystalline reaction (between humanity and dry cycle).


## 4.1. Durability Assessment

The corrosion of concrete structures is generally divided into three stages: induction period, progressive stage, and expiration period [40]. In fact, once chloride ion penetrates into concrete structures until the content reaches the critical concentration, the steel reinforcement begins to rust, resulting in a sharp expansion, and the concrete structure cracking will develop rapidly. Therefore, it is appropriate to use the duration of the above first stage to represent the service life of reinforced concrete structures [35]; thus, the service life time t can be described as follows:(2)t=TCx,t≤Ccr,where x is the concrete cover depth and Ccr is the critical value of chloride ion concentration at the beginning of reinforcement corrosion.According to the above RCM test and SEM image results, the cementitious permeable crystallization admixture (CPCA) plays an important role in reducing the corrosion risk of concrete structures, but the direct value of their service life is not given. Considering Fick’s second diffusion law and the boundary condition of constant chloride content, the variation of chloride concentrationC(x,t) with the chloride penetration depth x and time t can be determined as follows [29]:(3)Cx,t=C0+Cs−C01−erfx2Dt,where Cs represents the chloride concentration in an exposed environment of the concrete surface, C0 represents the initial chloride concentration in concrete, D is the chloride diffusion coefficient of concrete, and erf is the error function. In addition, Cs and D are assumed to be constants. However, the traditional Fick’s second diffusion law ignores the influence of concrete on chloride binding capacity and concrete structure defects, which may make calculated results deviate from the actual condition. Combining the pre-existing theories, one-dimensional diffusion equations can be derived, which can be used to calculate the service life directly [29, 35] as follows:(4)C=C0+Cs−C01−erfx2KDt0m/1+R1−m×t1−m,(5)t=1+R1−mx24KDt0merf−1Cs−Ccr/Cs−C021/1−m,where D is the chloride diffusion coefficient of concrete at hydration time t0, m is an experimental constant, t0 refers to the hydration time, K is a deterioration effect coefficient of chloride diffusion performance of concrete, and R is a coefficient that represents the chloride ion binding capacity of concrete.In practical engineering, the initial chloride contentC0 can be limited to 0.0125% for reinforcement concrete members in a marine environment [35]. When the critical chloride content Ccr and surface chloride concentration Cs are reached, the value of Ccr can be 0.05%, and the value of m is 0.64 [40–42], [43]. According to (5), combined with the commonly used coefficient R and K, the service life of the above four groups of concrete samples can be revealed. Finally, the influence of PA8 content (which reflects in the diffusion coefficient D) and the concrete cover depth x on the durability of concrete was obtained.According to experience, for plain concrete, the values of coefficientsR and K are 7 and 1, respectively; for the concrete with PA8, the values of R and K are 10 and 2, respectively. Therefore, combined with the values of all parameters, the durability of the above four groups of concrete samples can be calculated according to (5). Table 3 shows the calculation results of service life with different CA contents for concrete samples. It can be seen from Table 3 that the service life of plain concrete is 51.29 years, but the service life of concrete with 0.8%, 1.0%, and 1.2% PA8 content increases to about 130 years, 169 years, and 254 years, respectively. Therefore, PA8 can significantly improve the durability of concrete, and its service life increases with the increase of PA8 content. Since the design service life of concrete buildings is generally about 100 years, if PA8 is used to improve the durability of concrete, it is only necessary to add an appropriate amount, and there is no need to increase the service life of concrete too much. With the increase of PA8 content, the cost of concrete engineering will be partially increased. According to engineering experience, the recommended mixing ratio of PA8 is 1%.Table 3
Calculation results of service life with different PA8 contents for concrete samples.
PA8 (%)Ccr (%)mRC0 (%)Cs (%)DRCMKt0 (year)x (cm)Service life0.00.050.6470.01250.5014.4410.0767551.290.80.050.64100.01250.507.1020.07675130.151.00.050.64100.01250.506.4620.07675169.201.20.050.64100.01250.505.5820.07675254.13Figure8 shows the calculation results of the service life of concrete structures with different PA8 contents and Cs values. As shown in Figure 8, the service life of the concrete structure decreases with the increase of Cs value, which means that the increase of chloride ion concentration will reduce the service life of concrete structures. Therefore, it can be seen that the concrete structure in the marine environment is more vulnerable to seawater erosion, thus reducing the service life of the concrete structure, but adding an appropriate amount of PA8 can effectively enhance the durability of the concrete structure.Figure 8
Calculation results for the service life of concrete structures with different PA8 andCs contents.


## 4.2. Discussion

In fact, the corrosive ions in the external environment of concrete structures are not only chloride ions, but other destructive substances (such as sulfate) can also shorten the service life of concrete structures [29, 35, 41, 42], which means that the above analysis results only simulate the impact of chloride ions on the service life of concrete structures. However, with the change of PA8 content, the chloride diffusion coefficient D and service life of concrete samples are in line with the actual situation, that is, the overall trend is accurate.According to the above analysis of SEM images, the addition of PA8 can effectively improve the compactness of concrete internal structure, so it is reasonable to believe that the material may improve the mechanical properties of concrete partially. Therefore, the paper selected plain concrete and concrete samples with recommended dosage (1.0%) for uniaxial tensile strength and uniaxial compressive strength tests (as shown in Figure9).Figure 9
Mechanical strength test of plain concrete and optimized concrete samples: (a) electrohydraulic mechanical press, (b) electrohydraulic servo universal testing machine, and (c) mechanical strength test results (note: the optimized concrete represents 1.0% PA8 concrete structure).
(a)(b)(c)As shown in Figure9, standard cube specimens (150  mm × 150  mm × 150 mm) were used in the uniaxial compressive and tensile strength tests. The loading device used in the compressive strength test is an electrohydraulic mechanical press (Figure 9(a)), which continuously increases the pressure on the upper and lower surfaces of the concrete specimen until the specimen is damaged, so as to obtain its peak load and failure mode. However, the loading device used for the tensile strength test was an electrohydraulic servo universal testing machine (Figure 9(b)), and the tensile strength of concrete was measured by the splitting method. The tensile strength test is mainly based on the characteristics of low tensile strength and large elastic modulus of concrete. The ultimate tensile strain is very small when the specimen is subjected to splitting tensile failure. Therefore, the stress control method was used for loading in the test process. Based on the test results in Figure 9(c), it can be seen that the mechanical properties of concrete samples with the recommended addition amount have been significantly improved. The uniaxial tensile strength increased from 1.40 MPa to 1.70 MPa, which increased by about 21%; the uniaxial compressive strength increased from 28.70 MPa to 30.30 MPa, about 6%.In addition, deriving from the above experimental observations and analyses, the resistance to chloride penetration of concrete with the cementitious permeable crystallization admixture is greatly improved when compared with plain concrete. The main reason for this phenomenon is that there are a large number of active chemical particles in the admixture, which can react with water and cement hydration by-products in concrete. Combined with the analysis in Figure7, this chemical reaction can generate an insoluble crystal in the pores and capillary channels of concrete structure, which will permanently seal microcracks, pores, and capillary, and prevent water or liquid from penetrating in any direction. In short, the potential paths of chloride ion diffusion and water penetration can be blocked, so as to protect concrete from erosion, even in harsh environmental conditions. The chemical reaction can be described as follows:(6)3CaO−SiO2+MxRx+H2O⟶CaSixOxR−H2Ox+MxCaRx−H2Ox.The above complex physical and chemical reaction cycle can be summarized in Figure10. As shown in Figure 10, when the concrete structure is in a dry environment, due to the lack of necessary solution, the admixture will be in a dormant state. If the concrete is exposed to a wet environment or immersed in water, water will activate the admixture to react with calcium silicate and water to generate calcium silicate hydrate insoluble in water, thereby blocking the pores inside the concrete structure.Figure 10
Mechanism of circulation crystalline reaction (between humanity and dry cycle).


## 5. Conclusions

In the paper, the chloride ion permeability coefficient of concrete samples with different proportions of admixtures was proposed by rapid chloride migration (RCM) test, and the service life of concrete structures was calculated by the empirical formula. Finally, the influence of the admixture (CPCA) on the durability and some mechanical properties of concrete was obtained. The main conclusions are summarized as follows:(1)
The admixture (CPCA) has a significant effect on improving the chloride resistance of concrete structures. With the increase in admixture content, the chloride diffusion coefficient of concrete samples decreased sharply.(2)
According to the calculation results of concrete service life, the service life of plain concrete is 51.29 years, but the service life of concrete with 0.8%, 1.0%, and 1.2% PA8 content increases to about 130 years, 169 years, and 254 years, respectively. Therefore, the admixture (CPCA) can significantly improve the predicted service life of concrete structures.(3)
From the SEM images, the paper revealed that the pores in the microstructure of optimized concrete become smaller when adding the admixture (CPCA), and the growing acicular crystals can effectively improve the internal tightness of concrete structures.(4)
Moreover, through relevant mechanical experiments, the paper revealed that the admixture can significantly improve the compressive and tensile strength of concrete samples.(5)
Therefore, the application of the admixture (CPCA) can significantly prolong the corrosion resistance of steel bars in concrete, indicating that the use of the material has great potential advantages for special and important engineering structures (such as concrete waterproof structure and marine engineering structure).



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*Source: 4305613-2022-05-29.xml*