1. Introduction
Concrete has been widely used in construction for over 100 years [1,2]. It is often necessary to deploy a structural body that includes a concrete base to the seafloor and recover the concrete base after a period along with other structural body elements. As the concrete cures, it usually creates pores of varying sizes that allow the entry of water and ions from the environment, which can lead to some degree of concrete corrosion [3]. Seawater, in turn, is rich in chlorides and multiple types of electrolytes, rendering it a strong natural corrosive agent. Concrete in the marine environment is damaged by sodium and magnesium chlorides and harmful substances, such as sulfates [4,5,6], which mediate concrete expansion and cracking. The appearance of cracks in the concrete accelerates the rate of deterioration of the load-bearing capacity and reliability of structural elements. This affects the safety, durability, and service life of the structure [7,8,9,10]. The harsh corrosive environment in the ocean induces more serious damage to concrete elements than that observed on land [11,12,13]. Concrete structures, which are recognized for their durability, are often prematurely damaged owing to the lack of durability, thereby demonstrating serious economic and safety problems, which severely restrict the development of the marine industry [14,15].
More than 2000 species of bio-attachment organisms (i.e., fouling organisms) are found in the ocean. These organisms attach to the concrete matrix by secreting mucus, and they produce various chemicals during their metabolic activities. Among these chemicals, acids are very corrosive to concrete, and the biological metabolic acids react with the concrete body to produce ettringite, which leads to the expansion of the concrete volume, rupture, and spalling [16,17]. Furthermore, the attachment of benthic and borehole organisms in the ocean can significantly increase the weight of concrete elements, causing problems such as difficulties in the recycling [18]. The combination of biofouling and bio-corrosion poses a serious threat to the safety of concrete structures in large marine projects [19,20,21].
Anti-biofouling/anti-corrosion techniques have been significantly examined for concrete elements, and more protective methods have subsequently emerged. The commonly used methods include the surface coating method [22,23,24,25,26], the use of corrosion-resistant materials [27,28], the metal surface treatment [29,30], and electrochemical protection methods. However, all these methods have certain shortcomings. In recent times, the pursuit of smart, efficient, durable, and environmentally friendly solutions has garnered significant interest as a novel research strategy for the development of anti-biofouling/anti-corrosion of concrete structures in marine environments [31,32]. Adding admixtures to concrete is an economical, convenient, and proactive method, but it is necessary to find a suitable material for concrete corrosion protection.
Certain inorganic compounds of copper group elements, such as salts of Ag, Cu, Bi, Sb, and Sn, have good anti-biofouling and anti-corrosion properties. These salts are insoluble in water, and the trace cations they emit can kill bacteria and algal microorganisms, thus demonstrating a good biotoxic effect. Copper ions, for example, are highly biotoxic [33], which are very useful in sterilization. Among them, alkaline copper carbonate has a wide range of uses in production activities and is also used extensively as a component of agricultural fungicides. However, there needs to be more research on its application to concrete protection in the marine environment. In addition, polyaniline also has some antibacterial and antifouling properties [34] As an extensively studied corrosion protection agent [35], polyaniline is also a good choice as a coating material for reinforcing steel in concrete [36]. This provides the theoretical basis that polyaniline can be applied in concrete protection in the marine environment. Polyaniline can be prepared using a variety of techniques and is highly available while showing good environmental stability [37,38,39]. The solubility of polyaniline is extremely poor due to its chain rigidity and strong inter-chain interactions. With the action of surfactants, dispersants, anti-flocculants, and other additives, homogeneous water-dispersion systems with anti-biofouling and anti-corrosion functions can be produced. However, polyaniline is usually used as a coating material, and few studies have examined its direct use as an additive in concrete.
In summary, the current research on anti-biofouling and anti-corrosion additives in concrete is inadequate. In addition, most concrete additives with protective characteristics are available in a solid powder formulation. In the mixing process, additives are not often uniformly mixed with the concrete. Hence, it is difficult to ensure the dispersion of additives in the concrete. At the same time, the protection of concrete in the marine environment is required to address seawater corrosion. Less consideration is given to the effect of marine organisms, and the anti-biofouling performance is associated with certain shortcomings. In addition, relatively few actual tests have been conducted in the sea examining the anti-biofouling effect of concrete additives, and even fewer studies have evaluated the effect of anti-biofouling and anti-corrosion additives on the mechanical properties of concrete.
In the article, the polyaniline–alkaline copper carbonate complex is used as an additive for concrete, mainly from the point of view of protection against biofouling and biocorrosion. By preventing the adhesion of fouling organisms in the ocean, the concrete is protected from various chemical substances produced during the metabolic activities of organisms, thus coming to reduce the biochemical corrosion of concrete. At the same time, the compound has the function of antibacterial and anti-fouling. Microorganisms in the ocean produce corrosive substances such as sulfates, which react with the concrete body to produce ettringite, which can cause the concrete to expand in volume. With the protection of the additive, the concrete is less affected by corrosive substances. In addition, the compound contains polyaniline. When reinforcing steel is laid inside the concrete matrix, polyaniline provides good corrosion protection for the reinforcing steel. It protects the concrete from corrosion by harmful substances such as chloride ions. Thus, the bond between the reinforcement and concrete is maintained, cracks are reduced, the mass transmission of corrosive substances is reduced, and the safety of the concrete structure is protected.
This study principally aimed to investigate the effect of polyaniline–alkaline copper carbonate composites on concrete elements and their effectiveness in concrete applications in the marine environment. We also aimed to examine the solution for the preparation of these composites to achieve uniform dispersion and slow-release, long-lasting effects on concrete. By exploring these issues, this study sought to develop a new method of concrete protection in marine environments and analyze the value of this anticorrosive material in the field of concrete protection technology. This study is the first to hypothesize and examine the use of polyaniline–alkaline copper carbonate as an additive in concrete for the anti-biofouling and anti-corrosion in the marine environment. The high dispersion preparation of the composite was performed, and actual experiments of suspension in the sea and avoidance of biological organisms were conducted to test the effect of the composite on the mechanical properties of the concrete. The objectives of the study are listed as follows:
-. Successfully establish a process and method for the preparation of additives for polyaniline–alkaline copper carbonate composites.
-. Confirm that concrete elements with this compound can be used in practical applications in marine engineering. Furthermore, examine whether a concrete additive with good anti-biofouling and anti-corrosion properties can be formed that can effectively extend the service life of existing concrete elements.
-. Determine the optimum amount of this compound for use in concrete and analyze the effects of the compound on the mechanical properties of concrete.
2. Materials and Methods
2.1. Preparation Plan and Characterization Method of Polyaniline–Alkaline Copper Carbonate Composite Materials
The polyaniline–alkaline copper carbonate composites were prepared by a high dispersion method.
Reagents: aniline hydrochloride (C6H5NH2HCl); ammonium persulfate ((NH4)2S2O8); copper sulfate (CuSO4); sodium bicarbonate (NaHCO3); vinyl carbonate (EC, C3H4O3).
Apparatus: electronic analytical balance; 851 magnetic stirrers; SHZ-D(Ⅲ) circulating water vacuum pump; HH-S6-type water bath; electric thermostatic blast drying oven.
Experimental procedure:
(1). A total of 100 mL of aniline hydrochloride solution was mixed at a concentration of 2 mol/L with 500 mL of copper sulfate solution at a concentration of 0.4 mol/L. A total of 6 g (1% of the total weight of the solution) of ethylene carbonate was added to the mixed solution.
(2). A total of 100 mL of ammonium persulfate solution with a concentration of 2 mol/L was dropped in the aforementioned mixture while stirring (drop acceleration of 3 s to 5 s per drop; drop completion duration was approximately 70 min). Continuous stirring with an electromagnetic stirrer for 1 h after the drop was completed to obtain a dark green suspension (the temperature was maintained below 5 °C using an ice bath).
(3). While monitoring the pH, a saturated solution of sodium bicarbonate was added dropwise to the aforementioned dark green suspension while stirring until the pH of the suspension was higher than 8. The stirring was continued for 30 min. The pH of the suspension was confirmed to be higher than 8. In case the pH was not 8, the addition of additional drops of sodium bicarbonate solution was continued (approximately 33.604 g of sodium bicarbonate is required). This leads to the formation of a precipitate of copper alkali carbonate on the surface of the polyaniline, which is dark blue in color.
(4). After the solution is aged for six hours, polyaniline–alkaline copper carbonate composite powder (wet state) was obtained by centrifugation or extraction. The component was repeatedly rinsed to remove soluble impurities to obtain the polyaniline–alkaline copper carbonate composite product.
A total of 0.01 g of the dried composite product was added to a small centrifuge tube with 1–2 mL of water and shaken well to obtain a suspension of the composite. The suspension was evenly coated on a slide and air-dried to obtain a smear, and the contact angle was measured using a contact angle meter.
According to the study titled “A method for determining the dispersibility of insoluble substances in solution” [40], a composite product of mass M1 was added to water and ultrasonically dispersed, and then left to stand, with a total volume of V1. The volume of the upper suspension, which was approximately 70% of the total volume, was recorded as V2. The mass of the insoluble product obtained after drying was recorded as M2, i.e., the average concentration of the poured suspension was C. Comparing this with the concentration of the homogeneous suspension before standing, C0, the ratio of the two concentrations ΔC was obtained, which characterized the dispersibility of the composite product.
A surfactant was used in the preparation process to increase the dispersion of the product, and the role of the surfactant was elucidated based on the settling rate of the alkaline copper carbonate particles in the suspension (Figure 1). The properties of the products derived were compared with the surfactant-free preparation method using two different surfactants, with all other conditions being equal.
2.2. Concrete Specimen Preparation Method
After the polyaniline–alkaline copper carbonate composite was obtained, the composite was applied to the concrete. A batch of concrete specimens was prepared based on the standard test method for the performance of ordinary concrete mixes [41]. A batch of concrete specimens was made at Darch Construction Group Co., Ltd. (Zhoushan, China).
2.2.1. Experimental Setup
The specimens were divided into two groups: the test block group and the test plate group. The former is used for mechanical property testing and observations of the attachment of biological organisms after suspension experiments in the sea, while the latter is used for avoidance of biological organisms in laboratory experiments.
The test block group was set up with two influencing factors, which are listed as follows: The first was the method of using the compound in the specimen and the amount used. The method of using the compound was divided into additive application and coating application. When the compound was used as an additive, the specimens were divided into five control groups according to the amount used. The second was the number of days of suspension in sea experiments (Table 1).
Among them, 36 concrete test blocks need to be submerged in the sea for 30 days, 36 concrete test blocks need to be submerged in the sea for 90 days, and 36 test blocks do not need to be submerged in the sea.
The test plate group set an influencing factor based on the application method and the amount of compound used in the specimen (Table 2).
All 18 concrete test plates were used to test for an accelerated test for bioorganisms.
2.2.2. Specification and Preparation of Specimens
The specimens were prepared according to the specifications of C25 standard concrete with a density of 2356 kg/m3, slump at 60 ± 20 mm. The specific mix design is shown in Table 3. The specifications of the test block were 100 mm × 100 mm × 100 mm, 2.356 kg/block. The specifications of the test plate were 200 mm × 100 mm × 20 mm, 0.9424 kg/pc. Wire mesh sheets were placed inside the test plate to reinforce the test plate and render the test plates difficult to break.
During concrete preparation, the materials, except water and additives, are poured into the mixer in turn and mixed for 10 s. Unhydrated compound additive with water is mixed to obtain the compound suspension, and other admixtures can be added to the mixer simultaneously with the mixing water. Mix the concrete mixture for more than 120 s until it is evenly mixed, pour it out, and then mix it manually on the ground iron plate two times.
After the concrete mix was completed, the test blocks and test plates were molded separately.
The specimens were continued to be maintained for 28 days after solidification and removal from the mold. The curing environment was maintained at a temperature of 20 ± 2 °C, and humidity was maintained above 90% RH. The successfully prepared concrete specimens were obtained.
The group for which the compound was used as the coating was set up as a control and used as a reference to the additive groups. The analysis of the coating group specimens further elucidated the differences in the role of the polyaniline–alkaline copper carbonate composite when used as an additive and as a coating. After the specimens were cured, they were coated with the polyaniline–alkaline copper carbonate compound to form a protective film on the surface of the concrete specimens. The specific method is listed as follows: surfactant (propylene carbonate 5 g) was added followed by the addition of a wetting and dispersing agent, thickener, and other components to the suspension of polyaniline–alkaline copper carbonate composite (58.90 g, 500 mL), the surface of concrete specimens were treated (dip a brush into the paint and brush it evenly on the surface of the concrete specimen), and after the surfaces dried, the coating was prepared. The thickness of the obtained coating is between 200 μm and 300 μm.
2.3. Methodology for Actual Sea Experiments
The concrete test blocks were suspended in the low tide area of the intertidal zone of the offshore sea and were subjected to the scouring action of the waves. They were submerged in seawater for most of the time. The concrete blocks with different levels of compound additives and the coating groups were suspended in the intertidal zone of offshore waters at Zhoushan (Zhai Ruo Shan Island) (Figure 2) to simulate the marine environmental structure and test the actual anti-biofouling and anti-corrosion performance of the additive in concrete. The concrete blocks were removed after suspension for a period to observe the attachment of biological organisms. The mechanical properties were tested before and after suspension to determine the strength of the concrete and its changes.
Zhai Ruo Shan Island is located in the middle of Zhoushan Fishing Ground, Zhejiang Province, 29°56′ N, 122°5′ E, with a distance of 8 km from the seaport. It has a 7.27 km long coastline, an area of 2.7 km2, a marine monsoon climate on the southern edge of the northern subtropical zone, annual average temperatures of 16 °C, and humid climates [42,43]. The wharf is rich in biomass, with a suitable water depth (Figure 3). The wave scouring is strong, which is suitable for testing the attachment of biological organisms and seawater erosion resistance test of concrete test blocks in the marine environment.
During the suspension, the test blocks were tightly wrapped with fishing rope. Sets of six blocks were tied to the thicker main rope and immersed in the sea until the seawater completely submerged the test blocks (Figure 4). The blocks were retrieved after 30 d and 90 d of hanging.
2.4. Methodology of Mechanical Performance Test
The mechanical properties of the test blocks were tested separately (divided into compressive strength test and splitting tensile strength test) before and after the experiments in the sea according to the standard of physical and mechanical properties of the concrete test method [44].
A total of six concrete test blocks were included in each group, three of which were used for the compressive strength test. The test was conducted using a DY-2008DFZ type fully automatic pressure tester. The other three were used for the splitting tensile strength test conducted with custom fixtures using a WD-P6305 (300 KN) type microcomputer-controlled electronic universal testing machine.
2.5. Laboratory Test Methodology for the Avoidance of Biological Organisms
To better test the antibacterial and antifouling properties of the composite in seawater, accelerated tests for the avoidance of biological organisms were conducted in the laboratory. The avoidance of biological organisms toward concrete slabs containing the additive was accelerated by simulating an artificially enhanced marine wastewater environment in a laboratory pool, adding bacterial and algal organisms, and placing concrete slabs.
The artificially enhanced marine wastewater was prepared using seawater salt (36 g/L) as a solvent with the following formulation scheme (for 2 L):
(1) Inorganic nitrogen fraction: Using sodium nitrate and ammonium chloride, the total nitrogen content was three to five times the lower limit for Class V water. Total nitrogen content included ammonia nitrogen (N: 20 mg), of which, 60.7 mg was sodium nitrate and 38.2 mg was ammonium chloride.
(2) Phosphorus fraction: Potassium dihydrogen phosphate was used, and the dosage was calculated based on the natural ratio of C:N:P (the mass ratio of C:N:P is 100:5:1) (P: 4 mg), where potassium dihydrogen phosphate was 17.6 mg.
The above additions were made with reference to the environmental quality standards of the surface water [45] (total phosphorus was calculated as simple phosphorus).
(3) COD fraction: glucose was used at a dosage based on the natural ratio of C:N:P (100:5:1) (C: 400 mg); the amount of glucose was 1 g.
(4) Photosynthetic bacterial solution (concentration controlled at 100 mg/L).
In the aforementioned scenario, nitrate and nitrite were used for inorganic nitrogen content, and potassium phosphate solution was used for phosphorus content. The total amounts for phosphorus and inorganic nitrogen were 0.4 mg/L and 2 mg/L, respectively, which were the minimum standard for Class V water based on the environmental quality standard.
At a constant room temperature of 25 °C, the concrete test slabs were placed in a container vertically and submerged by adding artificially reinforced marine wastewater (before immersing the concrete test slabs into the containers, the test slabs were soaked with the same marine wastewater. After soaking through, the mass measurement of the test slab before immersion was carried out.). The surface adherence of the test slabs and changes in water quality were observed every 48 h from the time of submergence. After 14 days, the concrete test slabs were removed, and the total mass difference before and after submergence was measured.
3. Results and Discussion
3.1. Characterization of Polyaniline–Alkaline Copper Carbonate Composite Materials
Polyaniline–alkaline copper carbonate composites are insoluble in water. The polyaniline–alkaline copper carbonate composite was prepared as a stable dark green solid after drying and was mixed with water under sonication to re-form a uniformly dispersed suspension. The product of the composite was mixed with water, and the turbid solution was tested using a pH meter. The average pH result was 8.2, which was weakly alkaline.
The product of the synthesis was rinsed several times and dried in an oven at 70 °C for 36 h. After the complete evaporation of water, it was removed and weighed. After three calculations, the average value was obtained, and 11.05 g of basic copper carbonate could be obtained for every 15.96 g (0.1 mol) of anhydrous copper sulfate and 15.812 g (0.2 mol) of ammonium bicarbonate, with a yield of approximately 99.95% and almost no loss during the experiment. Meanwhile, 25 g of polyaniline could be obtained for every 25.918 g (0.2 mol) of aniline hydrochloride, with a polymerization rate of approximately 96.46%, which represented a good polymerization effect. The overall yield of the compound should be above 96.46%.
The results of the composite contact angle measurements are tabulated in Table 4.
The average contact angle of all three composite smear samples was less than 90°. This proves that the composite is hydrophilic, i.e., the composite is more likely to form a stable dispersion system in water, and hence, is more conducive to dispersion in concrete mixes.
For the composite product, the dispersibility was measured thrice, and the average value of ∆C was obtained as 0.974, which demonstrated a well-dispersed product. In addition, if the resting time was too long, the ∆C value fluctuated slightly in the range of ±0.020.
Under the same conditions, the height of the particles of alkaline copper carbonate (CuSC-1 (no surfactant added), CuSC-2-A (propylene carbonate added as surfactant), and CuSC-2-B (ethylene carbonate added as surfactant)) obtained by three different methods of preparation, depending on whether the surfactant was used or not, decreased after each period. These findings indicated that the heights of the supernatant were different, and the time nodes and heights after stabilization varied. The reduced heights of alkaline copper carbonate particles obtained by different preparation methods after different times are indicated by H1, H2-A, and H2-B, respectively, in Figure 5.
After the addition of the surfactant, the initial settling velocity of the product, basic copper carbonate, was subsequently reduced due to its more dispersed and finer particles, with higher final settling heights (all other conditions being equal).
We calculated the average settling velocity of CuSC-1, CuSC-2-A, and CuSC-2-B within 20 min after settling was initiated (Table 5). The initial settling velocity of the synthesized alkaline copper carbonate prepared by hyperdispersion was significantly lower than that of the conventionally synthesized alkaline copper carbonate. CuSC-2-B, which was prepared using ethylene carbonate as the active agent, had the slowest settling velocity, indicating that the basic copper carbonate particles were finer and best dispersed.
3.2. Results of Concrete Specimen Preparation
During the concrete mixing process, the addition of the compound additive affects the appearance of the mix to some extent. Generally, the mixture showed homogeneous properties. In the subsequent observations, it was found that the concrete mixes with the addition of the composite additive had a slightly longer setting time than the mixes without this additive. With the increase in setting time, the strength of the concrete after curing was also enhanced to some extent. This is due to the adsorption of the compound additive on the surface of the cement particles, forming a low permeability protective layer that prevents the reaction between water and cement, thus inhibiting the formation of calcium silicate and delaying the hydration reaction. These make the concrete set slower, and the setting time grows. Because the setting time is prolonged, the concrete can be more tightly bound internally, and the compactness is enhanced, so the concrete strength is increased slightly.
The laboratory-prepared complex additives were well dispersed and were added to the concrete in the form of complex suspensions during the concrete forming stage. Therefore, the prepared concrete specimens have a homogeneous dispersion of the additive in the concrete. In terms of environmental protection, the complex additive does not dissolve in the marine environment, thus being able to remain in the concrete for a long time and having environmentally friendly properties.
The concrete specimens obtained after preparation are shown in Table 6.
3.3. Analysis of Avoidance of Biological Organisms Test (Results and Discussion)
3.3.1. Attachment of Biological Organisms in Sea Experiments
The concrete specimens were suspended for a period, and after retrieval, the biological organisms that were attached to the specimens were mainly barnacles and sea anemones (Figure 6). Their numbers were separately counted, as shown in Table 7.
As shown in Table 7, the number of biological organisms attached to concrete specimens in the group with no additive was significantly higher than in other experimental groups. The number of biological organisms attached increased with longer periods. When the amount of compound additive was 0.05%, a small number of biological organisms was attached, and after 30 days of suspension, the number of barnacles attached was 20.0% of that observed in the group with no additive. Concurrently, no attachment of anemones was observed. After 90 days of suspension, the number of barnacles attached was only 7.0% of the group with no additive, and the number of sea anemones attached was 11.1% of the group with no additive. When the addition rate was more than 0.10%, no attachment of biological organisms to the concrete test block was observed. After 30 days of suspension, no attachment of biological organisms was observed, but after 90 days, a small number of biological organisms was attached. The number of barnacles attached was 9.3% of that of the group with no additive, and the number of sea anemones attached was 22.2% of that of the group with no additive. This shows that the polyaniline–alkaline copper carbonate composite has good effects in preventing the attachment of biological organisms during sea experiments. Additionally, when the composite was used as an additive in concrete, a better effect was observed rather than direct addition to the coating.
3.3.2. Accelerated Experiments for the Avoidance of Biological Organisms in the Laboratory and Discussion
The results of the accelerated experiments examining the avoidance of biological organisms in the laboratory showed that bacterial growth increased with increased time. The water became significantly turbid (Figure 7), with colonies and bacterial secretions appearing on the water surface (Figure 8), and attachment of biological organisms appeared on the test plates. After seven days of immersion, the concrete test plates were temporarily taken out. It was observed that more colonies and attachments of biological organisms appeared on the concrete test plates of the no-addition group. The attachment of biological organisms on the test plates of the other experimental groups was not apparent (Figure 9).
After 14 days of immersion, the concrete test plates were removed, and the mass of each test plate was measured. The mass before and after immersion was compared to obtain the difference, which represented the mass of the biological organisms attached to the test plate. The values of each group were averaged. As shown in Figure 10, the mass of biological organisms attached to concrete test plates after soaking was the highest in the group with no additives and the lowest in the groups with 0.05%, 0.10%, 0.15%, and 0.20% of additives. Furthermore, a similar mass was observed across the groups with 0.10%, 0.15%, and 0.20% of additives. The mass of biological organisms in the coated group was only 23.3% of that in the group with no additives. The mass of biological organisms in the 0.05%, 0.10%, 0.15%, and 0.20% groups were 43.8%, 43.8%, 37.0%, and 35.6% of that in the group with no additives, respectively. Based on these differences, it is clear from the results that the polyaniline–alkaline copper carbonate composite acted as an agent that prevented the attachments of biological organisms and biofouling in concrete test slabs.
3.3.3. Summary
The effects of polyaniline–alkaline copper carbonate composite in concrete were studied through the attachment of biological organisms to concrete blocks suspended in actual sea experiments and accelerated tests in the laboratory. The results showed that (1) concrete specimens with no addition of the composite were susceptible to the corrosive effects of organisms in the actual marine environment, which led to the loss in their strength, as well as susceptibility to attachment of marine organisms. In laboratory experiments of artificially enhanced marine wastewater environments, the concrete specimens were susceptible to biofouling, and more biological organisms were attached to the surface of the specimen plates. (2) The concrete specimens of the coating group showed good antibacterial and antifouling effects in 14-day accelerated laboratory experiments, and the composite coating on the surface played an evident role in preventing biofouling. When suspended in actual sea conditions, the concrete specimens in the coating group also showed good effects in preventing the attachment of biological organisms. However, as the suspension time in the actual sea environment increased, the coating was damaged, and a small number of biological organisms were attached to the concrete specimens. Considering that the concrete coating will flake off after being immersed in seawater for too long and will be damaged by the rigid, sharp, calcareous substrate of barnacles, oysters, and other creatures, the coating has a limited life span. Once the coating flakes off one after another, it can no longer provide adequate protection for the concrete. In addition, for coated concrete, transportation and construction can damage the fragile and thin coating, thus affecting its application. Therefore, in the long run, its effect was not as good as that of the concrete specimens in the additive groups. The additives are added directly into the concrete and do not suffer from the abovementioned problems compared to coatings. The compound additive in concrete can exert a long-lasting effect on preventing the attachment of biological organisms and has slow-release and stable features. (3) The concrete specimens with compound additives demonstrated positive effects when the concrete blocks were suspended in actual sea environments. When the additive rate was 0.10% or more, good effects were observed in preventing the attachment of biological organisms when the concrete blocks were suspended in actual sea environments for 90 days, with a very small number of organisms observed at 0.05% of the additive amount. In the 14-day laboratory experiments, the concrete specimens of all groups with the compound additive had good antibacterial and anti-biofouling effects, and these effects increased slightly with the increase in the amount of the compound additive.
3.4. Mechanical Performance Test (Results and Discussion)
The effectiveness of the compound in concrete was evaluated based on the following four aspects in mechanical performance tests:
Comparison of strength between groups with different addition rates.
Comparison of strength between groups for different suspension periods.
The degree of change in strength over time between groups with different addition rates.
Comparison of strength between the additive and coating groups.
The strength of each group of concrete specimens was measured before suspension in the sea (Table 8).
The test results show that the compound additive influenced the mechanical properties of concrete when the content exceeded a certain level, mainly in the form of a significant reduction in strength. This is due to the excessive amount of compound particles adsorbed on the cement surface, which excessively hinders the reaction between water and cement, making the concrete unable to set. In addition, during the preparation of concrete specimens, the curing time is greatly prolonged, and the hydration reaction is not complete, so the concrete sets less effectively. This makes it difficult to de-mold the specimens and reduces the integrity of the concrete specimens while affecting the strength of the concrete even more. However, the strength of the specimens did not change significantly when the additive rate was lower than 0.20%. Overall, the change in compressive strength of the specimens in the 0.05%, 0.10%, and 0.15% additive groups was within 6.8%, and the change in splitting tensile strength was within 2.1%, indicating that the compound additive in this range does not negatively affect the strength of concrete. Therefore, the additive rate should be controlled between 0.05% and 0.15%.
After 30 and 90 days of suspension in sea experiments, the concrete specimens were retrieved and tested again for mechanical properties. The changes in compressive strength and splitting tensile strength of concrete specimens are shown in Figure 11 and Figure 12, respectively.
The experimental group with a 0.20% addition rate had a more potent effect on the original strength of the concrete due to the addition of excessive compound additives. Its compressive strength and splitting tensile strength were significantly lower than those of the other experimental groups, but the strength did not change significantly again after 90 days of suspension in seawater. Simultaneously, the strength of the specimens in the group with no addition and the coated group decreased after 30 and 90 days of suspension compared to that of the concrete group with the compound additive. In particular, the compressive strength of the specimens in the group with no additives decreased by 4.8% and 11.1%, and the splitting tensile strength decreased by 20.6% and 32.5% after 30 and 90 days of suspension, respectively. The compressive strength of the specimens in the coated group decreased by 2.0% and 9.0%, and the splitting tensile strength decreased by 9.5% and 24.5% after 30 and 90 days of suspension, respectively, and the decrease in the coating group was lower than that of the group with no additives. The changes in compressive strength and splitting tensile strength of the compound additive group were significantly lower than those of the group with no additive and the coating group. When the compound was used as a coating, although the change in strength was higher after 90 days of suspension, it was still smaller than the change in the strength of the concrete in the group with no additive. The change in strength was not apparent after 30 days of suspension, and the decrease in strength in the later period was likely due to the peeling of the coating, which could not be continued to protect the concrete. In addition, the concrete coating can only prevent external harmful ions from continuing to penetrate the reinforced concrete but cannot inhibit the existing harmful ions on the continued damage to the concrete, especially for the use of sea sand for the concrete to play a limited role. Thus, the strength of the test block was reduced, and its application was not as effective as the compound additive.
4. Conclusions
This study proposed the use of polyaniline–alkaline copper carbonate composite as a concrete additive to prevent seawater corrosion and the attachment of marine biological organisms. Furthermore, we established a set of high-dispersion preparation methods for polyaniline–alkaline copper carbonate composite based on the sedimentation experiment of alkaline copper carbonate particles in suspension, and the role of surfactant in high-dispersion preparation was studied. Concrete specimens with polyaniline–alkaline copper carbonate composite were prepared, and the effect of the composite against the attachment of biological organisms was tested in 30-day and 90-day actual sea suspension experiments in Zhoushan sea waters. The concrete specimens with polyaniline–alkaline copper carbonate composite were tested in Zhoushan sea waters for 30 days and 90 days to check the preventive effect of attachment of biological organisms on the composite, and the mechanical properties of concrete specimens were tested in the laboratory before and after suspension. The following conclusions were obtained:
(1). The yield of the composite was above 96.46% in the laboratory, with good output. The average contact angles of the compound smear samples were all less than 90 °C, which proved that the compound was hydrophilic. The laboratory-prepared compound additive could ensure better dispersibility. The alkaline copper carbonate that was prepared by adding ethylene carbonate as the active agent had the slowest settling speed and the best dispersibility, which was more conducive to the uniform distribution of the compound in concrete.
(2). The real sea hanging test shows that the bio-attachment in the Zhoushan sea area is mainly barnacles and sea anemones. The polyaniline–alkaline copper carbonate complex has a good anti-bio-adhesion ability. Compared with the concrete specimens without compound additives, the concrete specimens containing the compound showed a significant reduction in bio-adhesion and a significant effect. The accelerated bio-avoidance experiments in the laboratory of concrete specimens showed that the complex materials had good antibacterial and antifouling effects. The antibacterial and antifouling effects of the specimens were improved to different degrees with the increase in the amount of the complexes.
(3). The experiments on the mechanical properties of concrete specimens before suspension showed that the high content of polyaniline–alkaline copper carbonate additive would lower the potency of the mechanical properties of concrete. However, no significant change was observed in the strength of specimens when the additive content was not higher than 0.20%, and the recommended additive content was 0.05~0.15%.
(4). Compared with the concrete specimens without the compound additive, the loss rates of compressive strength and the loss rates of splitting tensile strength of the specimens with the compound were significantly reduced after suspension in actual sea conditions. The compound effectively protected the concrete and improved its safety.
(5). The application effect of polyaniline–alkaline copper carbonate compound in concrete has been tested. Whether it is added directly to concrete as an additive or coated on concrete, it can effectively prevent concrete from being attached to organisms. However, when it is used as a coating rather than as an additive, it is applied less effectively.
Conceptualization, S.Z. and Y.Y.; data curation, S.Z., C.L. and J.Q.; formal analysis, Y.Y.; funding acquisition, X.L.; investigation, J.S.; methodology, S.Z.; project administration, X.L.; resources, X.L.; supervision, Y.Y.; validation, X.L., J.G. and B.Y.; writing—original draft, S.Z.; writing—review and editing, X.L. All authors have read and agreed to the published version of the manuscript.
Not applicable.
Not applicable.
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.
We thank Pingping Zhang, Institute of Marine Geology and Resources, Zhejiang University, for her help and support in completing the work of this study!
The authors declare no conflict of interest.
Footnotes
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
Enlarge this image.Figure 5. Folding line graph of the supernatant height of alkaline copper carbonate suspensions with the time required for different preparation methods.
Enlarge this image.Figure 9. Changes in water quality over time: (a) the group with no addition; (b) the group with coating; (c) the group with 0.05% addition; (d) the group with 0.10% addition; (e) the group with 0.15% addition; (f) the group with 0.20% addition.
Enlarge this image.Figure 11. Folding line graph of changes in compressive strength before and after the sea experiments for each group.
Enlarge this image.Figure 12. Folding line graph of changes in splitting tensile strength before and after sea experiments for each group.
Experimental setup of concrete specimens for the test block group.
| Usage and Amount Used | No Addition | Addition Rate of 0.05% | Addition Rate of 0.10% | Addition Rate of 0.15% | Addition Rate of 0.20% | Used as Coating | |
|---|---|---|---|---|---|---|---|
| Suspension Days | |||||||
| 0 days | 6 pieces | 6 pieces | 6 pieces | 6 pieces | 6 pieces | 6 pieces | |
| 30 days | 6 pieces | 6 pieces | 6 pieces | 6 pieces | 6 pieces | 6 pieces | |
| 90 days | 6 pieces | 6 pieces | 6 pieces | 6 pieces | 6 pieces | 6 pieces | |
A total of 18 experimental groups of 6 blocks each were examined, with a total of 108 test blocks.
Experimental setup of concrete specimens for the test plate group.
| Usage and Amount Used | No Addition | Addition Rate of 0.05% | Addition Rate of 0.10% | Addition Rate of 0.15% | Addition Rate of 0.20% | Used as Coating |
|---|---|---|---|---|---|---|
| Number of test plates | 3 | 3 | 3 | 3 | 3 | 3 |
There were 6 experimental groups of 3 plates each examined, with a total of 18 test plates.
Specific mix ratio design for concrete specimens.
| Constituents | Water | Cement | Desalinated Sand | Manufactured Sand | Crushed Stone (Φ5–15) | Fly Ash | Mineral Powder | High-Efficiency Water Reducing Agent | Catalyst |
|---|---|---|---|---|---|---|---|---|---|
| Portion | 167 | 208 | 297 | 445 | 1085 | 61 | 70 | 6.77 | 2.71 |
Compound smear contact angle.
| Smear Samples | Left Contact Angle/(°) | Right Contact Angle/(°) | Average Value |
|---|---|---|---|
| Smear 1 | 45.5 | 50.0 | 47.75 |
| Smear 2 | 42.0 | 40.5 | 41.25 |
| Smear 3 | 48.0 | 53.0 | 50.50 |
Average settling velocity of alkaline copper carbonate suspensions within 20 min after the start of settling for different preparation methods.
| Type of Synthesis | Supernatant Height after 20 min/(mm) | Average Speed/(mm·min−1) |
|---|---|---|
| CuSC-1 | 98 | 4.90 |
| CuSC-2-A | 77 | 3.85 |
| CuSC-2-B | 49 | 2.45 |
Test pieces.
| Groups | Number of Test Blocks/(blocks) | Number of Test Plates/(pcs) | Total Volume of Concrete/(L) | Total Additive Content/(g) |
|---|---|---|---|---|
| No addition | 18 | 3 | 25 | 0 |
| Coating group | 18 | 3 | 25 | 0 |
| Addition rate of 0.05% | 18 | 3 | 25 | 29.45 |
| Addition rate of 0.10% | 18 | 3 | 25 | 58.90 |
| Addition rate of 0.15% | 18 | 3 | 25 | 88.35 |
| Addition rate of 0.20% | 18 | 3 | 25 | 117.80 |
| Total | 108 | 18 | 150 | 294.50 |
Statistics on the number of biological organisms attached to concrete test blocks for sea experiments.
| Groups | Number of Barnacles |
Number of Barnacles |
Number of Sea |
Number of Sea |
|---|---|---|---|---|
| No addition | 15 | 43 | 5 | 9 |
| Coating group | 0 | 4 | 0 | 2 |
| Addition rate of 0.05% | 3 | 3 | 0 | 1 |
| Addition rate of 0.10% | 0 | 0 | 0 | 0 |
| Addition rate of 0.15% | 0 | 0 | 0 | 0 |
| Addition rate of 0.20% | 0 | 0 | 0 | 0 |
Results of mechanical tests before the suspension.
| Groups | Compressive Strength/(MPa) | Splitting Tensile Strength/(MPa) |
|---|---|---|
| No addition | 39.6 | 2.87 |
| Coating group | 41.0 | 2.73 |
| Addition rate of 0.05% | 40.3 | 4.14 |
| Addition rate of 0.10% | 38.4 | 3.67 |
| Addition rate of 0.15% | 36.9 | 2.81 |
| Addition rate of 0.20% | 30.7 | 2.10 |
References
1. Zhang, G.; Xie, Q.; Ma, C.; Zhang, G. Permeable epoxy coating with reactive solvent for anticorrosion of concrete. Prog. Org. Coat.; 2018; 117, pp. 29-34. [DOI: https://dx.doi.org/10.1016/j.porgcoat.2017.12.018]
2. Mangi, S.A.; Makhija, A.; Raza, M.S.; Khahro, S.H.; Jhatial, A.A. A comprehensive review on effects of seawater on engineering properties of concrete. Silicon; 2021; 13, pp. 4519-4526. [DOI: https://dx.doi.org/10.1007/s12633-020-00724-7]
3. Berrocal, C.G.; Löfgren, I.; Lundgren, K.; Tang, L. Corrosion initiation in cracked fibre reinforced concrete: Influence of crack width, fibre type and loading conditions. Corros. Sci.; 2015; 98, pp. 128-139. [DOI: https://dx.doi.org/10.1016/j.corsci.2015.05.021]
4. Huang, Y.; Zhang, X. Discussion on the chemical analysis method of chloride ions in admixtures. Heilongjiang Jiaotong Keji; 2013; 36, 70. [DOI: https://dx.doi.org/10.3969/j.issn.1008-3383.2013.05.057]
5. De Weerdt, K.; Justnes, H.; Geiker, M.R. Changes in the phase assemblage of concrete exposed to sea water. Cem. Concr. Compos.; 2014; 47, pp. 53-63. [DOI: https://dx.doi.org/10.1016/j.cemconcomp.2013.09.015]
6. Kaushik, S.K.; Islam, S. Suitability of sea water for mixing structural concrete exposed to a marine environment. Cem. Concr. Compos.; 1995; 17, pp. 177-185. [DOI: https://dx.doi.org/10.1016/0958-9465(95)00015-5]
7. Zhou, L.; Jiao, W.; Fan, H.; Liao, Y. Study of corrosion and rust inhibitors on the performance of reinforced concrete in marine environment. Merch. Qual.; 2016; 05, pp. 209-210.
8. Jia, H.; Yan, G.; Yan, G. Study on corrosion of reinforcement in concrete. China Saf. Sci. J.; 2005; 15, pp. 56-59. [DOI: https://dx.doi.org/10.3969/j.issn.1003-3033.2005.05.013]
9. Zhou, L.; Wu, N.; Jiang, L. Anti-corrosion measures for the durability of concrete structures of bridges in marine environment. Shanxi Archit.; 2012; 38, pp. 184-186. [DOI: https://dx.doi.org/10.3969/j.issn.1009-6825.2012.28.102]
10. Liu, K.; Zhang, J.; Li, Y.; Yu, F. Corrosion of reinforced concrete in marine environment. Equip. Environ. Eng.; 2020; 17, pp. 96-104. [DOI: https://dx.doi.org/10.7643/issn.1672-9242.2020.04.016]
11. Pour-Ali, S.; Dehghanian, C.; Kosari, A. Corrosion protection of the reinforcing steels in chloride-laden concrete environment through epoxy/polyaniline–camphorsulfonate nanocomposite coating. Corros. Sci.; 2015; 90, pp. 239-247. [DOI: https://dx.doi.org/10.1016/j.corsci.2014.10.015]
12. Goyal, A.; Pouya, H.S.; Ganjian, E.; Claisse, P. A review of corrosion and protection of steel in concrete. Arab. J. Sci. Eng.; 2018; 43, pp. 5035-5055. [DOI: https://dx.doi.org/10.1007/s13369-018-3303-2]
13. Shen, X.; Liu, Q.; Hu, Z.; Jiang, W.; Lin, X.; Hou, D.; Hao, P. Combine ingress of chloride and carbonation in marine-exposed concrete under unsaturated environment: A numerical study. Ocean Eng.; 2019; 189, 106350. [DOI: https://dx.doi.org/10.1016/j.oceaneng.2019.106350]
14. Lv, P. Studies on the Novei Polyaspartic Ester Based Poiyurea Coatings for Marine Concrete Protection. Master’s Thesis; Ocean University of China: Qingdao, China, 2007.
15. Li, X.; Zhao, Y.; Yang, B. Discussion of common problems in the testing of anti-corrosion coatings for concrete. Synth. Mater. Aging Appl.; 2014; 43, pp. 71-73. [DOI: https://dx.doi.org/10.3969/j.issn.1671-5381.2014.03.018]
16. Zhang, M.; Zhao, W.; Yu, S. Research progress of marine fouling organisms in China. Fish. Sci.; 2008; 27, pp. 545-549. [DOI: https://dx.doi.org/10.3969/j.issn.1003-1111.2008.10.012]
17. Wang, H.; Zhang, D.; Zheng, Y. Research progress on prevention and removal technology of marine fouling organisms. Chin. J. Fish.; 2018; 31, pp. 47-50. [DOI: https://dx.doi.org/10.3969/j.issn.1005-3832.2018.06.010]
18. Peres, R.S.; Armelin, E.; Alemán, C.; Ferreira, C.A. Modified tannin extracted from black wattle tree as an environmentally friendly antifouling pigment. Ind. Crops Prod.; 2015; 65, pp. 506-514. [DOI: https://dx.doi.org/10.1016/j.indcrop.2014.10.033]
19. Zhao, S. Research on Anti-Fouling Paint for Coastal Power Plants. Master’s Thesis; Harbin Institute of Technology: Harbin, China, 2017.
20. Huang, Z.; Li, C.; Zhang, L.; Li, F. Studies on the ecology of fouling and boring organisms in ZhouShan waters, China. Haiyang Xuebao; 1979; pp. 299-310.
21. Han, J.; Dai, C.; Gao, Z.; Chen, Z. Microorganism corrosion of concrete. Mater. Rep.; 2002; 16, pp. 42-44. [DOI: https://dx.doi.org/10.3321/j.issn:1005-023X.2002.10.014]
22. Yao, G.; Xu, M.; An, X.; Zhou, J.; Hua, P. Anti-attachment and anti-erosion performance test of anti-attachment coatings. J. Hydroelectr. Eng.; 2016; 35, pp. 31-39. [DOI: https://dx.doi.org/10.11660/slfdxb.20160504]
23. Wu, J. Pumped storage hydropower plant concrete anti-erosion epoxy spraying construction. Yunnan Water Power; 2016; 32, pp. 116-118. [DOI: https://dx.doi.org/10.3969/j.issn.1006-3951.2016.04.035]
24. Merachtsaki, D.; Fytianos, G.; Papastergiadis, E.; Samaras, P.; Yiannoulakis, H.; Zouboulis, A. Properties and performance of novel mg(oh)2-based coatings for corrosion mitigation in concrete sewer pipes. Materials; 2020; 13, 5291. [DOI: https://dx.doi.org/10.3390/ma13225291] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33238399]
25. Qiao, H.; Wen, S.; Wang, P.; Yang, Z.; Yang, T. Electrochemical characteristics of coated steel bars corrosion of magnesium oxychloride reinforced concrete. J. Build. Mater.; 2019; 22, pp. 999-1006. [DOI: https://dx.doi.org/10.3969/j.issn.1007-9629.2019.06.024]
26. Dong, Z.; Wu, G.; Xu, Y. Experimental study on the bond durability between steel-frp composite bars (sfcbs) and sea sand concrete in ocean environment. Constr. Build. Mater.; 2016; 115, pp. 277-284. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2016.04.052]
27. Dong, Z.; Wu, G.; Zhao, X.; Zhu, H.; Lian, J. The durability of seawater sea-sand concrete beams reinforced with metal bars or non-metal bars in the ocean environment. Adv. Struct. Eng.; 2020; 23, pp. 334-347. [DOI: https://dx.doi.org/10.1177/1369433219870580]
28. Tian, W.; Du, N.; Li, S.; Chen, S.; Wu, Q. Metastable pitting corrosion of 304 stainless steel in 3.5% nacl solution. Corros. Sci.; 2014; 85, pp. 372-379. [DOI: https://dx.doi.org/10.1016/j.corsci.2014.04.033]
29. Schwarz, W.; Pichlhöfer, A.; van den Hondel, A.; Esteves, H. Maintenance and repair of steel reinforced concrete structures by galvanic corrosion protection—Field experiences over 10 years. MATEC Web Conf.; 2018; 199, 5005. [DOI: https://dx.doi.org/10.1051/matecconf/201819905005]
30. Dong, S.; Zhao, B.; Lin, C.; Du, R.; Hu, R.; Zhang, G.X. Corrosion behavior of epoxy/zinc duplex coated rebar embedded in concrete in ocean environment. Constr. Build. Mater.; 2012; 28, pp. 72-78. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2011.08.026]
31. Hou, L.; Li, W.; Zheng, H.; Ji, T.; Tian, H. Recent advances and development of corrosion prevention technologies for marine reinforced concrete structures. Mater. Prot.; 2017; 50, pp. 62-67. [DOI: https://dx.doi.org/10.16577/j.cnki.42-1215/tb.2017.03.016]
32. Shi, H.; Wang, L. Discussion on nontoxic sodium silicateantifouling paint used in the netting of marine cage. Mar. Fish.; 2004; 26, pp. 239-242. [DOI: https://dx.doi.org/10.3969/j.issn.1004-2490.2004.03.016]
33. Pantano, D.; Neubauer, N.; Navratilova, J.; Scifo, L.; Civardi, C.; Stone, V.; von der Kammer, F.; Müller, P.; Sobrido, M.S.; Angeletti, B. et al. Transformations of nanoenabled copper formulations govern release, antifungal effectiveness, and sustainability throughout the wood protection lifecycle. Environ. Sci. Technol.; 2018; 52, pp. 1128-1138. [DOI: https://dx.doi.org/10.1021/acs.est.7b04130]
34. Quan, X.; Wang, J.; Souleyman, T.; Cai, W.; Zhao, S.; Wang, Z. Antibacterial and antifouling performance of bisphenol-a/poly(ethylene glycol) binary epoxy coatings containing bromine-benzyl-disubstituted polyaniline. Prog. Org. Coat.; 2018; 124, pp. 61-70. [DOI: https://dx.doi.org/10.1016/j.porgcoat.2018.08.010]
35. Xu, Y.; Li, L.; Zhang, J.; Liu, N.; Wang, J. Development of polyaniline and its anti-corrosion properties. Contemp. Chem. Ind.; 2017; 46, pp. 2285-2288. [DOI: https://dx.doi.org/10.3969/j.issn.1671-0460.2017.11.029]
36. Meroufel, A.; Deslouis, C.; Touzain, S. Electrochemical and anticorrosion performances of zinc-rich and polyaniline powder coatings. Electrochim. Acta; 2008; 53, pp. 2331-2338. [DOI: https://dx.doi.org/10.1016/j.electacta.2007.09.056]
37. Li, Z.; Hu, J.; Li, Y.; Zheng, F.; Liu, J. Self-healing active anticorrosion coatings with polyaniline/cerium nitrate hollow microspheres. Surf. Coat. Technol.; 2018; 341, pp. 64-70. [DOI: https://dx.doi.org/10.1016/j.surfcoat.2017.11.054]
38. Zhu, A.; Wang, H.; Sun, S.; Zhang, C. The synthesis and antistatic, anticorrosive properties of polyaniline composite coating. Prog. Org. Coat.; 2018; 122, pp. 270-279. [DOI: https://dx.doi.org/10.1016/j.porgcoat.2018.06.004]
39. Beygisangchin, M.; Abdul Rashid, S.; Shafie, S.; Sadrolhosseini, A.R.; Lim, H.N. Preparations, properties, and applications of polyaniline and polyaniline thin films—A review. Polymers; 2021; 13, 2003. [DOI: https://dx.doi.org/10.3390/polym13122003]
40. Zhang, L.; Song, J.; Li, J.; Song, J.; Shen, Q. A Method for Determining the Dispersibility of Insoluble Substances in Solution. China Patent; CN945277A, 2006.
41.
42. Wang, Y.; Sun, Y. Overview of the current situation and development of Ruo Mountain Marine Science and Technology Demonstration Island. Sci. Technol. Inf.; 2018; 16, pp. 222-224. [DOI: https://dx.doi.org/10.16661/j.cnki.1672-3791.2018.14.222]
43. Yan, Q.; Dong, C.; Dong, L.; Sun, Y.; Zu, G. The volcanic geology and the geological tourist resorce of Zhairuoshan island, Dinghai, Zhejiang Province. Bull. Sci. Technol.; 2006; 22, pp. 611-615. [DOI: https://dx.doi.org/10.13774/j.cnki.kjtb.2006.05.008]
44.
45.
© 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). Notwithstanding the ProQuest Terms and Conditions, you may use this content in accordance with the terms of the License.