Carbonation Resistance of Marine Concrete Containing Nano-SiO₂ under the Action of Bending Load ^†

Abstract

In order to study the influence of nanomaterials on the carbonation resistance of marine concrete under bending loads, an appropriate amount of nano-SiO₂ was added to plain concrete, and a self-developed carbonation box and bending loading device were used to conduct a coupling test. Four different stress levels were set to measure the carbonation depth of nano-concrete at different ages. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to analyze the concrete interfacial transition zone. The carbonation depth was used as the test index to evaluate the durability of nano-SiO₂-based concrete under the combined action of bending load and carbonation. The test results showed that the compressive and flexural strengths of concrete remarkably improved when the nano-SiO₂ concentration was 2%. Compared with regular concrete, the compressive and flexural strengths of nano-SiO₂ based concrete improved by 15.5% and 15.3%, respectively. When the stress level was 0.15 and 0.6, the carbonation depths of NS20 were 20.5 and 18.4% lower than those of PC in the tensile zone and 18.9 and 23.7% lower than those of PC in the compression zone. The carbonation depth of the NS20 tensile zone was lower by 31 and 18.4% at 3 and 28 days than that of PC. Compared with PC, the carbonation depth in the compression zone of NS20 decreased by 50 and 23.7%, and the carbonation depth of nano-concrete was significantly lower than that of conventional concrete under the same stress level and age. When the stress level is constant, the carbonation depth of the tension zone and compression zone increases gradually with the increase in age, and the carbonation depth of the concrete in the first 7 d was 50% that at 28 days. Under the same age, the carbonation depth in the tension zone increased with increasing stress levels, while the carbonation depth in the compression zone decreased with increasing stress levels. When the stress level was 0.3–0.45, the slope of the carbonation depth curve significantly increased. SEM and XRD analysis results revealed that nano-SiO₂ significantly improved the internal structure of concrete by reducing the width of the microcracks, the number of pores, and the number of microcracks. The number of C₃S/C₂S and CaCO₃ crystals in nano-SiO₂ based concrete was significantly less than that in plain concrete, and the amount of C-S-H gel was more than that in plain concrete. Under bending loads, the nano-SiO₂ significantly improved the carbonation resistance of concrete. When the dosage of nano-SiO₂ was 2%, its improvement effect was the most significant.

1. Introduction

As the growth rate of CO₂ concentration is much higher than ever before, the service condition of marine concrete at ports and wharfs is progressively declining, leading to the deterioration of the durability of marine concrete. Carbonation damage is a common problem in the deterioration of durability of marine concrete. CO₂ in the air gradually diffuses into the interior of marine concrete and reacts with Ca(OH)₂, resulting in the loss of protective film, reduced durability, and shortened service life, causing huge losses to the global economy. The quantitative need for ports and terminals rises along with the expansion of transportation and tourism, as does CO₂ emission. The CO₂ content in nature has grown significantly over the past ten years due to the rapid advancement of science and technology [1,2,3]. Marine concrete must operate in a challenging environment [4]. The endurance of concrete is impacted by a number of elements, including the extremely hostile service environment seen in ports and wharves. The analysis of concrete durability under a single factor cannot accurately recreate the actual service environment; however, the analysis of concrete durability under the combined action of bending loads and carbonation is more accurate [5].

When concrete is subjected to carbonation erosion, the passivation film disappears, resulting in acid–base imbalance. Scholars have divided the diffusion and reaction process of CO₂ in concrete into three regions: the complete carbonation zone, the carbonation reaction zone, and the non-carbonation zone. Ji et al. [6] pointed out that the variation curve of Ca(OH)₂ content in concrete after 28 days carbonation reaction was not exactly the same as that of pH value. According to the variation of Ca(OH)₂ and CaCO₃ content along the depth of the concrete, the carbonation reaction was divided into a partial carbonation zone, with a changing pH value, and an inward partial carbonation zone, with a stable pH value. Zhang et al. [7] found that in the complete carbonation zone and the carbonation reaction zone, Ca(OH)₂ is converted into CaCO₃, and the increase in CaCO₃ makes the interior of the concrete more dense. In this case, the compressive strength and freezing resistance of concrete are improved, while the bending strength and CL erosion resistance of concrete are reduced.

CO₂ concentration, humidity, temperature, and carbonation age are important factors affecting the depth of carbonation [8]. The following scholars have studied these factors. Dhir et al. [9] and Visser et al. [10] studied the carbonation depths of specimens at different CO₂ concentrations and found that the carbonation depths at higher CO₂ concentrations were basically consistent with those at standard concentrations. Andreas et al. [11] studied the influence of CO₂ concentration and humidity on the carbonation depth of concrete and found that when the CO₂ concentration was constant, concrete’s carbonation resistance became stronger and stronger with the increase in humidity. Von et al. [12] pointed out that the carbonation resistance of concrete is poor when the relative humidity is 70–75%. Zhang et al. [13] found that when the temperature increased from 20 °C to 30 °C, the carbonation depth of concrete increased by more than 50%. Branch et al. [14] found that after 28 days of carbonation, Ca²⁺ in the complete carbonation zone and the carbonation reaction zone gradually increased, and the content of Ca²⁺ in the complete carbonation zone was higher than that in the carbonation reaction zone. Zhao et al. [15] used XRD tests to analyze the microscopic substances of concrete at each age and found that there were a large amount of C₃S and C₂S in concrete at each age, proving that the increase in age only affected the carbonation depth, but not the cement hydration.

Under bending loads, cracks will gradually occur in the tensile zone of marine concrete, providing a channel for CO₂ intrusion and improving the carbonation rate. The structure of the concrete compression zone is gradually more dense, and the carbonation resistance is improved. Sanchun et al. [16] used a four-point bending loading device to compare the impact of loading methods on durability under instantaneous loading and continuous loading, and the results showed that continuous loading has a more obvious effect on the durability of concrete. Ren et al. [17] found that when the bending load stress level was 0.4–0.6, the carbonation depth of concrete showed the largest variation. Zhang et al. [18] found that when the stress level was 0.3–0.5, the carbonation depth of concrete in the tension zone was more obvious than that in the compression zone. The XRD test was used to study the change in mineral content before and after concrete carbonation. The results show that although the diffraction peak corresponding to Ca(OH)₂ gradually decreases, it will not disappear completely. With the gradual increase in stress level, the diffraction peak of CaCO₃ also increases, and the increase in the CaCO₃ diffraction peak in the late stage of carbonation reaction is less than that in the early stage. Basically consistent with the conclusions drawn by Zhou et al. [19], Tian et al. [20] used a continuous four-point bending device to carry out carbonation testing after loading and found that when the width of the concrete crack is less than 0.2 mm, the carbonation depth is basically consistent with that under the no-load condition, and when the width of the concrete crack is less than 0.4 mm, the carbonation depth near the side crack of the concrete surface increases significantly, but the carbonation depth of the remaining parts is the same as the carbonation degree of the non-cracked concrete. When the crack width of the concrete is greater than 0.4 mm, the carbonation depth of the concrete increases significantly, and CO₂ spreads inwards through cracks in the concrete, reducing its carbonation resistance. Chen et al. [21] found that when the stress was relatively small, there was little difference in carbonation depth between the pure bending zone and the transverse bending zone of concrete. With the increase in stress level, the pore diameter inside the pure bending zone of concrete became larger and the cracks gradually widened under the action of tensile stress, so that the cracks inside the concrete were connected with each other and CO₂ erosion was accelerated. Sullivangreen et al. [22] studied the relationship between the crack width in concrete and CO₂ diffusion and found that when the crack width was less than 0.009 mm, CO₂ would spread to the inside of the concrete along the crack, rather than eroding around it. When the crack width was more than 0.4 mm, the upper part of the crack would spread most significantly. Alahmad et al. [23] believe that when the crack width is less than 0.5 mm, concrete carbonation is not obvious and CaCO₃ crystals are fewer; when the crack width is greater than 0.5 mm, the carbonation depth will gradually deepen with the increase in crack width, and CaCO₃ crystals will increase. This conclusion is basically consistent with that of Liu et al. [24].

The impact of nanoparticles on the durability of concrete has attracted the attention of several researchers owing to the advancement in the study of nanomaterials. Wang et al. [25] discovered that the carbonation depths of concrete containing 1, 2, and 3% of nano-SiO₂ were 43.51, 51.32, and 55.44% lower than that of regular concrete, respectively, when the carbonation age was 28 days. After 28 days of carbonation, Zhang et al. [26] discovered that the carbonation depth of concrete with nano-SiO₂ was less than that of nano-Fe₂O₃ concrete. Compared with nano-Fe₂O₃, nano-SiO₂ had a better enhancement impact on the carbonation resistance of concrete. Zhang et al. [27] revealed that the carbonation resistance of concrete improved considerably when its nano-SiO₂ level was 7%; nevertheless, when the content was higher, the carbonation depth of concrete increased. Wang et al. [28] reported that the compressive strength of concrete with nano-SiO₂ after carbonation remarkably improved when the dosage of the nano-SiO₂ level was 1%. Zhang et al. [29] studied the porosity of nano-SiO₂-based concrete with different particle sizes after carbonation, and they found that nano-SiO₂ with smaller particle sizes improved the porosity of concrete better than that with large particle sizes. Gao et al. [30] investigated the resilience of nano-SiO₂ under various stresses. Hassan et al. [31] reported that nano-SiO₂ enhanced the internal microstructure and durability of concrete. For silica fume concrete, a significant amount of research has been conducted on its durability. Nano-SiO₂, with a finer particle size than silica fume, has a stronger volcanic ash effect and will show better improvement of concrete properties. Although the cost of silica fume is lower than that of nano-SiO₂, the amount of nano-SiO₂ doping is much smaller than that of silica fume, so nano-SiO₂ is more cost-effective. Qiu [32] reported that when the concrete is mixed with silica fume (5%), the compressive strength increases by 3.8%, from 102 MPa to 106 MPa, and when the concrete is mixed with nano-silica (0.5%), the compressive strength increases by 9.3%.

Therefore, the above studies show that bending load and carbonation simultaneously meet the actual service conditions of marine concrete. However, in the above experiments, there are few simultaneous tests of bending load and carbonation. In this paper, the coupling test of carbonation and bending load is realized, and we used a self-developed carbonation box and a bending loading device to conduct a coupling experiment. The effects of four different nanoparticle contents, different stress levels, and five different ages on the carbonation resistance ability of the concrete were investigated. These different ages, stress levels, and nano-SiO₂ contents all impacted the performance of the carbonation resistance of concrete. In order to examine the differences between the interface transition zone of nano-concrete and regular concrete, SEM and XRD were used to analyze the nano-SiO₂ effect on the internal structure of concrete. Based on the current research, some future prospects are proposed. The aim is to provide reference for future research on carbonation and bending load.

2. Materials and Methods

2.1. Text Materials

Swan P O 42.5 grade regular Portland cement was used for the test. The fineness modulus of the medium sand used as a fine aggregate was 2.43, Grade level II. The ratio of the two different gravels in the coarse aggregate was 7:3. The gravels had a gradation of 16 mm by 31.5 mm and 5 mm by 25 mm. Tributyl phosphate was used as a defoaming agent, and FDN naphthalene-based superplasticizer was used to reduce water. The nano-SiO₂ materials used were made by Hangzhou Hengge(China) Nano-Technology Co, LTD, and the key properties are displayed in

Table 1. Performance of nano-SiO₂.

Item	Diameter/nm	Specific Surface Area/(m2/g)	Purity/%	pH	Appearance
Nano-SiO2	25	580–630	99.9%	7–9	White-powder

. The chemical composition of Portland cement is shown in

Table 2. Chemical composition of Portland cement.

Chemical Composition	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	Alkali Content	Firing Loss
content	20.89	5.38	4.01	63.35	1.56	2.70	0.4	1.54

According to the code for the durability design of concrete structures (GB/T50476-2019), the environmental action grade for marine engineering concrete is I-C, and the concrete strength grade is C40. It is located in a carbonized environment. The water-binder ratio is 0.38, and the sand ratio is 35%. Nano-concrete was constructed following the design regulations of the ordinary concrete mixing ratio (JGJ55-2019). The amount of water used, the water–binder ratio, and the amount of nano-SiO₂ used in place of cement were the same as those of ordinary concrete. Nano-SiO₂ was added to the sample at dosages of 0.5, 1, 2, and 3% of mass of cement. Specific concrete mix ratio information is presented in

Table 3. Mix proportion of concretes (kg/m³).

Mixture Type	Water	Cement	Sand	Gravel	Nano-SiO2	FDN	Defoamer
PC	170.30	448.16	608.41	1129.91	—	1.75	—
NS05	170.30	445.92	608.41	1129.91	2.24	1.75	0.082
NS10	170.30	443.68	608.41	1129.91	4.48	1.75	0.082
NS20	170.30	439.20	608.41	1129.91	8.96	1.75	0.082
NS30	170.30	434.72	608.41	1129.91	13.44	1.75	0.082

.

2.2. Experiment Methods

2.2.1. Slump Test and Strength of Concrete

The “standard for performance test methods of ordinary concrete mixtures” (GB/T50080-2016) was followed when a slump test was conducted. The standard test methods for the physical and mechanical properties of ordinary concrete (GB/T 50081-2019) were used to test the compressive and flexural strengths of the specimens. Compressive strength was tested using a cubic test block with a size of 100 mm × 100 mm × 100 mm, while the flexural strength was tested using prismatic specimens with a size of 100 mm × 100 mm × 400 mm. The specimens were kept for 28 days under a standard curing environment and numbered as shown in Table 3, with three pieces in each group.

2.2.2. Bending Load and Carbonation Test

(1) Size and quantity of the specimens: there were 20 specimens in each group, and the specimens were made of prismatic test blocks with a size of 100 mm × 100 mm × 400 mm. Two opposing 100 mm × 400 mm surfaces were selected as carbonation depth test surfaces, and the forming surface, bottom surface, and end face were sealed with epoxy resin.

(2) Loading mechanism: a four-point loading approach was used to deliver bending force to the specimen using the test apparatus previously reported [32]. The schematic diagram of the loading device is displayed in Figure 1.

(3) Method of loading: the ultimate bending load was determined using the flexural strength test, and the specimen was subjected to various stress levels (0.15, 0.30, 0.45, and 0.60) by spinning nuts. The manometer was used to accurately measure the imposed load. The load on the specimen was monitored every three seconds, due to the prolonged test time, to prevent stress loss. The load was reloaded whenever its weight was reduced to make up for the lost weight.

(4) Carbonation unit: a straightforward carbonation box was independently developed to achieve the coupling effect of carbonation and bending load. The temperature sensor, humidity sensor, and specific long gas detection tube were used to monitor the temperature, humidity, and CO₂ concentration in the carbonation box in real time, according to the “standard test method for long-term performance and durability of ordinary concrete” (GB/T 50082-2020) specifications.

(5) Test for carbonation: the concrete test block was placed into the carbonation box after it was supported by a loading device, with the ultimate bending loads of 15%, 30%, 45%, and 60% applied. Then, CO₂ was injected into the carbonation box through the CO₂ gas cylinder, and the CO₂ concentration was maintained at (20 ± 3%). The temperature of CO₂, measured with a long gas detection tube, was maintained at (20 ± 2) °C using the heating plate. The humidifier was used to monitor the humidity within (70 ± 5)% in real time. The test subjects were aged 3, 7, 14, 21, and 28 days.

(6) Calculation of carbonation depth: The bending load device was removed and unloaded when the test reached the set age. Then, the specimen was cut from the middle of the span using a bench-cutting machine, and a dust ball was used to clean the powder on the cross section. The areas above and below the neutral axis are highlighted as the tension zone and compression zone, respectively. The samples were punctured every 10 mm to serve as measurement sites. Afterward, 1% of the phenolphthalein alcohol solution was sprayed on the sites. The carbonation depth area was the region that became colorless after 30 s. If the measuring point contains coarse aggregate, the average carbonation depth on both sides of the coarse aggregate can be used to obtain the carbonation depth point, which is then used to determine the average value of all measuring points. The average carbonation depth of each age can be determined using Equation (1).

$$\stackrel{-}{{\mathrm{d}}_{\mathrm{t}}}=\frac{1}{\mathrm{n}}\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{d}}_{\mathrm{i}}$$

(1)

where (${\mathrm{d}}_{\mathrm{i}}$) is the average carbonation depth (mm) at each measurement point, ($\stackrel{-}{{\mathrm{d}}_{\mathrm{t}}}$) is the average carbonation depth (mm) after t (days) of specimen carbonation, and $\mathrm{n}$ is the total number of measurement points.

2.3. Microscopic Test

(1) Scanning electron microscope (SEM) test: the microstructure of the interfacial transition zone of the concrete was examined using a Hitachi SU8020 cold field emission scanning electron microscope when it was coupled with bending load and carbonation.

(2) X-ray diffraction (XRD) test: before the XRD test, the carbonized concrete samples were dried at 70 °C for 24 h. The temperature range of 5 °C to 65 °C was the 2 θ.

3. Test Results and Analysis

3.1. Effect of Nano-SiO₂ on Slump and Strength of Concrete

The nano-concrete slump tests are shown in Figure 2. When the amount of nano-SiO₂ increased, the slump of concrete steadily decreased. Because of the huge specific surface area of nano-SiO₂ and the fact that cement particles require more water than nano-SiO₂ during hydration, an increase in nano-SiO₂ concentration causes an increase in viscosity. Small concrete slumps are characterized by numerous pores and low fluidity. Slump-wisely, the concentration of nano-SiO₂ should not be more than 3%. When the concentration of nano silica is greater than 3%, the concrete slump is low, and the fluidity is poor [33]. The agglomeration of nano materials in the concrete leads to the failure of normal hydration of cement and the reduction of compressive and flexural strength and durability of concrete [34].

The compressive strength and flexural strength of concrete are shown in Figure 3. When nanoparticles were added to the sample, the above properties of concrete were significantly higher than those of conventional concrete. The properties of the concrete improved because the addition of nano-SiO₂ to mixed concrete improved its compactness, and the reaction of nano-SiO₂ with Ca(OH)₂ in the concrete facilitated the hydration of the cement.

The optimal dosage of nanoparticles in concrete was observed. When the dosage was increased, the compressive and flexural strengths of nano-SiO₂ first increased and then decreased. This phenomenon was because nano-SiO₂ reacted with water and exhibited a large specific surface area and surface effect. The water demand increased with the excessive mixing of nano-SiO₂, which affected the hydration of the cement, thereby decreasing the compressive and flexural strengths of the concrete. Nano-SiO₂ also has poor dispersion and is easily agglomerated. The compressive and flexural strengths of concrete remarkably improved when the nano-SiO₂ concentration was 2%. Compared with regular concrete, the compressive and flexural strengths of nano-SiO₂-based concrete improved by 15.5% and 15.3%, respectively.

3.2. Effect of Nano-SiO₂ on Durability of Concrete under Coupling of Bending Load and Carbonation

3.2.1. Influence of Nano-SiO₂ Content

Figure 4 illustrates the link between concrete carbonation depth and nano-SiO₂ content for 28 days. T stands for the tension zone, and C represents the compression zone.

With the gradual increase in nano-SiO₂ content, the carbonation depth of the concrete in the tension zone and the compression zone first decreases and then increases. When the nano-SiO₂ content is 2%, the carbonation depth of concrete is the lowest, and the carbonation depth of concrete with nano-SiO₂ in the tension zone is greater than that in the compression zone. This indicates that ability of nano-SiO₂ to improve concrete carbonation resistance first increases and then decreases, and the concrete carbonation resistance is the best when the dosage is 2%.

When the stress level was 0.15 and 0.6, the carbonation depths of NS20 were 20.5 and 18.4% lower than that of PC in the tensile zone and 18.9 and 23.7% lower than that of PC in the compression zone, respectively. These results showed that the capacity of nano-SiO₂ to improve the carbonation resistance of concrete in the tensile zone progressively decreased with an increase in stress level. However, the impact of nano-SiO₂ on the improvement of the carbonation resistance of concrete gradually increased in the compression zone.

Compared with the compression zone, the carbonation depth of the tensile zone of NS20 at stress levels of 0.15 and 0.6 increased by 3.3 and 43.8%, respectively, indicating that the carbonation depth of the tensile zone and the compression zone exhibited a small difference at low-stress levels, but the difference became significant as the stress level rose. When the stress level was low, the number of cracks and microcracks in the tensile zone became equal to that in the compression zone. Due to more fractures and microcracks in the tensile zone than in the compression zone at a high-stress level, CO₂ rapidly diffused, thus increasing the carbonation depth.

3.2.2. Effect of Carbonation Age

The change curve of carbonation depth with carbonation age is depicted in Figure 5 (taking a stress level of 0.6 as an example, and other stress levels showed the same trend).

With the increase in carbonation age, the carbonation depth of nano-marine concrete in the tensile zone and compression zone increases gradually, and reaches the maximum carbonization depth at 28 days. The carbonation depth of nano-concrete is smaller than that of ordinary concrete. When the nano-material content is 2%, the carbonation depth of nano-concrete is the lowest. This shows that nano-SiO₂ can improve the carbonation resistance of concrete, and the optimal dosage is 2%.

The carbonation depth of the NS20 tensile zone was lower by 31 and 18.4% at 3 and 28 days than that of PC, respectively. Compared with PC, the carbonation depth in the compression zone of NS20 decreased by 50 and 23.7%. This finding shows that the beneficial impact of nano-SiO₂ on carbonation resistance is more pronounced at early ages.

When the age was 7 and 28 days, the carbonation depth of the PC tensile zone rose by 120 and 66.1%, respectively, compared with the compression zone, while the carbonation depth of the NS20 tensile zone rose by 176.5 and 77.7%, respectively compared with the compression zone. In the tensile and compression zones of 7 d concrete, the carbonation depths of regular and nano-based concretes were about 50% of that of 28 d concrete. This result showed that age significantly impacted the carbonation depth of concrete and that the tensile zone was more affected by age than was the compression zone. As concrete aged, CO₂ intrusion became more difficult because the tensile zone had more microcracks and cracks than did the compression zone. Conversely, CO₂ intrusion was more difficult in the compression zone when there were fewer fractures and microcracks in the concrete.

3.2.3. The Effect of Stress Level

The variation curve of carbonation depth with stress level is shown in Figure 6 (taking 28 days as an example, and the other ages have a similar trend).

As shown in Figure 6, the carbonation depth of concrete gradually increased with increasing tensile stress levels, while the carbonation depth of concrete gradually decreased with increasing compressive stress levels. In addition, the carbonation depth in the compression zone was smaller than that in the tensile zone. The carbonation depth of concrete reached its maximum and minimum points at a tensile stress level of 0.6 and a compressive stress level of 0.6, respectively, at the age of 28 days. Nano-concrete exhibited a lower carbonation depth than regular concrete, while NS20 had a significantly lower carbonation depth than the other three combinations.

When the compressive stress levels were 0.3 and 0.45, the carbonation depths of NS20 were 18.3 and 23.8% lower than those of PC, respectively. The carbonation depths of NS20 were 20.7 and 19.1% lower than those of PC when the tensile stress levels were 0.3 and 0.45, respectively. These results demonstrate that the addition of 2% nano-SiO₂ to the tensile and compression zones of concrete may more effectively increase the carbonation resistance of concrete under various stress conditions.

The concrete carbonation depth curve displayed a moderate slope when the stress level was less than 0.3. Then, the slope steepness increased when the stress level was between 0.3 and 0.45 and reduced when the stress level was greater than 0.45 (Figure 6). This phenomenon was because the microcracks in the concrete remained stable and unaltered when the stress level was less than 0.3, and when the concrete was in the elastic deformation stage, and the deformation was not significant. The concrete was in the transition stage whenever the stress level rose above 0.3. The initial microcracks in the transition zone of the concrete interface changed under the influence of tensile stress, and their length, breadth, and number increased as the stress ratio rose. The depth of carbonation gradually increased due to the diffusion and invasion of CO₂ caused by the gradually growing number of microcracks. Concrete microcracks became narrower as compressive stress levels rose, restricting the penetration of CO₂ into the cracks and decreasing the carbonation depth. This phenomenon indicates that most carbonation reactions in the tensile zone occur at the stress level of 0.45. The carbonation depths of the tensile zone of NS20 and PC were 95.9 and 95% of the carbonation depth of the tensile zone at the stress level of 0.6. The specimen reached the elastoplastic stage when the tensile stress level ratio exceeded 0.45. The stress had a minimal effect on the microcracks of concrete at this point, and the number of hydration products that might react with CO₂ decreased. As a result, the carbonation depth and slope of the carbonation depth curve both rose significantly. When the compressive stress level exceeded 0.45, the number and breadth of the microcracks in the concrete interface transition zone in the compression zone diminished, thereby slowing the carbonation depth curve by preventing CO₂ from diffusing into the concrete and lowering the CO₂ diffusion coefficient.

4. Enhancement of the Carbonation Resistance of Nano-Concrete under Bending Loads

4.1. SEM Test

Figure 7b,d shows magnified pictures from the ellipses of Figure 7a,c, respectively, illustrating the interface transition zone of PC and NS20 in the tension zone (stress level: 0.6 and age: 28 days). Figure 7a shows that the PC has many microcracks of various widths and numerous pores. Figure 7b shows that tiny fractures of varying widths form inside the concrete, and some of the larger cracks are related to the smaller cracks. As shown in Figure 7a,c, the internal structure of NS20 was denser than that of the PC, with fewer pores and microcracks overall. The reason is that the particle size of nano-SiO₂ is small, it can fill concrete pores and micro cracks, it has pozzolanic reaction, and it reduces free water content, making nano-concrete more dense inside. Nano-SiO₂ particles are more finely grained than cement particles. The addition of nano-SiO₂ enhanced the particle gradation of gel materials and the compactness of concrete by filling holes and microcracks in the interfacial transition zone [35].

As shown in Figure 7b,d, the number of CaCO₃ crystals and microcracks in NS20 was significantly lower than that in PC. CO₂ could not penetrate the concrete due to fewer microcracks in the NS20 interfacial transition zone and the interaction between the nano-SiO₂ and Ca(OH)₂. The failure of the carbonation reaction was attributed to the reduction of Ca(OH)₂ content. The lower level of CaCO₃ in NS20 in the interface transition zone than PC indicated that NS20 had a shallower carbonation than PC on a macro scale.

Nano-SiO₂ exhibited an uneven surface and a volcanic ash effect, which could facilitate cement hydration. Nano-SiO₂ can also interact with the hydration product Ca(OH)₂ to produce amorphous C-S-H gel. Figure 7b,d shows that the amount of C-S-H gel in NS20 is much higher than that in PC. At the margin of the NS20 microcrack, a flocculent-like C-S-H gel was observed. This gel filled the microcrack and created a new network structure, with nano-SiO₂ particles as the crystal core, to enhance the pore structure of concrete [33], thereby lessening the concrete porosity, preventing internal CO₂ movement, and reducing the carbonation depth.

4.2. XRD Test

When the stress level and the carbonation age were 0.6 and 28 days, the interfacial transition occured in PC and NS20. Ca(OH)₂ shows obvious diffraction peaks at 2θ of 18.09° and 35.06°, CaCO₃ shows obvious diffraction peaks at 2θ of 20.08° and 29.38°, C₃S/C₂S shows obvious diffraction peaks at 2θ of 23.55°, and SiO₂ shows obvious diffraction peaks at 2θ of 26.66°.The apparent diffraction peak 2 corresponded to an θ of SiO₂ of 26.66°. The peak value of SiO₂ in NS20 and PC was almost the same because both the coarse and fine aggregates in the concrete contained significant amounts of SiO₂, but they did not participate in the carbonation reaction.

This phenomenon is shown in Figure 8: (1) the amount of Ca(OH)₂ is modest and almost equal in PC and NS20. The reasons could be explained as follows: The surface damage of the PC tensile zone was more severe, and the number of macroscopic fractures rose under high-stress levels. When CO₂ penetrated concrete, it combined with Ca(OH)₂ to consume a significant quantity of Ca(OH)₂, thus reducing the concentration of Ca(OH)₂ in concrete. There were fewer microcracks in NS20 at a tensile stress level of 0.6, restricting CO₂ migration and reducing Ca(OH)₂ consumption. The amount of Ca(OH)₂ at the interface transition zone in PC and NS20 was the same because nano-SiO₂ and Ca(OH)₂ combine to produce C-S-H gel [36].

(2) CaCO₃ concentration in NS20 was significantly lower than in PC, which was related to the phenomena illustrated in Figure 7b,d. The low CaCO₃ concentration in NS20 is because NS20 has a denser internal structure than PC, thus restricting CO₂ erosion. In PC, Ca(OH)₂ and CO₂ combined to form CaCO₃. Nano-SiO₂ and Ca(OH)₂ interact in NS20 to produce C-S-H gel. Because the C-S-H gel is amorphous, no discernible diffraction peaks are observed in Figure 8. However, Figure 7d,b show that NS20 has a larger C-S-H gel composition than does PC. C-S-H gel was used to fill the internal defect, thus decreasing the porosity of the concrete. Nano-SiO₂ can fill pores and micro-cracks and improve concrete compactness, resulting in a lower depth of carbonation of NS20 than normal concrete. Therefore, NS20 has a shallower carbonation depth than PC.

(3) Because PC hydration was insufficient and there were more dehydrated particles, NS20 exhibited a lower C₃S/C₂S ratio than PC. In addition to increasing cement hydration and lowering C₃S/C₂S concentration, the action and high activity of nano-pozzolanic SiO₂ also allow it to participate in secondary cement hydration. Ca(OH)₂, which is mostly formed by hydrating C₃S/C₂S, was generated in large quantities. The internal pores and micro-cracks of NS20 concrete are less than PC, the diffusion coefficient of CO₂ decreases, and the carbonation depth of NS20 is less than PC. As a result, NS20 exhibited a lower carbonation depth than PC, while the C₃S/C₂S of nano-concrete was lower than that of PC.

The carbonation resistance of NS20 was much higher than that of PC under bending force.

5. Conclusions

The following conclusions were drawn:

(1) As the concentration of nano-SiO₂ increased, the compressive and flexural strengths of nano-SiO₂-based concrete were enhanced to various degrees. The improved strength of the concrete was optimal when the optimal dosage (2%) of nano-SiO₂ was used. Compared with regular concrete, the compressive and flexural strengths of nano-SiO₂-based concrete both improved by 15.5 and 15.3%, respectively.

(2) With the gradual increase in nano-SiO₂ content, the carbonation depth of concrete in the tension zone and compression zone first decreased and then increased. The carbonation depth of nano marine concrete is less than that of ordinary marine concrete. When the stress levels were 0.15 and 0.6, the carbonation depths in the tensile zone of NS20 were 20.5% and 18.4% lower than those of PC, respectively. In addition, the carbonation depths in the compression zone were 18.9 and 23.7% lower than those of PC. Under bending strain, the carbonation depth of NS20 became the lowest.

(3) The carbonation resistance of concrete was significantly influenced by the carbonation age. The carbonation depth of concrete increased the most for the first 7 d, and roughly reached half of that depth after 28 days. The concrete carbonation depth in the tensile zone was greater than that in the compression zone at each age, and the carbonation depth of NS20 was lower than that of PC. The lowest carbonation depth was observed in NS20. At 3 d, the carbonation depth in the tensile zone and compression zone of NS20 and PC reduced by 31 and 50%, respectively. At 28 days, the carbonation depths in the tensile and compression zones of NS20 were lower than those of the PC by 18.4 and 23.7%, respectively. When the nano-SiO₂ doping amount is less than or equal to 2%, the carbonation resistance of concrete under loading increased.

(4) Bending load impacted the carbonation depth of concrete. With the gradual increase in stress level, the carbonation depth of the concrete tension zone gradually increased, while the carbonation depth of the concrete compression zone gradually decreased. Under each stress condition, the carbonation depth of concrete in the tensile zone was larger than that in the compression zone. The slope of the carbonation depth curve increased when the stress level was between 0.3 and 0.45, indicating that nano-SiO₂-based concrete had a lower carbonation depth than conventional concrete. The carbonation depths in the tensile zone of NS20 decreased by 31 and 18.4% at tensile stress levels of 0.3 and 0.45, respectively, compared with that of PC. The NS20 exhibited the best carbonation resistance, reducing the carbonation depths in the compression zone by 50 and 23.7% compared with PC.

(5) SEM testing revealed that the internal structure of NS20 was superior to PC. NS20 had fewer CaCO₃ crystals and more C-S-H gel than PC in the interfacial transition zone. To prevent the diffusion of CO₂ inside the concrete and decrease the depth of carbonation, nano-SiO₂ effectively enhanced the internal structure of concrete by reducing the width of the microcracks, the number of pores, and the number of microcracks. Nano-SiO₂ effectively enhanced the carbonation resistance ability of concrete under loading.

(6) XRD testing revealed that NS20 and PC had almost the same amounts of Ca(OH)₂. Compared with PC, NS20 had less CaCO₃ and C₃S/C₂S in the interfacial transition zone. In addition to the reduced amount of Ca(OH)₂ and inhibited carbonation, nano-SiO₂ improved cement hydration, tightened the internal structure of concrete, and increased the resistance of the concrete to carbonation under loading.

6. Prospect

Due to global warming, carbon dioxide concentrations are steadily rising, carbonation erosion is increasingly more harmful to concrete, and bending load is an important reason for the reduction of concrete carbonation resistance. In this study, two-factor research on marine concrete durability has been carried out, and in the future, we will carry out three-factor research on concrete durability.