Effect of SiO2 Sol/Silane Emulsion in Reducing Water and Chloride Ion Penetration in Concrete

Here, a new concrete hydrophobic treatment method is developed using SiO2 sol and silane emulsion. The effectiveness of the modification for concrete protection is evaluated through testing water absorption and chloride diffusion. Two types of concrete with different strength grades (C40, C50) are used as the research object. The results show that the water capillary absorption coefficient and chloride ion diffusion coefficient of concrete decrease greatly under the protection of SiO2 sol and silane emulsion. Additionally, the protection effect is better with the increase of SiO2 consumption. Contact angle test results reveal that when the coating amount of SiO2 sol and silane emulsion is 300 g/m2, respectively, the contact angle reaches 150.2◦, indicating the concrete (C40) surface reaches the superhydrophobic state. Through scanning electron microscope (SEM) observation, it is found that the hydrophobic effect of the SiO2 sol/silane emulsion is mainly due to the change in the surface morphology of concrete (C40).


Introduction
Concrete is a porous material with a large amount of pores and micro cracks on its surface, allowing water to penetrate into the concrete easily due to the capillary forces. Although there are many factors that can lead to concrete damage, water transport properties are still the direct factor leading to the decline of concrete durability [1]. Accordingly, chloride, sulphate, and other soluble aggressive agents can be absorbed by concrete, and their reaction with oxygen can rust the steel bars to cause the cracking of concrete [2][3][4][5]. The corrosion will propagate further, which results in structure failure which influences the durability of the structure. Therefore, if certain measures are not taken, it is inevitable that water will enter the concrete and cause corrosion.
Although many studies have been conducted on the evaluation of the waterproof performance of concrete, there is a necessity for more attention in this area. Previously, various strategies were used to protect concrete from water penetration [6][7][8]. Nowadays, several chemicals on the market are being applied to extend the service life of concrete structures. The impregnating agents (water repellents, hydrophobic agents) such as silane are a kind of very effective material to protect concrete from water penetration [9]. Silane can penetrate into concrete for a certain depth and interact with hydrated cement particles to form a water repellent layer which can repel water without sealing the pores of the concrete [10][11][12]. Thus, impregnating agents have almost the same service life as normal concrete.

Concrete
Cement used in this experiment is a commercial Portland cement type P.O. 42.5. The proportion of the concrete used in this study is shown in Table 1. Natural sand with a 2 mm maximum size and gravel with a 25 mm maximum size were used as fine and coarse aggregates, respectively. The concrete specimens were prepared using 100 mm × 100 mm × 100 mm molds (for the capillary water absorption test and the capillary chloride salt suction experiment) and 100 mm × 300 mm 2 molds (for the chloride ion permeability coefficient test). The samples were demolded after 24 h of curing, then they were placed in a curing room (20 ± 2 • C, relative humidity (RH) ≥ 90%) for 14 days. After 14 days, the samples were cut into two halves. Subsequently, these half cubes were placed in the humid room until they were 28 days. The compressive strength of the concrete with 100 mm × 100 mm × 100 mm after 28 days curing is shown in Table 1. Triethoxy(isobutyl)silane monomers were selected to synthesize the silane emulsion. Deionized water was used as the solvent. Span 80 (100% purity) and Peregal-O (100% purity) were used as the emulsifier. Polyethylene glycol 2000 (PEG2000) was used as the stabilizer. The synthesizing procedure was as follows: (i) a certain amount of monomer and emulsifier were added to a round-bottom flask; (ii) an aqueous solution of PEG2000 (50% concentration) was added into the flask over a magnetic Coatings 2020, 10, 682 3 of 12 stirrer maintained at 55 • C, and the mixture was stirred at the speed of 2000 r min −1 ; (iii) a silane emulsion was obtained after 7 h of reaction followed by cooling.
SiO 2 sol was prepared as normal from tetraethyl orthosilicate (TEOS, 100% purity), ethanol (EtOH, 99.5% purity), H 2 O and HNO 3 (65% purity). First, TEOS and EtOH were mixed with a molar ratio of 1:1, and stirred at room temperature for 10 min. Second, 0.1 M HNO 3 in water was added dropwise to the solution until the H 2 O and TEOS molar ratio of 4:1 was attained. Finally, the solution was then stirred at 50 • C for 2 h to obtain SiO 2 sol. Table 2 shows the characteristic of the silane emulsion and SiO 2 sol. The pH value was measured by a pH-meter (PHS-3C, INESA, Shanghai, China). The viscosity of the silane emulsion and SiO 2 sol were measured using a viscosity meter (DV-S, Brookfield, Middleboro, MA, USA). emulsifier. Polyethylene glycol 2000 (PEG2000) was used as the stabilizer. The synthesizing procedure was as follows: (i) a certain amount of monomer and emulsifier were added to a roundbottom flask; (ii) an aqueous solution of PEG2000 (50% concentration) was added into the flask over a magnetic stirrer maintained at 55 °C, and the mixture was stirred at the speed of 2000 r min −1 ; (iii) a silane emulsion was obtained after 7 h of reaction followed by cooling. SiO2 sol was prepared as normal from tetraethyl orthosilicate (TEOS, 100% purity), ethanol (EtOH, 99.5% purity), H2O and HNO3 (65% purity). First, TEOS and EtOH were mixed with a molar ratio of 1:1, and stirred at room temperature for 10 min. Second, 0.1 M HNO3 in water was added dropwise to the solution until the H2O and TEOS molar ratio of 4:1 was attained. Finally, the solution was then stirred at 50 °C for 2 h to obtain SiO2 sol. Table 2 shows the characteristic of the silane emulsion and SiO2 sol. The pH value was measured by a pH-meter (PHS-3C, INESA, Shanghai, China). The viscosity of the silane emulsion and SiO2 sol were measured using a viscosity meter (DV-S, Brookfield, Middleboro, MA, USA).

Formation of Hydrophobic Layer on Concrete Surface
Concrete specimens were dried in an oven at 60 °C for 48 h after being removed from the curing room. The surfaces of the specimens were polished and cleaned. The lateral side of the concrete specimens were sealed with epoxy resin, leaving the two opposite surfaces as coating-hydrophobic layers. First, SiO2 sol was applied in different amounts (Table 3) carefully using a brush on the surfaces of the specimens. Second, a silane emulsion was coated on the surface of the specimens after 3 h, and repeated two times, 6 h apart. Finally, the specimens were put aside in ambient conditions for 7 days before carrying out the tests. emulsifier. Polyethylene glycol 2000 (PEG2000) was used as the stabilizer. The synthesizing procedure was as follows: (i) a certain amount of monomer and emulsifier were added to a roundbottom flask; (ii) an aqueous solution of PEG2000 (50% concentration) was added into the flask over a magnetic stirrer maintained at 55 °C, and the mixture was stirred at the speed of 2000 r min −1 ; (iii) a silane emulsion was obtained after 7 h of reaction followed by cooling. SiO2 sol was prepared as normal from tetraethyl orthosilicate (TEOS, 100% purity), ethanol (EtOH, 99.5% purity), H2O and HNO3 (65% purity). First, TEOS and EtOH were mixed with a molar ratio of 1:1, and stirred at room temperature for 10 min. Second, 0.1 M HNO3 in water was added dropwise to the solution until the H2O and TEOS molar ratio of 4:1 was attained. Finally, the solution was then stirred at 50 °C for 2 h to obtain SiO2 sol. Table 2 shows the characteristic of the silane emulsion and SiO2 sol. The pH value was measured by a pH-meter (PHS-3C, INESA, Shanghai, China). The viscosity of the silane emulsion and SiO2 sol were measured using a viscosity meter (DV-S, Brookfield, Middleboro, MA, USA).

Formation of Hydrophobic Layer on Concrete Surface
Concrete specimens were dried in an oven at 60 °C for 48 h after being removed from the curing room. The surfaces of the specimens were polished and cleaned. The lateral side of the concrete specimens were sealed with epoxy resin, leaving the two opposite surfaces as coating-hydrophobic layers. First, SiO2 sol was applied in different amounts (Table 3) carefully using a brush on the surfaces of the specimens. Second, a silane emulsion was coated on the surface of the specimens after 3 h, and repeated two times, 6 h apart. Finally, the specimens were put aside in ambient conditions for 7 days before carrying out the tests. Table 3. Consumption of SiO2 sol and silane emulsion on concrete surface (g/m 2 ).

Type
SiO2 Sol Silane Emulsion

Formation of Hydrophobic Layer on Concrete Surface
Concrete specimens were dried in an oven at 60 • C for 48 h after being removed from the curing room. The surfaces of the specimens were polished and cleaned. The lateral side of the concrete specimens were sealed with epoxy resin, leaving the two opposite surfaces as coating-hydrophobic layers. First, SiO 2 sol was applied in different amounts (Table 3) carefully using a brush on the surfaces of the specimens. Second, a silane emulsion was coated on the surface of the specimens after 3 h, and repeated two times, 6 h apart. Finally, the specimens were put aside in ambient conditions for 7 days before carrying out the tests.

Capillary Water Absorption
To evaluate the effectiveness of the hydrophobic layer, a capillary water absorption test was carried out according to standard UNI EN 1015-18:2004 ( Figure 1a) [32]. Figure 1b shows concrete specimens were placed in a pan where they were immersed in deionized water at a depth of 3-6 mm above the exposed surface, at room temperature. The pan was covered to prevent water evaporation. The amount of absorbed water over time was measured by weighing the samples after 1, 2, 4, 6, 8, 12, 24, and 36 h. The water capillary coefficient typically fits the following equation [33]: ∆W is the mass of water absorbed per unit exposed area (g·m −2 ), t is the time (h).

Capillary Water Absorption
To evaluate the effectiveness of the hydrophobic layer, a capillary water absorption test was carried out according to standard UNI EN 1015-18:2004 ( Figure 1a) [32]. Figure 1b shows concrete specimens were placed in a pan where they were immersed in deionized water at a depth of 3-6 mm above the exposed surface, at room temperature. The pan was covered to prevent water evaporation. The amount of absorbed water over time was measured by weighing the samples after 1, 2, 4, 6, 8, 12, 24, and 36 h. The water capillary coefficient typically fits the following equation [33]: ΔW is the mass of water absorbed per unit exposed area (g·m −2 ), t is the time (h).
Water capillary sorption tests were carried out on three replicates of each sample.

Chloride Penetration
The procedure for the chloride penetration test was similar to that for the water absorption test, except that the water was replaced by a 3% NaCl solution. During the soaking test, the container was covered to prevent the change in density caused by evaporation. After soaking, the specimens were dried at 105 ± 5 °C. The chloride content of the concrete was analyzed using the Volhard Titration Method (BS EN 12469 (2007) [34]). The concrete powder was collected by a mill. The chloride content was reported as wt.% by cement weight. The chloride content values were averaged among the values obtained by the analyses carried out on three specimens of each concrete type.

Chloride Diffusion Coefficients
The chloride diffusion coefficient test was conducted in accordance to the procedure of the rapid chloride migration (RCM) method described by Luping Tang. Concrete slices of 50 mm in thickness were obtained by sawing the cylindrical specimen (100 mm × 300 mm 2 ). The slices were kept in an oven at 105 ± 5 °C until obtaining a constant mass. Then, the hydrophobic layer was applied to the circular surfaces of the specimens. After seven days, the specimens were saturated using the procedure of ASTM C 1202-97 [35]. The slice was placed between two cells. One cell was filled with a 0.3 mol L −1 NaOH solution and the other with a 10.0% NaCl solution. The cells were connected to a 30 V power source. The temperature of the test specimens and solution was 20-25 °C. The penetration depth of the chloride ions into the concrete was measured based on the depth of the colour change of a surface of concrete broken in the direction of the chloride flow using a 0.1 M AgNO3 solution.

Contact Angle Measurement
To evaluate the hydrophobic layer properties on concrete, the contact angle measurements were performed using an OCA-20 Contact Angle Instrument (Data Physics Instruments, Inc., Filderstadt, Germany). The optical system of the device adopts a digital CMOS camera. Water drops, with the Water capillary sorption tests were carried out on three replicates of each sample.

Chloride Penetration
The procedure for the chloride penetration test was similar to that for the water absorption test, except that the water was replaced by a 3% NaCl solution. During the soaking test, the container was covered to prevent the change in density caused by evaporation. After soaking, the specimens were dried at 105 ± 5 • C. The chloride content of the concrete was analyzed using the Volhard Titration Method (BS EN 12469 (2007) [34]). The concrete powder was collected by a mill. The chloride content was reported as wt.% by cement weight. The chloride content values were averaged among the values obtained by the analyses carried out on three specimens of each concrete type.

Chloride Diffusion Coefficients
The chloride diffusion coefficient test was conducted in accordance to the procedure of the rapid chloride migration (RCM) method described by Luping Tang. Concrete slices of 50 mm in thickness were obtained by sawing the cylindrical specimen (100 mm × 300 mm 2 ). The slices were kept in an oven at 105 ± 5 • C until obtaining a constant mass. Then, the hydrophobic layer was applied to the circular surfaces of the specimens. After seven days, the specimens were saturated using the procedure of ASTM C 1202-97 [35]. The slice was placed between two cells. One cell was filled with a 0.3 mol L −1 NaOH solution and the other with a 10.0% NaCl solution. The cells were connected to a 30 V power source. The temperature of the test specimens and solution was 20-25 • C. The penetration depth of the chloride ions into the concrete was measured based on the depth of the colour change of a surface of concrete broken in the direction of the chloride flow using a 0.1 M AgNO 3 solution.

Contact Angle Measurement
To evaluate the hydrophobic layer properties on concrete, the contact angle measurements were performed using an OCA-20 Contact Angle Instrument (Data Physics Instruments, Inc., Filderstadt, Germany). The optical system of the device adopts a digital CMOS camera. Water drops, with the size of about 5 µL, were placed at five different points on the specimen surface and the contact angle was measured immediately (Figure 2). The reported contact angle values were averages of five measurements conducted for each test. During the test, an automatic ring fitting method was used to calculate the contact angle.
Coatings 2020, 10, x FOR PEER REVIEW 5 of 12 size of about 5 μL, were placed at five different points on the specimen surface and the contact angle was measured immediately (Figure 2). The reported contact angle values were averages of five measurements conducted for each test. During the test, an automatic ring fitting method was used to calculate the contact angle.

SEM
The morphologies of the treated concrete surfaces by hydrophobic layer, prepared in this study, were investigated using SEM (S3500N, HITACH, Tokyo, Japan). SEM test samples were taken from the concrete surface to a 4 mm depth using a cutting method.

Effect of SiO2 Sol/Silane Emulsion on Water Contact Angle on Concrete
During the contact angle test, C40 concrete samples were taken as the test object. The main reason is that the porosity of C40 concrete is larger than that of C50 concrete, thus it more easily absorbs water. When a superhydrophobic structure can be constructed on the surface of a C40 concrete, it will be more practical. Figure 3a shows the contact angle of the untreated concrete. We can see that the water drop spread on the concrete surface and the contact angle values were just 65.6°. It is believed that concrete is a hydrophilic material. When water contacts with concrete, it can penetrate into the concrete surface by means of capillary forces spontaneously.
When the silane emulsion was used on the concrete surface, the concrete achieved hydrophobicity. Figure 3b shows the images of water droplets on the concrete surface treated with silane emulsion, as well as the value of the contact angle. It was found that the contact angle increased by more than 95%, to achieve 128.4°. Moreover, when the SiO2 sol was applied onto the concrete surface only, the contact angle increased slightly to 96.2° compared with the untreated surface ( Figure  3c). This shows that the hydrophobic effect of concrete is not good if SiO2 sol is used alone.

SEM
The morphologies of the treated concrete surfaces by hydrophobic layer, prepared in this study, were investigated using SEM (S3500N, HITACH, Tokyo, Japan). SEM test samples were taken from the concrete surface to a 4 mm depth using a cutting method.

Effect of SiO 2 Sol/Silane Emulsion on Water Contact Angle on Concrete
During the contact angle test, C40 concrete samples were taken as the test object. The main reason is that the porosity of C40 concrete is larger than that of C50 concrete, thus it more easily absorbs water. When a superhydrophobic structure can be constructed on the surface of a C40 concrete, it will be more practical. Figure 3a shows the contact angle of the untreated concrete. We can see that the water drop spread on the concrete surface and the contact angle values were just 65.6 • . It is believed that concrete is a hydrophilic material. When water contacts with concrete, it can penetrate into the concrete surface by means of capillary forces spontaneously.
Coatings 2020, 10, x FOR PEER REVIEW 5 of 12 size of about 5 μL, were placed at five different points on the specimen surface and the contact angle was measured immediately (Figure 2). The reported contact angle values were averages of five measurements conducted for each test. During the test, an automatic ring fitting method was used to calculate the contact angle.

SEM
The morphologies of the treated concrete surfaces by hydrophobic layer, prepared in this study, were investigated using SEM (S3500N, HITACH, Tokyo, Japan). SEM test samples were taken from the concrete surface to a 4 mm depth using a cutting method.

Effect of SiO2 Sol/Silane Emulsion on Water Contact Angle on Concrete
During the contact angle test, C40 concrete samples were taken as the test object. The main reason is that the porosity of C40 concrete is larger than that of C50 concrete, thus it more easily absorbs water. When a superhydrophobic structure can be constructed on the surface of a C40 concrete, it will be more practical. Figure 3a shows the contact angle of the untreated concrete. We can see that the water drop spread on the concrete surface and the contact angle values were just 65.6°. It is believed that concrete is a hydrophilic material. When water contacts with concrete, it can penetrate into the concrete surface by means of capillary forces spontaneously.
When the silane emulsion was used on the concrete surface, the concrete achieved hydrophobicity. Figure 3b shows the images of water droplets on the concrete surface treated with silane emulsion, as well as the value of the contact angle. It was found that the contact angle increased by more than 95%, to achieve 128.4°. Moreover, when the SiO2 sol was applied onto the concrete surface only, the contact angle increased slightly to 96.2° compared with the untreated surface ( Figure  3c). This shows that the hydrophobic effect of concrete is not good if SiO2 sol is used alone.   When the silane emulsion was used on the concrete surface, the concrete achieved hydrophobicity. Figure 3b shows the images of water droplets on the concrete surface treated with silane emulsion, as well as the value of the contact angle. It was found that the contact angle increased by more than 95%, to achieve 128.4 • . Moreover, when the SiO 2 sol was applied onto the concrete surface only, the contact angle increased slightly to 96.2 • compared with the untreated surface (Figure 3c). This shows that the hydrophobic effect of concrete is not good if SiO 2 sol is used alone.
The contact angles for the concrete surfaces treated by the SiO 2 sol/silane emulsion with different SiO 2 sol amounts are given in Figure 4a-c. It can be seen that, with the use of SiO 2 sol, the contact angle of the concrete surface was enhanced to about 140 • . Moreover, with the growing amount of SiO 2 sol, the contact angle increased constantly. Moreover, the Figures show when the 100 g/m 2 SiO 2 sol was used on the concrete surface, the contact angle was 133.76 • . When the SiO 2 sol amount increased to 300 g/m 2 , however, the superhydrophobic surface was obtained because the contact angle achieved was about 150.2 • .
Coatings 2020, 10, x FOR PEER REVIEW 6 of 12 The contact angles for the concrete surfaces treated by the SiO2 sol/silane emulsion with different SiO2 sol amounts are given in Figure 4a-c. It can be seen that, with the use of SiO2 sol, the contact angle of the concrete surface was enhanced to about 140°. Moreover, with the growing amount of SiO2 sol, the contact angle increased constantly. Moreover, the Figures show when the 100 g/m 2 SiO2 sol was used on the concrete surface, the contact angle was 133.76°. When the SiO2 sol amount increased to 300 g/m 2 , however, the superhydrophobic surface was obtained because the contact angle achieved was about 150.2°. Regarding the Cassie-Baxter regime mentioned previously [36], a two-length-scale, micro-and nano-hierarchical surface roughness, and a hydrophobic surface are important to achieve superhydrophobicity. Here, the super-hydrophobic effect was the consequence of the combination of the SiO2 sol, which introduced the micro-and nano-roughness, and the silane emulsion, which provided the hydrophobic surface on the concrete. Concurrently, it should be noted that the SiO2 sol/silane emulsion is not considered a type of paint, but an impregnation agent.

Effect of SiO2 Sol/Silane Emulsion on Water Absorption in Concrete
The decrease in water uptake of concrete surfaces can be a criterion for the evaluation of the efficacy of the hydrophobic treatment. To verify if the SiO2 sol/silane emulsion layers could improve the resistance of the treated concrete to the penetration of water, capillary absorption tests were performed. Figure 5a,b shows the results of water capillary absorption tests obtained for concrete specimens of C50 and C40, respectively. The curves demonstrate that the application of SiO2 sol/silane emulsion produced a drastic decrease in the capillary absorption values compared to the untreated samples. To compare, the treatment with the SiO2 sol/silane emulsion was more efficient than silane emulsion alone. Table 4 presents the results of the capillary water absorption coefficients, which is the weight of water that penetrates in concrete per unit of area and time. The water absorption coefficients of the untreated concrete specimens of C50 and C40 at 24 h was 45.83 g·m −2 ·h −0.5 and 53.71 g·m −2 ·h −0.5 , respectively. When the silane emulsion was coated on the concrete, the water absorption coefficients reduced by about 70%. However, the most effective treatments were those based on the SiO2 sol/silane emulsion layer, which reduced the water absorption amount by more than 75% compared to the control. The superiority of the SiO2 sol/silane emulsion is believed to be the result of the rise in the contact angle between the water and the concrete surface. These changes are coherent with the contact angle data discussed above.
It should be pointed out that the enhancement of the protection efficiency depends on the amount of SiO2 sol coated on the concrete surface. Initially (S0), silane emulsion forms a thin protective film on the surface. The addition of SiO2 sol (S1, S2, S3) leads to the formation of superficial nano-sized protrusions caused by the reaction of SiO2 and Ca(OH)2 on the concrete surface [37]. Using Regarding the Cassie-Baxter regime mentioned previously [36], a two-length-scale, micro-and nano-hierarchical surface roughness, and a hydrophobic surface are important to achieve super-hydrophobicity. Here, the super-hydrophobic effect was the consequence of the combination of the SiO 2 sol, which introduced the micro-and nano-roughness, and the silane emulsion, which provided the hydrophobic surface on the concrete. Concurrently, it should be noted that the SiO 2 sol/silane emulsion is not considered a type of paint, but an impregnation agent.

Effect of SiO 2 Sol/Silane Emulsion on Water Absorption in Concrete
The decrease in water uptake of concrete surfaces can be a criterion for the evaluation of the efficacy of the hydrophobic treatment. To verify if the SiO 2 sol/silane emulsion layers could improve the resistance of the treated concrete to the penetration of water, capillary absorption tests were performed. Figure 5a,b shows the results of water capillary absorption tests obtained for concrete specimens of C50 and C40, respectively. The curves demonstrate that the application of SiO 2 sol/silane emulsion produced a drastic decrease in the capillary absorption values compared to the untreated samples. To compare, the treatment with the SiO 2 sol/silane emulsion was more efficient than silane emulsion alone. Table 4 presents the results of the capillary water absorption coefficients, which is the weight of water that penetrates in concrete per unit of area and time. The water absorption coefficients of the untreated concrete specimens of C50 and C40 at 24 h was 45.83 g·m −2 ·h −0.5 and 53.71 g·m −2 ·h −0.5 , respectively. When the silane emulsion was coated on the concrete, the water absorption coefficients reduced by about 70%. However, the most effective treatments were those based on the SiO 2 sol/silane emulsion layer, which reduced the water absorption amount by more than 75% compared to the control. The superiority of the SiO 2 sol/silane emulsion is believed to be the result of the rise in the contact Coatings 2020, 10, 682 7 of 12 angle between the water and the concrete surface. These changes are coherent with the contact angle data discussed above.

Effect of SiO2 Sol/Silane Emulsion on Chloride Ion Diffusion in Concrete
It is believed that the water capillary action during the transport is the fundamental role of Chloride ions entering into the concrete, especially near the unsaturated concrete surface. To understand the influence of the concrete strength grade on the protection efficacy of the SiO2 sol/silane emulsion on preventing chloride ion penetration, C40 and C50 concrete were used. Figure  6a,b shows chloride profiles in the various depth ranges of C40 and C50, respectively. Considering the Figures, it is observed that both silane emulsion and SiO2 sol/silane emulsion can reduce deep chloride ion penetration relative to the control sample. Similar to the capillary water absorption test, SiO2 sol/silane emulsion performs better than silane emulsion. Regarding a 6 mm depth in C40 concrete, for example, SiO2 sol/silane emulsion gives an approximately 29.6%-56.6% reduction in chloride ion concentration, while silane emulsion gives approximately 14.8%.
Moreover, chloride ion content was decreased gradually with the increased coating amount of SiO2 sol. The observed effect also can be explained by changes in the surface contact angle of the treated concrete caused by the addition of SiO2 sol.   It should be pointed out that the enhancement of the protection efficiency depends on the amount of SiO 2 sol coated on the concrete surface. Initially (S0), silane emulsion forms a thin protective film on the surface. The addition of SiO 2 sol (S1, S2, S3) leads to the formation of superficial nano-sized protrusions caused by the reaction of SiO 2 and Ca(OH) 2 on the concrete surface [37]. Using the 300 g/m 2 of SiO 2 sol, the SiO 2 sol/silane emulsion offers sufficient protection for concrete.

Effect of SiO 2 Sol/Silane Emulsion on Chloride Ion Diffusion in Concrete
It is believed that the water capillary action during the transport is the fundamental role of Chloride ions entering into the concrete, especially near the unsaturated concrete surface. To understand the influence of the concrete strength grade on the protection efficacy of the SiO 2 sol/silane emulsion on preventing chloride ion penetration, C40 and C50 concrete were used. Figure 6a,b shows chloride profiles in the various depth ranges of C40 and C50, respectively. Considering the Figures, it is observed that both silane emulsion and SiO 2 sol/silane emulsion can reduce deep chloride ion penetration relative to the control sample. Similar to the capillary water absorption test, SiO 2 sol/silane emulsion performs better than silane emulsion. Regarding a 6 mm depth in C40 concrete, for example, SiO 2 sol/silane emulsion gives an approximately 29.6%-56.6% reduction in chloride ion concentration, while silane emulsion gives approximately 14.8%.
Moreover, chloride ion content was decreased gradually with the increased coating amount of SiO 2 sol. The observed effect also can be explained by changes in the surface contact angle of the treated concrete caused by the addition of SiO 2 sol. Table 5 presents the results of chloride ion penetration depth and diffusion coefficients determined using the RCM method. The results indicate that the application of silane emulsion and SiO 2 sol/silane emulsion on the concrete surface causes the reduction of the chloride ion penetration depth and diffusion coefficient. Regarding C50 concrete treated with a SiO 2 sol/silane emulsion, the chloride diffusion coefficient, ranged only by 1.4-1.1 × 10 −12 m 2 ·s −1 , is about one-tenth of that of the concrete without treatment. Concurrently, the penetration depth of the chloride ions is reduced about 64.1%-57.5%. 6a,b shows chloride profiles in the various depth ranges of C40 and C50, respectively. Considering the Figures, it is observed that both silane emulsion and SiO2 sol/silane emulsion can reduce deep chloride ion penetration relative to the control sample. Similar to the capillary water absorption test, SiO2 sol/silane emulsion performs better than silane emulsion. Regarding a 6 mm depth in C40 concrete, for example, SiO2 sol/silane emulsion gives an approximately 29.6%-56.6% reduction in chloride ion concentration, while silane emulsion gives approximately 14.8%.
Moreover, chloride ion content was decreased gradually with the increased coating amount of SiO2 sol. The observed effect also can be explained by changes in the surface contact angle of the treated concrete caused by the addition of SiO2 sol.  However, when concrete with a lower strength grade is used, the surface treatment seems more effective in reducing chloride penetration. Particularly, the concrete with a SiO 2 sol/silane emulsion treatment is always able to reduce the chloride diffusion coefficient significantly by at least 83.7% when C40 concrete is used.
It should be noted that this type of result is only related to the penetration of chloride ions by diffusion (saturated concrete), excluding chloride penetration by absorption of contaminated water. Therefore, the SiO 2 sol/silane emulsion treatment does not primarily inhibit chloride ion penetration. The main effect is to reduce the water penetration, which may be contaminated with chloride ions. It is important to verify again that the SiO 2 sol/silane emulsion presented efficiency greater than the silane emulsion.

Microstructure Analysis
To analyze the changes of the concrete surface morphology, untreated and treated concrete samples using silane emulsion and SiO 2 sol/silane emulsion were examined using SEM. Figure 7a shows the SEM image of the untreated concrete sample surface, which is rough and some pores or cracks are present. Figure 7b is the SEM image of the concrete surface treated by silane emulsion. The image clearly shows that the silane emulsion does not significantly modify the substrate morphology, as expected. However, there was a remarkable change in the surface morphology of the concrete treated by the SiO 2 sol/silane emulsion (Figure 7c). The pre-treated by SiO 2 sol on concrete surface improves their pore coverage ability since the nano-SiO 2 tend to fill in the pores in a concrete surface [38,39]. Moreover, SiO 2 sol can react with the hydration product Ca(OH) 2 to form a secondary hydration C-S-H gel, further increasing the density of the concrete surface [40,41]. Consequently, the pore size and number for absorption or permeation of water into the concrete are effectively reduced compared to the concrete treated with the silane emulsion only. This verified that the treatment with a SiO 2 sol/silane emulsion has a higher protective effectiveness than silane emulsion alone.  Figure 8 shows the results of an energy dispersive spectrometer (EDS) test of the concrete treated by SiO2 sol/silane emulsion (S3). Figure 8 shows that the contents of the Si and C elements are greatly increased. Among them, the C element mainly comes from carbon in silane. Table 6 provides the test results of spot scanning and surface scanning during EDS analysis. Considering the table we can see that, when the concrete (C40) was treated with SiO2 sol only, the silicon content increased by 81.7% compared to that of the untreated concrete but, when the SiO2 sol/silane emulsion (S3) was used, the silicon content increased by 249.5%. Similarly, the content of silicon on the concrete surface also was greatly improved in the results of the surface scanning test, indicating that the SiO2 sol/silane emulsion (S3) could be distributed evenly on the concrete surface.    Figure 8 shows that the contents of the Si and C elements are greatly increased. Among them, the C element mainly comes from carbon in silane. Table 6 provides the test results of spot scanning and surface scanning during EDS analysis. Considering the table we can see that, when the concrete (C40) was treated with SiO 2 sol only, the silicon content increased by 81.7% compared to that of the untreated concrete but, when the SiO 2 sol/silane emulsion (S3) was used, the silicon content increased by 249.5%. Similarly, the content of silicon on the concrete surface also was greatly improved in the results of the surface scanning test, indicating that the SiO 2 sol/silane emulsion (S3) could be distributed evenly on the concrete surface.  Figure 8 shows the results of an energy dispersive spectrometer (EDS) test of the concrete treated by SiO2 sol/silane emulsion (S3). Figure 8 shows that the contents of the Si and C elements are greatly increased. Among them, the C element mainly comes from carbon in silane. Table 6 provides the test results of spot scanning and surface scanning during EDS analysis. Considering the table we can see that, when the concrete (C40) was treated with SiO2 sol only, the silicon content increased by 81.7% compared to that of the untreated concrete but, when the SiO2 sol/silane emulsion (S3) was used, the silicon content increased by 249.5%. Similarly, the content of silicon on the concrete surface also was greatly improved in the results of the surface scanning test, indicating that the SiO2 sol/silane emulsion (S3) could be distributed evenly on the concrete surface.

Conclusions
A new method of concrete surface hydrophobic treatment was prepared using SiO 2 sol and silane emulsion. The consumption of SiO 2 sol is essential for the hydrophobization of the treated concrete surfaces. Regarding the case of an appropriate coating amount (SiO 2 sol and silane emulsion is 300 g/m 2 , respectively), with application using the same methodology, the superhydrophobic effect was achieved and the water contact angle reached 150.2 • . Concerning concrete of either C50 or C40, the water capillary absorption coefficient was reduced by 80.5% and 73.4% at most, respectively. Therefore, the combination of SiO 2 sol and silane emulsion can reduce the water capillary absorption of concrete effectively. Due to the decrease in water absorption, it is difficult for chloride ions to diffuse into the concrete. The chloride ion diffusion coefficient of concrete of either C50 or C40 was only 1.1 × 10 −12 m 2 ·s −1 and 1.2 × 10 −12 m 2 ·s −1 , respectively. The excellent hydrophobic effect mainly comes from the improvement of the microstructure of the concrete surface.