Preparation and Evaluation of Exhaust-Purifying Cement Concrete Employing Titanium Dioxide

To address the increasing air pollution caused by vehicle exhaust, environment-friendly pavement materials that possesses exhaust-purifying properties were prepared using common cement concrete and porous cement concrete as the base of photocatalyst nano-titanium dioxide (TiO2), respectively. Firstly, Fe3+-doped TiO2 powder was prepared by applying planetary high-energy ball milling in order to improve the efficiency of the semiconductor photocatalyst for degrading vehicle exhausts. Two nano-TiO2, namely the original and modified nanomaterials, were adopted to produce the photocatalytic cement concretes subsequently. The physicochemical properties of the modified powder, as well as the mechanical and photocatalytic properties of TiO2-modified concrete, were characterized using a suite of complementary techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), compressive strength and degradation efficiency tests. The results show that the ball milling method not only successfully doped Fe3+ into catalysts but also caused significant changes in: (1) decreased particle sizes, (2) more amorphous morphology, (3) decreased percentage of the most thermodynamically stable crystal facet, and (4) increased percentage of other high gas sensing crystal facets. Both the original and modified nano-TiO2 can improve the concrete strength while the strengthening effect of modified nanomaterials is superior. It is pronounced that the photocatalytic property of the modified nano-TiO2 is much better than that of the original nano particles, and the degradation rate of porous concrete is also better than common concrete when exposed to the same photocatalyst content. In a comprehensive consideration of both mechanical performance and degradation efficiency, the recommended optimum dosage of TiO2 is 3% to 4% for exhaust-purifying concrete.


Introduction
With the acceleration of the urbanization process and car ownership, vehicle exhaust has become one of the main causes of air pollutions all over the world, especially in the developing and densely populated countries, such as China, for which the vehicle exhaust that comes from fossil fuels has accounted for about 25% of the total industrial exhaust emissions [1,2]. Air pollutants, mainly represented by nitrogen oxides (NO x ) and sulfur oxides (SO x ) in the emissions, can cause photochemical smog, haze, and acid rains. This poses great threat to ecosystems as well as the wellbeing of human and other species if it reaches a certain level of concentration in the atmosphere [3][4][5]. During the past decade, various countermeasures have been adopted to reduce vehicle emissions, such as using cleaner fuel, installing a gas-cleaning device in vehicles, and even developing electromobiles [2]. Nevertheless, vehicle exhaust is still a major problem, especially due to the rapid growth of the transportation sector.
The Chinese 42.5 ordinary Portland cement from Jidong (Tangshan, China) was used to prepare concrete samples, along with coarse aggregate (maximum size of 20.0 mm) and fine aggregate (clean, natural sand, maximum size of 4.75 mm, fineness modulus of 2.3). The chemical agent superplasticizer from Subote (Nanjing, China), with a 25% solid concentration, was used at a dosage of 0.8% by mass of cement. The grading of porous concrete coarse aggregate is the key factor affecting its permeability and mechanical properties. After determining the target porosity and water-cement ratio of concrete, the mixing ratio was calculated according to the volume method. The coarse aggregate gradation used in this study, combined with those in previous researches, is shown in Table 2 [20][21][22]. In contrast to the commonly used chemical modification method for nano-TiO 2 that requires complex raw materials and operation processes, the planetary high energy ball milling method was used in this study to prepare the iron-doped nano-TiO 2 photocatalyst, of which the process is simpler and the production is continuous. During the milling process, TiO 2 powder undergoes severe plastic deformation due to strong collision, resulting in serious distortion of the lattice of TiO 2 , a large increase of internal defects, and improved catalyst activity. At the same time the ball milling process will produce a lot of heat, coupled with mechanical force to induce the separation of Fe 3+ from ferric nitrate and its diffuse into the distorted TiO 2 lattice, resulting in the formation of a new iron-doped photocatalyst which can reduce the probability of electron recombination and finally achieve the improvement of the photocatalytic performance [19,23].
Based on the above principle, the preparation process involves firstly weighing nano-TiO 2 and ferric nitrate in the amount of 100 g and 15 g, respectively. Then the starting materials were manually premixed and poured into the ball mill jar. The rotational speed and the ball-to-powder weight ratio were 300 rpm and 30:1, respectively. After 3 h of ball milling process, the mixtures were put into a muffle furnace, and calcined at 400 • C for 3 h. Subsequently, the obtained samples were washed by deionized water three times to remove any residual ferric nitrate. At last, the samples were dried at 90 • C for 6 h and then ground into powders. The high energy ball milling setup is shown in Figure 1. The original and modified nano-TiO 2 are shown in Figure 2.

Preparation of Concrete Samples
Two types of photocatalytic cement concretes were prepared, namely, the common concrete and the porous concrete, and both of the two TiO2 products (original and modified) were used for each type of concrete. That is to say, a total of four sets of photocatalytic concretes were prepared. Nanomaterials were used to replace the quantity of cement in the amount of 0%, 1%, 2%, 3%, 4%, and 5%, respectively. The common concrete reference ratio is cement:water:sand:gravel:water-reducing agent, 395:178:621:1205:4. The porous concrete reference ratio is cement:water:gravel:water-reducing agent, 252:100:1542:2. The experiment program is summarized in Table 3, and the contents for the two TiO2 products were the same.
The preparation process of concrete in this study is described as follows: Firstly, mixing the powder and coarse aggregate for 1 min in dry situation, then adding water slowly and continually mixing for 2 min, and finally forming a 100 mm × 100 mm × 100 mm cube test block. After 24 h of curing, cube test blocks were removed from molds and put into a standard curing room to keep curing at 20 ± 2 °C and relative humidity ≥ 95%. When the curing age arrived at 28 days, the blocks were taken out to carry on experiments of mechanical and photocatalytic properties. The compressive strength of each group of concrete samples was tested in accordance with T 0553 in JTG E30-2005 "Test Methods of Cement and Concrete for Highway Engineering" [24]. Table 3. Experiment program. Nano-TiO2 content/%  0  1  2  3  4  5  0  1  2  3  4  5  Code C-0 C-1 C-2 C-3 C-4 C-5 P-0 P-1 P-2 P-3 P-4 P-5 2.2.3. Test of Photocatalytic Degradation Efficiency

Preparation of Concrete Samples
Two types of photocatalytic cement concretes were prepared, namely, the common concrete and the porous concrete, and both of the two TiO2 products (original and modified) were used for each type of concrete. That is to say, a total of four sets of photocatalytic concretes were prepared. Nanomaterials were used to replace the quantity of cement in the amount of 0%, 1%, 2%, 3%, 4%, and 5%, respectively. The common concrete reference ratio is cement:water:sand:gravel:water-reducing agent, 395:178:621:1205:4. The porous concrete reference ratio is cement:water:gravel:water-reducing agent, 252:100:1542:2. The experiment program is summarized in Table 3, and the contents for the two TiO2 products were the same.
The preparation process of concrete in this study is described as follows: Firstly, mixing the powder and coarse aggregate for 1 min in dry situation, then adding water slowly and continually mixing for 2 min, and finally forming a 100 mm × 100 mm × 100 mm cube test block. After 24 h of curing, cube test blocks were removed from molds and put into a standard curing room to keep curing at 20 ± 2 °C and relative humidity ≥ 95%. When the curing age arrived at 28 days, the blocks were taken out to carry on experiments of mechanical and photocatalytic properties. The compressive strength of each group of concrete samples was tested in accordance with T 0553 in JTG E30-2005 "Test Methods of Cement and Concrete for Highway Engineering" [24]. C-0 C-1 C-2 C-3 C-4 C-5 P-0 P-1 P-2 P-3 P-4 P-5

Preparation of Concrete Samples
Two types of photocatalytic cement concretes were prepared, namely, the common concrete and the porous concrete, and both of the two TiO 2 products (original and modified) were used for each type of concrete. That is to say, a total of four sets of photocatalytic concretes were prepared. Nanomaterials were used to replace the quantity of cement in the amount of 0%, 1%, 2%, 3%, 4%, and 5%, respectively. The common concrete reference ratio is cement:water:sand:gravel:water-reducing agent, 395:178:621:1205:4. The porous concrete reference ratio is cement:water:gravel:water-reducing agent, 252:100:1542:2. The experiment program is summarized in Table 3, and the contents for the two TiO 2 products were the same.
The preparation process of concrete in this study is described as follows: Firstly, mixing the powder and coarse aggregate for 1 min in dry situation, then adding water slowly and continually mixing for 2 min, and finally forming a 100 mm × 100 mm × 100 mm cube test block. After 24 h of curing, cube test blocks were removed from molds and put into a standard curing room to keep curing at 20 ± 2 • C and relative humidity ≥ 95%. When the curing age arrived at 28 days, the blocks were taken out to carry on experiments of mechanical and photocatalytic properties. The compressive strength of each group of concrete samples was tested in accordance with T 0553 in JTG E30-2005 "Test Methods of Cement and Concrete for Highway Engineering" [24]. C-0 C-1 C-2 C-3 C-4 C-5 P-0 P-1 P-2 P-3 P-4 P-5

Test of Photocatalytic Degradation Efficiency
The photocatalytic degradation efficiency was tested in a custom-designed testing system that contains a continuous gas flow reactor according to the Italian Standard UNI 11247 [7,25], as shown in Figure 3 [26]. The test samples were the pre-prepared concrete blocks. The device mainly consisted of the following four parts: A sample reaction chamber, a simulation photo source, a standard gas source, and a gas analyzer. Among these four parts, the reaction chamber was made of borosilicate glass in order to prevent the chamber from absorbing pollutant gas and UV light. The surface of the samples placed in the reaction chamber was kept parallel to the lamp to ensure that the surfaces of samples were uniformly illuminated. The simulation photo source comprised of a mercury vapor lamp and an incandescent lamp on the upper side of the reaction chamber, with the wavelength range of 300 to 700 nm, which covers most of the visible range of sunlight [27]. The radiant energy was 48 W/m 2 , which is close to the sun radiation intensity at mid-latitudes [3]. On both side of the reaction chamber, there was a gas inlet and a gas outlet, respectively, which were used to supply gas for tests and extract gas for the analyzer. The gas analyzer was located at the same side with gas outlet. A small fan was placed at the corner of the reaction chamber to ensure test gas reacts completely on the specimen surface.
According to the main components of automobile exhaust gas, NO was used as the main pollutant model of photocatalytic degradation experiment. The concentration of NO used during the test was 200.0 ppb according to former studies [4,9]. The gas analyzer NHA-508 (Anhua, Foshan, China) was used as the detection device for NO concentration. The photocatalytic degradation efficiency was tested in a custom-designed testing system that contains a continuous gas flow reactor according to the Italian Standard UNI 11247 [7,25], as shown in Figure 3 [26]. The test samples were the pre-prepared concrete blocks. The device mainly consisted of the following four parts: A sample reaction chamber, a simulation photo source, a standard gas source, and a gas analyzer. Among these four parts, the reaction chamber was made of borosilicate glass in order to prevent the chamber from absorbing pollutant gas and UV light. The surface of the samples placed in the reaction chamber was kept parallel to the lamp to ensure that the surfaces of samples were uniformly illuminated. The simulation photo source comprised of a mercury vapor lamp and an incandescent lamp on the upper side of the reaction chamber, with the wavelength range of 300 to 700 nm, which covers most of the visible range of sunlight [27]. The radiant energy was 48 W/m 2 , which is close to the sun radiation intensity at mid-latitudes [3]. On both side of the reaction chamber, there was a gas inlet and a gas outlet, respectively, which were used to supply gas for tests and extract gas for the analyzer. The gas analyzer was located at the same side with gas outlet. A small fan was placed at the corner of the reaction chamber to ensure test gas reacts completely on the specimen surface.
According to the main components of automobile exhaust gas, NO was used as the main pollutant model of photocatalytic degradation experiment. The concentration of NO used during the test was 200.0 ppb according to former studies [4,9]. The gas analyzer NHA-508 (Anhua, Foshan, China) was used as the detection device for NO concentration.

Microstructure Analysis
The morphologies of the powdered TiO2 and the inner structure of concrete samples were observed with scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan). The surface elemental analysis of the samples was performed using energy dispersive X-ray spectroscopy (EDS) Figure 3. Photocatalytic degradation test equipment [26]. (a) Device appearance and (b) Device principle.

Microstructure Analysis
The morphologies of the powdered TiO 2 and the inner structure of concrete samples were observed with scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan). The surface elemental analysis of the samples was performed using energy dispersive X-ray spectroscopy (EDS) [13,28]. After the mechanical test, the samples subject to SEM analysis were taken from the fragments caused by the mechanical test, and conductive coating with gold was applied after drying treatment to prevent charging effects. In addition, the powder X-ray diffraction method (XRD, Bruker D8 Advance, Berlin, Germany) was employed to investigate the influence of modification on the crystal phase of the powdered TiO 2 with Ni-filtered Cu Kα radiation from 15 • to 80 • .

Characterization of TiO 2 Samples
The effect of the planetary high energy ball milling treatment on the catalyst morphology and crystal structure was investigated by SEM and XRD tests, prior to preparing concrete samples. The SEM results are shown in Figure 4 and its energy spectrum is shown in Figure 5. It can be seen from Figure 4 that the amorphous nanoparticles were distributed unevenly and there were obvious agglomerations in both samples. However, the TiO 2 particles were more prone to agglomeration after ball milling, which can be partially attributed to the reduction of particle sizes, as well as the high surface energy that was caused by ball milling. Therefore, the modified TiO 2 appears to be more amorphous and finer, providing more surface area for the activation of photocatalytic activity. The EDS spectrum verifies that the ball milling procedure successfully introduces iron into TiO 2 while the spectrum of original TiO 2 has no iron.
According to the XRD patterns of the original and modified TiO 2 shown in Figure 6, the diffraction peaks of both samples showed fingerprint features of an anatase TiO 2 structure with no other characteristic peaks observed, indicating the phase purity of the samples. The percentage of diffraction intensity at different crystal facets is depicted in Table 4. It can be seen that the percentage of the (101) peak, which is the most thermodynamically stable and has relatively low chemical activity [19], decreases sharply from 51.84% to 48.47% after applying the ball milling procedure. The percentages of the other peaks for Fe 3+ -doped TiO 2 , such as (004), (105), (211), and (116), which have high gas sensing activity [29,30], are all increased to some degree. Correspondingly, the percentage of (101) crystal facet is decreased, which indicates that the preferred orientation of (101) has been compromised.
On the basis of these data, the average crystal sizes were calculated according to Scherrer's equation [31]. For the original and modified TiO 2 , the results were 21.16 nm and 20.78 nm, respectively. It demonstrates that ball milling favors the formation of small crystallites, leading to more intense diffraction peaks, with the decrease of (101) crystal facet. In summary, it can be concluded that the planetary high energy ball mill can dope Fe 3+ ions into nano-TiO 2 , making it successfully modified.
Materials 2018, 11, x FOR PEER REVIEW 6 of 13 [13,28]. After the mechanical test, the samples subject to SEM analysis were taken from the fragments caused by the mechanical test, and conductive coating with gold was applied after drying treatment to prevent charging effects. In addition, the powder X-ray diffraction method (XRD, Bruker D8 Advance, Berlin, Germany) was employed to investigate the influence of modification on the crystal phase of the powdered TiO2 with Ni-filtered Cu Kα radiation from 15° to 80°.

Characterization of TiO2 Samples
The effect of the planetary high energy ball milling treatment on the catalyst morphology and crystal structure was investigated by SEM and XRD tests, prior to preparing concrete samples. The SEM results are shown in Figure 4 and its energy spectrum is shown in Figure 5. It can be seen from Figure 4 that the amorphous nanoparticles were distributed unevenly and there were obvious agglomerations in both samples. However, the TiO2 particles were more prone to agglomeration after ball milling, which can be partially attributed to the reduction of particle sizes, as well as the high surface energy that was caused by ball milling. Therefore, the modified TiO2 appears to be more amorphous and finer, providing more surface area for the activation of photocatalytic activity. The EDS spectrum verifies that the ball milling procedure successfully introduces iron into TiO2 while the spectrum of original TiO2 has no iron.
According to the XRD patterns of the original and modified TiO2 shown in Figure 6, the diffraction peaks of both samples showed fingerprint features of an anatase TiO2 structure with no other characteristic peaks observed, indicating the phase purity of the samples. The percentage of diffraction intensity at different crystal facets is depicted in Table 4. It can be seen that the percentage of the (101) peak, which is the most thermodynamically stable and has relatively low chemical activity [19], decreases sharply from 51.84% to 48.47% after applying the ball milling procedure. The percentages of the other peaks for Fe 3+ -doped TiO2, such as (004), (105), (211), and (116), which have high gas sensing activity [29,30], are all increased to some degree. Correspondingly, the percentage of (101) crystal facet is decreased, which indicates that the preferred orientation of (101) has been compromised.
On the basis of these data, the average crystal sizes were calculated according to Scherrer's equation [31]. For the original and modified TiO2, the results were 21.16 nm and 20.78 nm, respectively. It demonstrates that ball milling favors the formation of small crystallites, leading to more intense diffraction peaks, with the decrease of (101) crystal facet. In summary, it can be concluded that the planetary high energy ball mill can dope Fe 3+ ions into nano-TiO2, making it successfully modified.

Mechanical Property Analysis
The compressive strength test was conducted to ensure that the incorporation of nano-TiO2 did not compromise the mechanical performance of concrete, as shown in Figure 7. It can be seen from the figure that the compressive strength of two types of concrete both increased firstly and then reduced with the continuous increment of nano-TiO2 content. It indicates that the incorporation of nano-TiO2 is beneficial to the improvement of concrete strength, which is mainly due to the microfilling effect of nano-TiO2, making the internal structure of concrete more compact and leading to the improvement of strength. The maximum increase in strength is about 3% and 5%, respectively. As the particle size of modified TiO2 is smaller than that of the original TiO2, the strength of modified TiO2 concrete is higher than that of the original TiO2 concrete with the same content of TiO2. In addition, the incorporation of Fe 3+ can motivate the formation of iron-containing hydration products [32,33], which makes the internal structure more compact, and therefore improves the strength.
Nonetheless, when the nano-material content exceeds a certain range, the strength of each group begins to decline. At first, it can be ascribed to the agglomeration tendency as illustrated in Section 3.1, which could easily cause uneven distribution within the structure. On the other hand, the replacement of cement by TiO2 makes the corresponding cement content decrease when the nano-

Mechanical Property Analysis
The compressive strength test was conducted to ensure that the incorporation of nano-TiO2 did not compromise the mechanical performance of concrete, as shown in Figure 7. It can be seen from the figure that the compressive strength of two types of concrete both increased firstly and then reduced with the continuous increment of nano-TiO2 content. It indicates that the incorporation of nano-TiO2 is beneficial to the improvement of concrete strength, which is mainly due to the microfilling effect of nano-TiO2, making the internal structure of concrete more compact and leading to the improvement of strength. The maximum increase in strength is about 3% and 5%, respectively. As the particle size of modified TiO2 is smaller than that of the original TiO2, the strength of modified TiO2 concrete is higher than that of the original TiO2 concrete with the same content of TiO2. In addition, the incorporation of Fe 3+ can motivate the formation of iron-containing hydration products [32,33], which makes the internal structure more compact, and therefore improves the strength.
Nonetheless, when the nano-material content exceeds a certain range, the strength of each group begins to decline. At first, it can be ascribed to the agglomeration tendency as illustrated in Section 3.1, which could easily cause uneven distribution within the structure. On the other hand, the replacement of cement by TiO2 makes the corresponding cement content decrease when the nano-

Mechanical Property Analysis
The compressive strength test was conducted to ensure that the incorporation of nano-TiO 2 did not compromise the mechanical performance of concrete, as shown in Figure 7. It can be seen from the figure that the compressive strength of two types of concrete both increased firstly and then reduced with the continuous increment of nano-TiO 2 content. It indicates that the incorporation of nano-TiO 2 is beneficial to the improvement of concrete strength, which is mainly due to the micro-filling effect of nano-TiO 2 , making the internal structure of concrete more compact and leading to the improvement of strength. The maximum increase in strength is about 3% and 5%, respectively. As the particle size of modified TiO 2 is smaller than that of the original TiO 2 , the strength of modified TiO 2 concrete is higher than that of the original TiO 2 concrete with the same content of TiO 2 . In addition, the incorporation of Fe 3+ can motivate the formation of iron-containing hydration products [32,33], which makes the internal structure more compact, and therefore improves the strength.
Nonetheless, when the nano-material content exceeds a certain range, the strength of each group begins to decline. At first, it can be ascribed to the agglomeration tendency as illustrated in Section 3.1, which could easily cause uneven distribution within the structure. On the other hand, the replacement of cement by TiO 2 makes the corresponding cement content decrease when the nano-materials content increases excessively. In terms of the material strength, the best nano-TiO 2 content of cement were 3% and 4% for the two types of concrete, respectively.

Photocatalytic Property Analysis
The photocatalytic degradation test of concrete was carried out using the custom-designed test device proposed in this study. It should be noted that the device was subjected to a blank control verification prior to the start of the formal test, and the change of NO concentration was recorded. During the equipment verification, NO concentration was almost unchanged, proving that the sealing of the reaction chamber was good. The parameters used in the testing process are shown in Table 5. Keeping the above test parameters remaining unchanged, photocatalytic degradation tests of all samples were conducted. The residual rate of NO concentration after different periods of photocatalysis was calculated and displayed in Figure 8. It should be pointed out that the NO concentration results of concrete samples without TiO2 almost stayed constant. Thus, these results were not depicted in Figure 8. As it shows, the degradation rate of all the samples increases with the extension of irradiation time. Overall, the degradation rate of samples with modified nano-TiO2 was much higher than that of the original nano-TiO2 samples. The reason can be partially attributed to the smaller particle size that causes more contact area with pollutants. Also, the doped Fe 3+ in the crystal of TiO2, by the mechanical chemistry theory of ball milling as mentioned above, can act as an acceptor which can trap photogenerated charges and reduce the metal ion species into a lower oxidation state. Then, due to the close energy level of the Ti 4+ /Ti 3+ couple, the transformation of Ti 4+ into Ti 3+ is accelerated, which has been reported to improve the photodegradation by acting as a hole trap [15,34]. Another point is that Fe 3+ dopant, which has been reported to be effective to the batho-chromic shift, promotes a possible extension of the activity of TiO2 towards visible light range [35,36]. Therefore, the photocatalytic activities of TiO2 are enhanced significantly.
For further comparison, the photocatalytic degradation rates of all the samples at 60 min are shown in Figure 9. At the same content of nano materials, the degradation rate of concretes with modified nano-TiO2 is about 13% to 20% higher than that with original nano-TiO2. At the same time, the photocatalytic degradation rate of porous cement concrete is better than that of common cement concrete, which is because of the greater contact area with light caused by the connected pores of

Photocatalytic Property Analysis
The photocatalytic degradation test of concrete was carried out using the custom-designed test device proposed in this study. It should be noted that the device was subjected to a blank control verification prior to the start of the formal test, and the change of NO concentration was recorded. During the equipment verification, NO concentration was almost unchanged, proving that the sealing of the reaction chamber was good. The parameters used in the testing process are shown in Table 5. Keeping the above test parameters remaining unchanged, photocatalytic degradation tests of all samples were conducted. The residual rate of NO concentration after different periods of photocatalysis was calculated and displayed in Figure 8. It should be pointed out that the NO concentration results of concrete samples without TiO 2 almost stayed constant. Thus, these results were not depicted in Figure 8. As it shows, the degradation rate of all the samples increases with the extension of irradiation time. Overall, the degradation rate of samples with modified nano-TiO 2 was much higher than that of the original nano-TiO 2 samples. The reason can be partially attributed to the smaller particle size that causes more contact area with pollutants. Also, the doped Fe 3+ in the crystal of TiO 2 , by the mechanical chemistry theory of ball milling as mentioned above, can act as an acceptor which can trap photogenerated charges and reduce the metal ion species into a lower oxidation state. Then, due to the close energy level of the Ti 4+ /Ti 3+ couple, the transformation of Ti 4+ into Ti 3+ is accelerated, which has been reported to improve the photodegradation by acting as a hole trap [15,34]. Another point is that Fe 3+ dopant, which has been reported to be effective to the batho-chromic shift, promotes a possible extension of the activity of TiO 2 towards visible light range [35,36]. Therefore, the photocatalytic activities of TiO 2 are enhanced significantly.
For further comparison, the photocatalytic degradation rates of all the samples at 60 min are shown in Figure 9. At the same content of nano materials, the degradation rate of concretes with modified nano-TiO 2 is about 13% to 20% higher than that with original nano-TiO 2 . At the same time, the photocatalytic degradation rate of porous cement concrete is better than that of common cement concrete, which is because of the greater contact area with light caused by the connected pores of porous concrete. In the case of photocatalytic property, the optimized content of TiO 2 is suggested to be 4%.

Microstructure Analysis of Modified Concrete
The microstructural test of C-0 and C-4 are shown in Figure 10. It can be seen from Figure 10 that the internal structure of C-0 concrete is looser, and the cracks or pores can be clearly seen between the hydration products. C-4 concrete is relatively dense inside, and agglomerated particulate matters are filled between the hydration products. In order to further confirm the composition of the material, the two samples were analyzed by EDS. The results are shown in Figure 11. It can be seen from Figure  11 that the main elements in C-0 are calcium, silicon, and oxygen, whereas for C-4, calcium, silicon, and oxygen elements, as well as a lot of titanium. It indicates that the inner structure of C-4 has titanium-containing substances except common hydration products, which is TiO2. Therefore, based on the above analysis, it can be seen that the introduction of modified nano-TiO2 can be filled in the cracks or pores between the hydration, so that the structural density increased, which in turn improves the strength of C-4.
In this paper, we can see that the introduction of nano-TiO2, especially the modified nano-TiO2, has a positive impact on the mechanics and degradation properties of concrete. But there is an optimal amount. Considering the effects of mechanical properties and degradation efficiency, the recommended optimum dosage is 3% to 4%.

Microstructure Analysis of Modified Concrete
The microstructural test of C-0 and C-4 are shown in Figure 10. It can be seen from Figure 10 that the internal structure of C-0 concrete is looser, and the cracks or pores can be clearly seen between the hydration products. C-4 concrete is relatively dense inside, and agglomerated particulate matters are filled between the hydration products. In order to further confirm the composition of the material, the two samples were analyzed by EDS. The results are shown in Figure 11. It can be seen from Figure 11 that the main elements in C-0 are calcium, silicon, and oxygen, whereas for C-4, calcium, silicon, and oxygen elements, as well as a lot of titanium. It indicates that the inner structure of C-4 has titanium-containing substances except common hydration products, which is TiO 2 . Therefore, based on the above analysis, it can be seen that the introduction of modified nano-TiO 2 can be filled in the cracks or pores between the hydration, so that the structural density increased, which in turn improves the strength of C-4.
In this paper, we can see that the introduction of nano-TiO 2 , especially the modified nano-TiO 2 , has a positive impact on the mechanics and degradation properties of concrete. But there is an optimal amount. Considering the effects of mechanical properties and degradation efficiency, the recommended optimum dosage is 3% to 4%.

Microstructure Analysis of Modified Concrete
The microstructural test of C-0 and C-4 are shown in Figure 10. It can be seen from Figure 10 that the internal structure of C-0 concrete is looser, and the cracks or pores can be clearly seen between the hydration products. C-4 concrete is relatively dense inside, and agglomerated particulate matters are filled between the hydration products. In order to further confirm the composition of the material, the two samples were analyzed by EDS. The results are shown in Figure 11. It can be seen from Figure  11 that the main elements in C-0 are calcium, silicon, and oxygen, whereas for C-4, calcium, silicon, and oxygen elements, as well as a lot of titanium. It indicates that the inner structure of C-4 has titanium-containing substances except common hydration products, which is TiO2. Therefore, based on the above analysis, it can be seen that the introduction of modified nano-TiO2 can be filled in the cracks or pores between the hydration, so that the structural density increased, which in turn improves the strength of C-4.
In this paper, we can see that the introduction of nano-TiO2, especially the modified nano-TiO2, has a positive impact on the mechanics and degradation properties of concrete. But there is an optimal amount. Considering the effects of mechanical properties and degradation efficiency, the recommended optimum dosage is 3% to 4%.

Conclusions
This paper highlights the feasibility of using the planetary high energy ball milling method in modifying TiO2, as well as subsequent preparation of exhaust-purifying cement concrete. The photocatalytic efficiency and mechanical performance of exhaust-purifying cement concrete were both evaluated. The main conclusions can be summarized as follows: (1) The modified nano-TiO2 doped with Fe 3+ was successfully prepared by planetary high energy ball milling. The ball milling process caused significant changes to the feature parameters of TiO2, including decreased particle size, the more amorphous morphology, the compromised percentage of the most thermodynamically stable crystal facet, and the increased percentage of other high gas sensing crystal facets. (2) The incorporation of nano-TiO2 is beneficial to the improvement of concrete strength because of the micro-filling effect of nano particles. The incorporation of Fe 3+ can motivate the formation of iron-containing hydration products, leading to the further increase of concrete strength. In terms of the material strength, the best nano-TiO2 content of cement was 3% and 4% for the two types of concrete, respectively. (3) The employment of nano-TiO2 endow the concrete the functional properties of purifying exhaust gas. Due to the doping of Fe 3+ in the crystal, the photocatalytic degradation effect of modified nano-TiO2 is better than that of the original nano-TiO2. The degradation rate of porous concrete is much higher than that of common concrete with the same content of photocatalyst. For both types of concrete, the photocatalytic degradation rate comes to the peak when the nano-TiO2 content is 4%. (4) The internal structure of the concrete was modified to be more compact with the adoption of nano-TiO2, so the strength is improved. In a comprehensive consideration of both mechanical performance and degradation efficiency, the recommended optimum dosage of TiO2 is 3% to 4% for concrete. It is the most promising strategy to develop functional pavements using photocatalytic porous concrete to purify the urban traffic pollutions.
At last, it is strongly suggested that the modified nano-TiO2 doped with Fe 3+ may be further fused with other granular metamaterial components in concrete, to explore potential enhancement in both mechanical and chemical properties in further studies on the basis of the dispersive behavior of granular materials [37,38].

Conclusions
This paper highlights the feasibility of using the planetary high energy ball milling method in modifying TiO 2 , as well as subsequent preparation of exhaust-purifying cement concrete. The photocatalytic efficiency and mechanical performance of exhaust-purifying cement concrete were both evaluated. The main conclusions can be summarized as follows: (1) The modified nano-TiO 2 doped with Fe 3+ was successfully prepared by planetary high energy ball milling. The ball milling process caused significant changes to the feature parameters of TiO 2 , including decreased particle size, the more amorphous morphology, the compromised percentage of the most thermodynamically stable crystal facet, and the increased percentage of other high gas sensing crystal facets. (2) The incorporation of nano-TiO 2 is beneficial to the improvement of concrete strength because of the micro-filling effect of nano particles. The incorporation of Fe 3+ can motivate the formation of iron-containing hydration products, leading to the further increase of concrete strength. In terms of the material strength, the best nano-TiO 2 content of cement was 3% and 4% for the two types of concrete, respectively. (3) The employment of nano-TiO 2 endow the concrete the functional properties of purifying exhaust gas. Due to the doping of Fe 3+ in the crystal, the photocatalytic degradation effect of modified nano-TiO 2 is better than that of the original nano-TiO 2 . The degradation rate of porous concrete is much higher than that of common concrete with the same content of photocatalyst. For both types of concrete, the photocatalytic degradation rate comes to the peak when the nano-TiO 2 content is 4%. (4) The internal structure of the concrete was modified to be more compact with the adoption of nano-TiO 2 , so the strength is improved. In a comprehensive consideration of both mechanical performance and degradation efficiency, the recommended optimum dosage of TiO 2 is 3% to 4% for concrete. It is the most promising strategy to develop functional pavements using photocatalytic porous concrete to purify the urban traffic pollutions.
At last, it is strongly suggested that the modified nano-TiO 2 doped with Fe 3+ may be further fused with other granular metamaterial components in concrete, to explore potential enhancement in both mechanical and chemical properties in further studies on the basis of the dispersive behavior of granular materials [37,38].

Conflicts of Interest:
The authors declare no conflict of interest.