Evaluation of Nitrogen Oxide Reduction Performance in Permeable Concrete Surfaces Treated with a TiO2 Photocatalyst

Fine dust, recently classified as a carcinogen, has raised concerns about the health effects of air pollution. Vehicle emissions, particularly nitrogen oxide (NOx), contribute to ultrafine dust formation as a fine dust precursor. A photocatalyst, such as titanium dioxide (TiO2), is a material that causes a catalytic reaction when exposed to light, has exceptional characteristics such as decomposition of pollutants, and can be used permanently. This study aimed to investigate NOx reduction performance by developing ecofriendly permeable concrete with photocatalytic treatment to reduce fine dust generated from road mobile pollution sources. Permeable concrete specimens containing an activated loess and zeolite admixture were prepared and subjected to mechanical and durability tests. All specimens, including the control (CTRL) and admixture, met quality standard SPS-F-KSPIC-001-2006 for road pavement. Slip resistance and permeability coefficient also satisfied the standards, while freeze–thaw evaluation criteria were met only by CTRL and A1Z1 specimens. NOx reduction performance of the permeable concrete treated with TiO2 photocatalyst was assessed using ISO standard and tank chambers. NOx reduction efficiency of up to 77.5% was confirmed in the permeable concrete specimen with TiO2 content of 7.5%. Nitrate concentration measurements indirectly confirmed photolysis of nitrogen oxide. Incorporating TiO2 in construction materials such as roads and sidewalks can improve the atmospheric environment for pedestrians near roads by reducing NOx levels through photocatalysis.


Introduction
Air pollution due to rapid economic growth is one of the problems facing the world, mainly in the form of fine dust in the process of burning fossil fuels such as coal and oil in factories and automobiles, etc. [1].Particulate matter (PM) refers to atmospheric substances that exist in the form of particles.It is generally classified as fine dust (PM 10 ) when the diameter of the largest particle is below 10 µm or as ultrafine dust (PM 2.5 ) when the diameter of the largest particle is below 2.5 µm.In particular, PM can easily infiltrate bronchial tubes and lungs; therefore, it causes various respiratory diseases as well as cardiovascular, skin, and eye diseases.Samet et al. (2000) reported that the total mortality rate increases by 0.51% if the concentration of atmospheric PM 10 increases [2].Other studies have reported that the total mortality rate could increase up to 4-18% upon long-term exposure to PM [3,4].Therefore, the International Agency for Research on Cancer (IARC), an organization affiliated with the World Health Organization (WHO), has classified PM as a Group 1 carcinogen [5], and measures for reducing atmospheric PM are urgently needed.
Nitrogen oxide (NO x ) accounts for approximately 60% of PM originating from the roadside.NO x is a pollution source that secondarily generates PM 2.5 by reacting with vapor (H 2 O), ozone (O 3 ), and organic compounds under certain conditions in the atmosphere.NO x is mainly generated by the combustion of fossil fuels such as coal or petroleum.In particular, it is estimated that a significant amount of NO x is emitted from road mobility pollution sources in large cities with significant vehicle transit, and it has been reported that approximately 45% of NO x from Seoul has been emitted through road mobility pollution sources [6].Therefore, for reducing NO x in urban areas, controlling NO x emitted through road mobility sources or using road facilities is effective.Among various road facilities, those composed of concrete, such as boundary stones, bagged and sacked concrete, and crash walls, have large surface areas; therefore, the use of infrastructure composed of functional concrete is an efficient method of reducing roadside NO x [7].
TiO 2 photocatalysts, which are most commonly used as functional materials for reducing NO x as a precursor to PM, can be used semi-permanently because it does not change under light, and it is mainly included in concrete, asphalt mixture, paint for mortar, coating materials, and packing materials owing to its excellent capability to decompose pollutants [8][9][10].TiO 2 photocatalysts have a 3.0-3.2eV band gap, and electrons (e − ) and electron holes (h + ) are formed when TiO 2 is irradiated using light with a wavelength of 380 nm (ultraviolet rays) or below.The generated e − and h + are strong reducing agents and oxidizing agents, respectively; thus, hydroxyl radical (OH) and superoxide anion (O 2 − ) can be created through reaction with water and oxygen from the surrounding atmosphere, demonstrating excellent efficiency for decomposing atmospheric pollutants [11].The NO x decomposition reaction mediated by TiO 2 is presented in Equation (1), and Figure 1 shows a schematic representation of the NO x decomposition process [12].Nitrogen oxide (NOx) accounts for approximately 60% of PM originating from the roadside.NOx is a pollution source that secondarily generates PM2.5 by reacting with vapor (H2O), ozone (O3), and organic compounds under certain conditions in the atmosphere.NOx is mainly generated by the combustion of fossil fuels such as coal or petroleum.In particular, it is estimated that a significant amount of NOx is emitted from road mobility pollution sources in large cities with significant vehicle transit, and it has been reported that approximately 45% of NOx from Seoul has been emitted through road mobility pollution sources [6].Therefore, for reducing NOx in urban areas, controlling NOx emitted through road mobility sources or using road facilities is effective.Among various road facilities, those composed of concrete, such as boundary stones, bagged and sacked concrete, and crash walls, have large surface areas; therefore, the use of infrastructure composed of functional concrete is an efficient method of reducing roadside NOx [7].
TiO2 photocatalysts, which are most commonly used as functional materials for reducing NOx as a precursor to PM, can be used semi-permanently because it does not change under light, and it is mainly included in concrete, asphalt mixture, paint for mortar, coating materials, and packing materials owing to its excellent capability to decompose pollutants [8][9][10].TiO2 photocatalysts have a 3.0-3.2eV band gap, and electrons (e − ) and electron holes (h + ) are formed when TiO2 is irradiated using light with a wavelength of 380 nm (ultraviolet rays) or below.The generated e − and h + are strong reducing agents and oxidizing agents, respectively; thus, hydroxyl radical (OH) and superoxide anion (O2 − ) can be created through reaction with water and oxygen from the surrounding atmosphere, demonstrating excellent efficiency for decomposing atmospheric pollutants [11].The NOx decomposition reaction mediated by TiO2 is presented in Equation (1), and Figure 1 shows a schematic representation of the NOx decomposition process [12].Various studies have investigated the reduction in atmospheric NO x using TiO 2 photocatalysts that demonstrate excellent decomposition of organic matter.Guo et al. (2020) assessed the photocatalytic decomposition efficiency of concrete consisting of nano-TiO 2 photocatalysts [13].They utilized two methods for integrating nano-TiO 2 into the concrete: (1) directly mixing nano-TiO 2 particles with concrete and (2) spreading nano-TiO 2 particles onto the concrete surface by spraying.They reported that the photocatalytic efficiency improved when concrete was not polished according to the increase in nano-TiO 2 content increase as well as the increase in the concentration of pollutants.Gopala Krishna Sastry et al. (2021) experimentally studied the effect of nano-TiO 2 on fly ashbased geopolymer concrete durability and strength [14].In their research, the compressive strength, bending strength, and splitting tensile strength of geopolymer concrete in which fly ash was substituted with nano-TiO 2 were assessed; the fly ash-based geopolymer concrete was soaked in magnesium sulfate and sodium chloride to assess durability.They reported that the strength and durability of the fly ash-based geopolymer concrete increased as the amount of substituted TiO 2 increased.Kim et al. (2018) investigated the NO x removal efficiency of the highway-retaining wall of South Korea's Gyeongbu Expressway, where TiO 2 was spread onto the concrete surface through a surface penetrant [15].They reported that the NO x concentration was reduced by approximately 12% following the addition of TiO 2 .Additionally, they reported that the NO x removal efficiency increased as the amount of sunshine and traffic increased.Beeldens (2014) investigated the photocatalyst block pavement in Antwerp, Belgium [16], and Th.Maggos et al. (2007) performed an onsite experiment by applying TiO 2 photocatalyst paint in a parking lot [17].Furthermore, Gian Lsuca Guerrini (2012) used cement-based paint treated with TiO 2 in the Umberto 1 tunnel in Rome to identify its NO x reduction capacity [18].On the other hand, since the activity of the TiO 2 photocatalyst is only active in UV light, which is about 5% of sunlight [19,20], various methods have been studied to improve the activity, such as bonding with other semiconductors, noble metal loading, doping, and heterojunction configuration [21][22][23].Islam Ibrahim et al. (2022) reported that the component combination of TiO 2 /g-C3N4@Ag NPs showed visible light activation and improved photocatalytic performance under not only UV but also visible light irradiation [24].
Thus, this study investigated the NO x reduction performance of road pavement concrete coated with TiO 2 photocatalysts to reduce fine dust generated from roadsides for pedestrians.Unlike asphalt or general concrete pavement, rainwater is drained to prevent aquaplaning, enabling smooth passage of vehicles and moving objects, and a permeable concrete containing ecofriendly materials was developed.In addition, a tank photoreactor was fabricated and the NO x reduction efficiency of the specimen that utilized real-size construction material was determined.For this purpose, the mixing ratio of permeable concrete for sidewalk pavement was derived, and the compressive strength, bending strength, freeze-thawing resistance, skid resistance, and permeability performance were determined.Next, TiO 2 photocatalyst was coated onto the fabricated permeable concrete mixed with the derived optimal mixing ratio to assess the NO x reduction efficiency according to the TiO 2 content.NO x reduction assessment involved analysis using a newly developed tank chamber that can accommodate large-scale specimens and an ISO standard chamber that uses small specimens.Additionally, a water quality analyzer was used to identify the generated nitrate concentration after NO x reduction assessment.

Materials and Methods
Most road pavements are made up of asphalt or concrete; hence, the surface layer and base layer are impervious.Consequently, rainwater wells up when there is heavy rainfall, resulting in hydroplaning.Hydroplaning reduces skid resistance, threatens safety, and disturbs smooth transit.Permeable concrete has increased permeability owing to the mixture of single-size particle aggregate, cement, and water.Paving roads with this permeable concrete can smoothly drain rainwater unlike general concrete/asphalt pavements and facilitate smooth transit of moving objects.In particular, using permeable concrete where TiO 2 photocatalyst is mixed/spread as pavement material for sidewalks and bicycle roads reduces NO x generated from road mobility pollution sources such as vehicles/motorcycles, and the effect of NO x on pedestrians can be directly reduced.To this end, the durability of permeable concrete containing a mixture of active loess and zeolite and the NO x reduction efficiency of TiO 2 spray coating was assessed at the laboratory level.

Materials 2.1.1. Permeable Concrete
Table 1 shows the chemical composition of cement for the mixture of permeable concrete for road pavement.The cement used in precast concrete pavement (PCP) is a Type 1 Portland cement fabricated in South Korea with a density of 3.15 g/m 3 and Blaine of 3000 cm 2 /g.The active loess is red clay that activates SiO 2 and Al 2 O 3 by rapidly cooling natural red clay after high-temperature heating.According to previous research, concrete containing active loess is known to have strong acid resistance.In particular, active loess as a material that forms a porous structure is an environmentally friendly material that has excellent far-infrared radiation emissivity and deodorant performance.Zeolite is mainly composed of SiO 2 and Al 2 O 3 similar to other pozzolan material and forms a porous structure similar to active loess.Zeolite is known to have outstanding absorption performance owing to its excellent cation exchange properties.Therefore, this study used porous active loess and zeolite as mixing materials for the permeable concrete to improve the coating performance of TiO 2 photocatalyst.Figure 2 shows the active loess and zeolite used in this study, and Table 2 shows the chemical composition of active loess and zeolite provided by the suppliers.The coarse aggregate used in this study was a single-size particle thick aggregate with a diameter of 10 mm or above and 13 mm or below.The density and absorption rate of this aggregate were 2.34 g/m 3 and 1.27%, respectively.and the NOx reduction efficiency of TiO2 spray coating was assessed at the laboratory level.

Permeable Concrete
Table 1 shows the chemical composition of cement for the mixture of permeable concrete for road pavement.The cement used in precast concrete pavement (PCP) is a Type 1 Portland cement fabricated in South Korea with a density of 3.15 g/m 3 and Blaine of 3,000 cm 2 /g.The active loess is red clay that activates SiO2 and Al2O3 by rapidly cooling natural red clay after high-temperature heating.According to previous research, concrete containing active loess is known to have strong acid resistance.In particular, active loess as a material that forms a porous structure is an environmentally friendly material that has excellent far-infrared radiation emissivity and deodorant performance.Zeolite is mainly composed of SiO2 and Al2O3 similar to other pozzolan material and forms a porous structure similar to active loess.Zeolite is known to have outstanding absorption performance owing to its excellent cation exchange properties.Therefore, this study used porous active loess and zeolite as mixing materials for the permeable concrete to improve the coating performance of TiO2 photocatalyst. Figure 2 shows the active loess and zeolite used in this study, and Table 2 shows the chemical composition of active loess and zeolite provided by the suppliers.The coarse aggregate used in this study was a single-size particle thick aggregate with a diameter of 10 mm or above and 13 mm or below.The density and absorption rate of this aggregate were 2.34 g/m 3 and 1.27%, respectively.

Photocatalyst
The photocatalyst used as surface coating material of permeable concrete in this study was a product of AERODISP ® W740X by Evonik Industries AG, and an anatase-type TiO 2 with outstanding photolytic efficiency was used.The characteristics of AERODISP ® W740X used in this study are summarized in Table 3.

Experimental Variable
Table 4 shows the mixing ratio of permeable concrete used in this study.The waterbinder rate used in the mixture was 36.4%.Previous studies have reported that when the rate of substitution of porous pozzolan material for cement exceeds 20%, the strength of permeable concrete significantly decreases [25,26].Therefore, the rate of substitution of porous pozzolan material for cement was set to 15%.As an experimental variable, several mixture ratios (2:1, 1:1, and 1:2) of active loess and zeolite were considered.Among the test specimen names in Table 4, CTRL refers to the permeable concrete in which porous material was not substituted, and the Arabic numbers behind A and Z refer to the mixture ratio of active loess and zeolite, respectively.For example, A2Z1 indicated a specimen where 10% of cement was substituted with active loess and 5% was substituted with zeolite.The AERODISP ® W740X product used as a surface coating material for permeable concrete has a high solid fraction with 40% TiO 2 content.To determine the optimal TiO 2 content for reducing NO x , photocatalysts with varying TiO 2 content were used as shown in Table 5. Distilled water was used to dilute AERODISP ® W740X.T0 refers to the permeable concrete where the photocatalyst was not spread.In the SPS-F-KSPIC-001-2006(2018) standard (Korea standard), the evaluation criteria and quality required performances of permeable concrete for road pavement are presented as shown in Table 6 [27].In this standard, the performance of permeable concrete is differentiated according to the location of the road pavement (sidewalk, bicycle road, and parking lot), and the minimum required performances for the compressive strength, bending strength, residual compressive strength after 100 cycles of freezing and thawing, skid resistance, and permeability coefficient of permeable concrete are presented.In this study, the compressive strength, bending strength, residual compressive strength after 100 cycles of freezing and thawing, skid resistance, and permeability coefficient were tested to assess the performance of the permeable concrete containing a mixture of active loess and zeolite.The effect of the method of compaction and compaction intensity of permeable concrete on its strength properties and durability are significant.Therefore, this study compacted permeable concrete by performing the free fall of a rammer that is 50 mm in diameter and 2.5 kg in mass at 300 mm height, as shown in Figure 3.The number of compaction layers and compaction frequency for each layer followed the presented method in the SPS-F-KSPIC-001-2006 standard.Table 7 shows the size of specimen for each test, number of compaction layers, and the compaction frequency for each layer.The compressive strength of permeable concrete was assessed on days 7, 14, and 28 using a single-axis compression test according to the KS F 2405 standard.The bending strength, skid resistance, and permeability coefficient of permeable concrete were assessed on day 28 according to the KS F 2408, KS F 2375, and KS F 4001 standards, respectively.Lastly, the freeze-thaw resistance was assessed on day 28 using a single-axis compression test according to the KS F 2405 standard with the specimen that underwent 100 cycles of freezing and thawing according to the KS F 2456 standard.The compressive strength of permeable concrete was assessed on days 7, 14, and 28 using a single-axis compression test according to the KS F 2405 standard.The bending strength, skid resistance, and permeability coefficient of permeable concrete were assessed on day 28 according to the KS F 2408, KS F 2375, and KS F 4001 standards, respectively.Lastly, the freeze-thaw resistance was assessed on day 28 using a single-axis compression test according to the KS F 2405 standard with the specimen that underwent 100 cycles of freezing and thawing according to the KS F 2456 standard.

NO x Reduction Performance Evaluation Test
To assess the NO x removal performance of the photocatalyst, the schematic of nitrogen oxide reduction evaluation system that complies with ISO 22197-1(2016) is shown in Figure 4 [28].The NO x reduction evaluation system consists of a test gas supplier, a photoreactor (test chamber), and a test gas analyzer.The test gas supplier consists of a flow controller, a humidifier, a gas mix tank, etc.A mixture of dry air, moist air that has passed through the humidifier, and polluted gas supplies 1ppm polluted gas with 50% relative humidity.The test chamber for testing NO x reduction was fabricated in two sizes: an ISO standard photoreactor (sample size 100 mm × 50 mm × 5 mm) for small specimens and a tank photoreactor (tank size 240 mm × 140 mm × 200 mm) for large specimens such as concrete and asphalt construction materials.The upper part of the chamber was composed of tempered glass that ultraviolet (UV) could penetrate.As the size of the specimen for testing NO x reduction, a small specimen of 50 mm × 10 mm × 5 mm in size is used for the ISO standard photoreactor, but the ISO standard photoreactor specimen guideline was corrected to the size of the aggregate used in permeable concrete fabrication (10 mm or above), and permeable concrete 50 mm × 10 mm × 25 mm in size was fabricated for use.In the case of the tank photoreactor, a permeable concrete compressive strength test piece (100 mm diameter × 50 mm height) was used.Images of each chamber and specimens are shown in Figure 5 (ISO standard photoreactor) and Figure 6 (tank-type photoreactor).For specimen preprocessing, all specimens were soaked in distilled water for 2 h or above and dried at 40 • C before testing.The preprocessed specimens were coated with a photocatalyst using an automatic spray device.As experimental pollutant gas, NO gas and air were mixed, and 1 ppm NO gas with 50% relative humidity at 25 • C was used.The specimen coated with photocatalyst was located within the chamber and pollutant gas was injected at a flow rate of 3 L min −1 and stabilized for 1 h.Then, the pollutant gas concentration was documented for 5 h while irradiating with a UV lamp at a wavelength of 325 nm.Lastly, the UV lamp was turned off and the pollutant gas concentration was observed for 1 h.The NO x reduction efficiency of the photocatalyst could be determined using the NO x concentration (NO xequil ) at equilibrium during the photocatalyst reaction and the initial NO x concentration (NO xinitial ) (Equation ( 2)).

Nitrate Assessment
As an indirect measurement method for validating the photodecomposition reaction of nitrogen oxide, the nitrate concentration on the surface of photocatalyst-coated permeable concrete was analyzed [29].Distilled water was used to dissolve nitrate on the surface of the permeable concrete.To extract nitrate, specimens that completed the NOx reduction experiment (T0, T7.5, and T10) in both reactors were used.Figure 7 shows the water quality analyzer (a) for measuring nitrate concentration and the process of dissolving nitrate in specimens of ISO standard photoreactor (b) and tank-type

Nitrate Assessment
As an indirect measurement method for validating the photodecomposition reaction of nitrogen oxide, the nitrate concentration on the surface of photocatalyst-coated permeable concrete was analyzed [29].Distilled water was used to dissolve nitrate on the surface of the permeable concrete.To extract nitrate, specimens that completed the NOx reduction experiment (T0, T7.5, and T10) in both reactors were used.Figure 7 shows the water quality analyzer (a) for measuring nitrate concentration and the process of dissolving nitrate in specimens of ISO standard photoreactor (b) and tank-type

Nitrate Assessment
As an indirect measurement method for validating the photodecomposition reaction of nitrogen oxide, the nitrate concentration on the surface of photocatalyst-coated permeable concrete was analyzed [29].Distilled water was used to dissolve nitrate on the surface of the permeable concrete.To extract nitrate, specimens that completed the NOx reduction experiment (T0, T7.5, and T10) in both reactors were used.Figure 7 shows the water quality analyzer (a) for measuring nitrate concentration and the process of dissolving nitrate in specimens of ISO standard photoreactor (b) and tank-type

Nitrate Assessment
As an indirect measurement method for validating the photodecomposition reaction of nitrogen oxide, the nitrate concentration on the surface of photocatalyst-coated permeable concrete was analyzed [29].Distilled water was used to dissolve nitrate on the surface of the permeable concrete.To extract nitrate, specimens that completed the NO x reduction experiment (T0, T7.5, and T10) in both reactors were used.Figure 7 shows the water quality analyzer (a) for measuring nitrate concentration and the process of dissolving nitrate in specimens of ISO standard photoreactor (b) and tank-type photoreactor (c).Given the properties of permeable concrete, the distilled water poured onto the surface would be lost downwards; thus, 40 mL of distilled water was poured into a rectangular container (approximately 63 mL for tank photoreactor specimens) and the coated surface of permeable concrete was soaked into the distilled water to dissolve nitrate for 5 min.The collected solution was filtered through a 0.22 µm filer, and nitrate concentration was measured through a water quality analyzer (SPECTROPHOTOMETER, HS-3700, HUMAS, Daejeon, Korea).
photoreactor (c).Given the properties of permeable concrete, the distilled water poured onto the surface would be lost downwards; thus, 40 mL of distilled water was poured into a rectangular container (approximately 63 mL for tank photoreactor specimens) and the coated surface of permeable concrete was soaked into the distilled water to dissolve nitrate for 5 min.The collected solution was filtered through a 0.22 μm filer, and nitrate concentration was measured through a water quality analyzer (SPECTROPHOTOMETER, HS-3700, HUMAS, Daejeon, Korea).

Mechanical Properties of Permeable Concretes
Figure 8 shows the compressive strength test results of permeable concrete according to the number of curing days.The CTRL sample, without the active loess and zeolite mixture, achieved an average compressive strength of 21.64 MPa after 7 days of curing.It exhibited 96.3% of the compressive strength of 22.47 MPa after 28-day curing.In contrast, after curing for 7 days, the compressive strength of A1Z2, A1Z1, and A2Z1, containing a mixture of active loess and zeolite, was 16.74, 17.95, and 15.75 MPa, respectively; on average, they exhibited 84.3% of the 28-day compressive strength after curing.The compressive strength of A1Z2, A1Z1, and A2Z1 at day 28 was 18.95, 20.72, and 20.23 MPa, respectively, which were 84.33%, 92.21%, and 90.03% compared to the CTRL, respectively.In this recipe, the compressive strength of concrete in which some of the cement was replaced with pozzolanic materials (activated loess, natural zeolite) showed a decrease, and when the content of zeolite was higher than that of activated loess, the compressive strength of concrete was greatly reduced.It is judged that when cement is replaced with pozzolanic materials such as zeolite and activated loess, the absolute amount of cement clinker decreases and the pozzolanic reaction of pozzolanic materials such as activated loess and zeolite does not sufficiently proceed.In addition, it is judged that the content of constituent minerals that affect the development of strength, such as SiO2, is also reduced [30][31][32][33].However, the average compressive strength of A1Z2, A1Z1, and A2Z1 after a curing period of 28 days satisfied the quality standard for permeable concrete for road pavement presented by SPS-F-KSPIC-001-2006.

Mechanical Properties of Permeable Concretes
Figure 8 shows the compressive strength test results of permeable concrete according to the number of curing days.The CTRL sample, without the active loess and zeolite mixture, achieved an average compressive strength of 21.64 MPa after 7 days of curing.It exhibited 96.3% of the compressive strength of 22.47 MPa after 28-day curing.In contrast, after curing for 7 days, the compressive strength of A1Z2, A1Z1, and A2Z1, containing a mixture of active loess and zeolite, was 16.74, 17.95, and 15.75 MPa, respectively; on average, they exhibited 84.3% of the 28-day compressive strength after curing.The compressive strength of A1Z2, A1Z1, and A2Z1 at day 28 was 18.95, 20.72, and 20.23 MPa, respectively, which were 84.33%, 92.21%, and 90.03% compared to the CTRL, respectively.In this recipe, the compressive strength of concrete in which some of the cement was replaced with pozzolanic materials (activated loess, natural zeolite) showed a decrease, and when the content of zeolite was higher than that of activated loess, the compressive strength of concrete was greatly reduced.It is judged that when cement is replaced with pozzolanic materials such as zeolite and activated loess, the absolute amount of cement clinker decreases and the pozzolanic reaction of pozzolanic materials such as activated loess and zeolite does not sufficiently proceed.In addition, it is judged that the content of constituent minerals that affect the development of strength, such as SiO 2 , is also reduced [30][31][32][33].However, the average compressive strength of A1Z2, A1Z1, and A2Z1 after a curing period of 28 days satisfied the quality standard for permeable concrete for road pavement presented by SPS-F-KSPIC-001-2006. Figure 9 demonstrates the results of the flexural strength test for each specimen after a curing period of 28 days.After curing for 28 days, the flexural strength of the CTRL was 3.88 MPa, while those of A1Z2, A1Z1, and A2Z1, which contain a mixture of zeolite and Figure 9 demonstrates the results of the flexural strength test for each specimen after a curing period of 28 days.After curing for 28 days, the flexural strength of the CTRL was 3.88 MPa, while those of A1Z2, A1Z1, and A2Z1, which contain a mixture of zeolite and active loess, were 3.01, 3.63, and 3.02 MPa, respectively.They were, on average, 82.9% of that of the CTRL.Similar to the compressive strength experiment results, the A1Z1 specimen showed high strength compared to A1Z2 and A2Z1.All specimens satisfied the flexural strength standard of permeable concrete for road pavements presented by SPS-F-KSPIC-001-2006.The evaluation results of compressive strength and flexural strength of permeable concrete specimens are summarized in Table 8. Figure 9 demonstrates the results of the flexural strength test for each specimen after a curing period of 28 days.After curing for 28 days, the flexural strength of the CTRL was 3.88 MPa, while those of A1Z2, A1Z1, and A2Z1, which contain a mixture of zeolite and active loess, were 3.01, 3.63, and 3.02 MPa, respectively.They were, on average, 82.9% of that of the CTRL.Similar to the compressive strength experiment results, the A1Z1 specimen showed high strength compared to A1Z2 and A2Z1.All specimens satisfied the flexural strength standard of permeable concrete for road pavements presented by SPS-F-KSPIC-001-2006.The evaluation results of compressive strength and flexural strength of permeable concrete specimens are summarized in Table 8.

Durability of Permeable Concretes
Table 9 summarizes the permeability coefficient and skid resistance assessment results.The average skid resistance of specimens CTRL, A1Z2, A1Z1, and A2Z1 is 48.3, 50.3, 48, and 49.7 BPN, respectively.All specimens satisfied the standard of 30 BPN for sidewalk concrete and 40 BPN for bicycle road and parking lot concrete presented by SPS-F-KSPIC-001-2006. Therefore, the skid resistance was found to not be significantly affected by the binder type.In contrast, the permeability coefficient was the greatest in A1Z2 at 6.93 × 10 −3 cm/s with the lowest compressive and flexural strengths.However, this was determined not to be a significant difference between other specimens.The skid resistance of all specimens satisfied the standard of 1 × 10 −3 cm/s of permeable concrete presented by SPS-F-KSPIC-001-2006. Figure 10 displays the specimens subjected to the freeze-thaw action of the manufactured permeable concretes.CTRL and A1Z1 did not show significant differences before and after 100 cycles of freezing and thawing, as shown in Figure 10a,c.In contrast, as shown in Figure 10b,d, the surface aggregate was severely eliminated, and cracks were observed.Therefore, residual compressive strength testing was performed for CTRL and A1Z1, excluding A1Z2 and A2Z1, after completing the freezing and thawing testing, and the results are summarized in Table 10.The test results showed that, on average, the residual compressive strengths of CTRL and A1Z1 were 19.6 and 16.8 MPa, respectively, which are 87.1% and 82.4% of the compressive strength on day 28, respectively.Thus, they were found satisfy 80% or above the SPS-F-KSPIC-001-2006 standard.Therefore, in subsequent studies, A1Z1 specimens that satisfied all permeable concreate evaluation criteria were used.50.3, 48, and 49.7 BPN, respectively.All specimens satisfied the standard of 30 BPN for sidewalk concrete and 40 BPN for bicycle road and parking lot concrete presented by SPS-F-KSPIC-001-2006. Therefore, the skid resistance was found to not be significantly affected by the binder type.In contrast, the permeability coefficient was the greatest in A1Z2 at 6.93 × 10 −3 cm/s with the lowest compressive and flexural strengths.However, this was determined not to be a significant difference between other specimens.The skid resistance of all specimens satisfied the standard of 1 × 10 −3 cm/s of permeable concrete presented by SPS-F-KSPIC-001-2006. Figure 10 displays the specimens subjected to the freeze-thaw action of the manufactured permeable concretes.CTRL and A1Z1 did not show significant differences before and after 100 cycles of freezing and thawing, as shown in Figure 10a,c.In contrast, as shown in Figure 10b,d, the surface aggregate was severely eliminated, and cracks were observed.Therefore, residual compressive strength testing was performed for CTRL and A1Z1, excluding A1Z2 and A2Z1, after completing the freezing and thawing testing, and the results are summarized in Table 10.The test results showed that, on average, the residual compressive strengths of CTRL and A1Z1 were 19.6 and 16.8 MPa, respectively, which are 87.1% and 82.4% of the compressive strength on day 28, respectively.Thus, they were found to satisfy 80% or above the SPS-F-KSPIC-001-2006 standard.Therefore, in subsequent studies, A1Z1 specimens that satisfied all permeable concreate evaluation criteria were used.

NO x Reduction Performance Evaluation According to TiO 2 Content
To assess the nitrogen oxide removal performance of the photocatalyst according to TiO 2 content, the photocatalyst was coated with varying TiO 2 contents onto the A1Z1 permeable concrete supporter, and a nitrogen oxide removal experiment was performed in the ISO standard photoreactor chamber according to ISO 22197-1:2016.
Figure 11 shows the results of assessing NO x reduction performance according to photocatalyst content, and the average NO x reduction efficiency values of each specimen are summarized in Table 11.In A1Z1 (T0) without photocatalyst coating, it was found that photodecomposition reaction was nonexistent with NO x reduction efficiency of 1.6%.In all specimens with photocatalyst coating, the NO gas concentration rapidly decreased owing to the activation of the photocatalyst reaction after lighting the UV lamp, the photocatalyst reaction was deactivated after turning off the UV lamp, and recovery to the initial concentration was shown.Additionally, the tendency of an increase in nitrogen oxide removal efficiency was shown according to the increase in photocatalyst content, and the NO x reduction effect was the greatest at 77.5% with 7.5% TiO 2 content.Meanwhile, with 10% TiO 2 content, the NO x reduction efficiency was 67.7%, which was lower than that with 7.5% TiO 2 content despite the higher TiO 2 content.In this result, the reason for the maximum NO x reduction effect of permeable concrete using T7.5 photocatalyst is that TiO 2 particles of 70 nm show the most appropriate dispersion and adsorption at a content of 7.5% in macro-and micropores on the surfaces of active loess and zeolite as porous materials and coarse aggregate.

Sample Code
Compressive

NOx Reduction Performance Evaluation according to TiO2 Content
To assess the nitrogen oxide removal performance of the photocatalyst according to TiO2 content, the photocatalyst was coated with varying TiO2 contents onto the A1Z1 permeable concrete supporter, and a nitrogen oxide removal experiment was performed in the ISO standard photoreactor chamber according to ISO 22197-1:2016.
Figure 11 shows the results of assessing NOx reduction performance according to photocatalyst content, and the average NOx reduction efficiency values of each specimen are summarized in Table 11.In A1Z1 (T0) without photocatalyst coating, it was found that photodecomposition reaction was nonexistent with NOx reduction efficiency of 1.6%.In all specimens with photocatalyst coating, the NO gas concentration rapidly decreased owing to the activation of the photocatalyst reaction after lighting the UV lamp, the photocatalyst reaction was deactivated after turning off the UV lamp, and recovery to the initial concentration was shown.Additionally, the tendency of an increase in nitrogen oxide removal efficiency was shown according to the increase in photocatalyst content, and the NOx reduction effect was the greatest at 77.5% with 7.5% TiO2 content.Meanwhile, with 10% TiO2 content, the NOx reduction efficiency was 67.7%, which was lower than that with 7.5% TiO2 content despite the higher TiO2 content.In this result, the reason for the maximum NOx reduction effect of permeable concrete using T7.5 photocatalyst is that TiO2 particles of 70 nm show the most appropriate dispersion and adsorption at a content of 7.5% in macro-and micropores on the surfaces of active loess and zeolite as porous materials and coarse aggregate.For the NO x reduction assessment of construction materials with large volumes, such as concrete and asphalt, a tank photoreactor chamber that modified the ISO 22197-1:2016 standard experiment regulation was fabricated, and the NO x reduction experiment was performed with the compressive strength test piece.For the photocatalyst used in surface coating, T7.5 and T10, which demonstrated high efficiency in previous experiments, were used, and the results are shown in Figure 12 and Table 12.The NO x reduction efficiency value in the sample using the compressive strength test piece in the tank photoreactor was lower than that in the ISO standard photoreactor despite the broad coating area.The observed low NO x reduction efficiency can be attributed to the larger volume of the tank photoreactor compared to the specimen in the ISO standard photoreactors.As the volume inside the chamber increased, the NO x reduction efficiency decreased due to the increase in the amount of pollutants to be reduced compared to the photocatalyst coating unit area.In addition, as shown in Figure 12, higher efficiency was shown with 7.5% TiO 2 content than with 10% TiO 2 content.
T2.5 49.8 T5 59.6 T7.5 77.5 T10 67.7 For the NOx reduction assessment of construction materials with large volumes, such as concrete and asphalt, a tank photoreactor chamber that modified the ISO 22197-1:2016 standard experiment regulation was fabricated, and the NOx reduction experiment was performed with the compressive strength test piece.For the photocatalyst used in surface coating, T7.5 and T10, which demonstrated high efficiency in previous experiments, were used, and the results are shown in Figure 12 and Table 12.The NOx reduction efficiency value in the sample using the compressive strength test piece in the tank photoreactor was lower than that in the ISO standard photoreactor despite the broad coating area.The observed low NOx reduction efficiency can be attributed to the larger volume of the tank photoreactor compared to the specimen in the ISO standard photoreactors.As the volume inside the chamber increased, the NOx reduction efficiency decreased due to the increase in the amount of pollutants to be reduced compared to the photocatalyst coating unit area.In addition, as shown in Figure 12, higher efficiency was shown with 7.5% TiO2 content than with 10% TiO2 content.Table 12.NOx reduction efficiencies according to the TiO2 content in the tank photoreactor.

Nitrate Analysis after Photolysis of Nitrogen Oxides
As an indirect method of identifying the TiO2 photocatalyst decomposition reaction, the concentration of nitrate generated directly after the NOx reduction experiment was analyzed.The nitrate content accumulated on the surface of T0, T7.5, and T10 specimens

Nitrate Analysis after Photolysis of Nitrogen Oxides
As an indirect method of identifying the TiO 2 photocatalyst decomposition reaction, the concentration of nitrate generated directly after the NO x reduction experiment was analyzed.The nitrate content accumulated on the surface of T0, T7.5, and T10 specimens that completed the NO x reduction experiment from the ISO standard photoreactor and tank photoreactor was dissolved in distilled water for extraction.Nitrate concentrations in the specimens in both photoreactor types measured through the water quality analyzer are summarized in Table 13.Compared to T0, which did not undergo photocatalyst treatment, nitrate concentration measurement following the NO x reduction experiment revealed that a photodecomposition reaction occurred in the specimen treated with TiO 2 photocatalyst.In addition, higher nitrate concentration was shown in specimen T7.5 than in T10; this was attributed to the greater amount of nitrate generated in specimen T7.5 with higher NO x reduction efficiency than in T10, as shown in the NO x reduction experiment results.The specimens in the tank photoreactor exhibited a similar trend to the results observed in the ISO standard photoreactor specimens.

Conclusions
The application of TiO 2 photocatalyst to construction materials for enhancing air quality has garnered significant interest among researchers in both industry and academia.In line with the objective of creating functional construction materials to reduce NO x emissions from roadside sources, this study focused on developing permeable concrete containing porous materials like active loess and zeolite for sidewalk pavements.The NO x reduction performance was then evaluated with the application of TiO 2 .Based on our findings, the following conclusions can be drawn: A decrease in the physical performance of permeable concrete that includes active loess and zeolite was observed in comparison to the CTRL, but they all satisfied the quality standard of permeable concrete for sidewalk and road pavement.

2.
The skid resistance and permeability coefficient showed results that satisfy standards for sidewalk concrete in all permeable concrete specimens, but the quality standard was satisfied at 80% or above of residual compressive strength in only CTRL and A1Z1 specimens in the freezing and thawing experiment.

3.
Considering the physical and durability performances of this study, A1Z1 mixture was determined as the optimal mixing ratio for permeable concrete for applying TiO 2 photocatalyst.4.
The NO x reduction efficiency tended to increase according to the increase in TiO 2 content.With 7.5% TiO 2 content, a maximum NO x reduction efficiency of 77.5% was observed.

5.
The tank photoreactor of the specimen using the compressive strength test piece also showed similar results to the NOx reduction tendency in the ISO standard photoreactor.

6.
By assessing the nitrate concentration generated after NO x reduction assessment, it was found that a photocatalyst reaction occurred during UV irradiation on the surface of the photocatalyst-coated permeable concrete.Additionally, higher nitrate concentration and higher NO x reduction efficiency were observed with 7.5% TiO 2 content than with 10% TiO 2 content.

Figure 3 .
Figure 3.An overview of specimen casting.

Figure 3 .
Figure 3.An overview of specimen casting.

Figure 4 .
Figure 4. Schematic diagram of the NOx reduction performance assessment system.

Figure 4 .Figure 4 .
Figure 4. Schematic diagram of the NO x reduction performance assessment system.

Figure 7 .
Figure 7. Images of nitrate concentration measurement equipment and nitrate collection of specimens: (a) water quality analyzer; (b) specimens of ISO standard photoreactor; (c) specimens of tank-type photoreactor.

Figure 7 .
Figure 7. Images of nitrate concentration measurement equipment and nitrate collection of specimens: (a) water quality analyzer; (b) specimens of ISO standard photoreactor; (c) specimens of tank-type photoreactor.

Figure 8 .
Figure 8. Results of compressive strength of permeable concrete specimens.

Figure 8 .
Figure 8. Results of compressive strength of permeable concrete specimens.

Figure 9 .
Figure 9. Results of flexural strength of permeable concrete specimens.

Figure 9 .
Figure 9. Results of flexural strength of permeable concrete specimens.

Figure 11 .
Figure 11.Changes in NO concentration according to TiO2 content.

Figure 11 .
Figure 11.Changes in NO concentration according to TiO 2 content.

Figure 12 .
Figure 12.Changes in NO concentration according to the TiO2 content in the tank photoreactor.

Figure 12 .
Figure 12.Changes in NO concentration according to the TiO 2 content in the tank photoreactor.

Table 1 .
Chemical composition of cement.

Table 1 .
Chemical composition of cement.

Table 2 .
Chemical composition of active loess and zeolite.

Table 2 .
Chemical composition of active loess and zeolite.

Table 4 .
Mix proportions of permeable concrete.

Table 5 .
Mix proportions of the diluted photocatalyst.

Table 6 .
Evaluation criteria and quality requirement of permeable concrete for pavement.

Table 7 .
Specimen size and compaction method details.

Table 8 .
Compressive strength and flexural strength of permeable concrete specimens.

Table 8 .
Compressive strength and flexural strength of permeable concrete specimens.

Table 9 .
Skid resistance and permeability coefficient of permeable concrete specimens.

Table 9 .
Skid resistance and permeability coefficient of permeable concrete specimens.

Table 10 .
Freezing and thawing resistance of permeable concrete specimens.

Table 11 .
NO x reduction efficiencies according to TiO 2 content.

Table 13 .
Nitrate concentrations of NO x reduction specimens.