Development of TiO2–CaCO3 Based Composites as an Affordable Building Material for the Photocatalytic Abatement of Hazardous NOx from the Environment

This study explores the depollution activity of a photocatalytic cementitious composite comprising various compositions of n-TiO2 and CaCO3. The photocatalytic activity of the CaCO3–TiO2 composite material is assessed for the aqueous photodegradation efficiency of MB dye solution and NOx under UV light exposure. The catalyst CaCO3–TiO2 exhibits the importance of an optimal balance between CaCO3 and n-TiO2 for the highest NOx removal of 60% and MB dye removal of 74.6%. The observed trends in the photodegradation of NOx removal efficiencies suggest a complex interplay between CaCO3 and TiO2 content in the CaCO3–n-TiO2 composite catalysts. This pollutant removal efficiency is attributed to the synergistic effect between CaCO3 and n-TiO2, where a higher percentage of n-TiO2 appeared to enhance the photocatalytic activity. It is recommended that CaCO3–TiO2 photocatalysts are effectiveness in water and air purification, as well as for being cost-effective construction materials.


Introduction
A flurry of research activities is directed at the removal of nitrogen oxides (NO x ), one of the main pollutants in the atmosphere, due to their toxic effects on health and a series of other ecological and environmental problems [1].Various methodologies have been developed, such as selective non-catalytic reduction, selective catalytic reduction, scrubbing, and adsorption, to control NO x emissions [2].However, the above-mentioned technologies for NO x degradation are costly and cause secondary pollution [3].One of the promising techniques for enhancing the photocatalytic performance of heterojunction semiconductor catalysis has been attempted to resolve these issues [4][5][6].In the process of development to real applications, titanium dioxide (TiO 2 ) photocatalysts have also been included in concrete engineering toward degradation of NO x in the atmosphere [7].Despite a few advancements in the production of commercial photocatalytic types of cement with the inclusion of nano titanium dioxide (n-TiO 2 ) particles, the utility of photocatalytic cement/concrete is limited, especially in terms of its efficiency in dealing with air pollution and cost-effectiveness [8,9].Many reports revealed the efficacy of TiO 2 photocatalysts in cementitious materials (with TiO 2 loadings from 1 to 10 wt.% as a fraction of the cement content) [10].These TiO 2 preparations of photocatalytic cement/concrete materials have been proven to be efficient in depolluting the environment by degrading NO x (de-NO x effect).The successful application of n-TiO 2 cement-based materials in engineering has been well demonstrated.The pavement prepared with n-TiO 2 cement-based materials in China exhibited a good ability to remove NO x exhaust from vehicles [11].The road exhaust index of nano-TiO 2 concrete pavement materials in Milan, Italy, was determined, and it was found that a reduction of 60-70% in the pollution index has been witnessed [12].A cricket stadium in Dubai sports city is coated with nano-TiO 2 white cement in buildings to provide improved air quality [13].However, there are a few key questions regarding maximizing the photocatalytic efficiencies and cost-effectiveness of n-TiO 2 -incorporated photocatalytic cement/concrete in real environmental conditions.
The n-TiO 2 has several advantages and features, like stable physical and chemical properties, non-toxic nature, low cost, and adequate photocatalytic activities under ultraviolet light [14][15][16][17][18]. n-TiO 2 can be easily agglomerated when it is mixed with cementitious materials [19].The highly alkaline and calcium-rich cement environment promotes TiO 2 agglomeration, decreases the catalyst surface area, and causes the surface precipitation of calcium hydroxide/calcium carbonate [20].As a consequence of poor dispersibility and/or occlusion of TiO 2 by cement hydrates, high loadings of n-TiO 2 are required to achieve high photocatalytic efficiency, which leads to higher costs.These aspects make the effective dispersion of TiO 2 in cement/concrete a challenging task [21].It should also be kept in mind that n-TiO 2 has a poor response under visible light, which limits the development of photocatalytic cement-based materials in natural light scenarios [22].We envisage that there needs to be a good strategy for balancing high photocatalytic efficiency and cost-effectiveness in real-world conditions.
The aim of this study is to develop an efficient and cost-effective photocatalytic cement material along with TiO 2 as a promotor and immobilize the resultant composite onto the surface of a pre-formed mortar (which can absorb the composite particles on its surface).Calcium carbonate (CaCO 3 ) is a kind of inorganic, nonmetallic mineral (mostly as calcite) material with abundant reserves and also derived from biowaste chicken eggshells; calcite materials have advantageous characteristics, such as high whiteness and low cost, and are considered superior to most non-metallic minerals in cost-effective performance [23,24].The development of CaCO 3 -nano-TiO 2 composite photocatalysts using CaCO 3 as the additive to TiO 2 has been demonstrated to reduce the agglomeration of TiO 2 particles, improve the recyclability and reusability performance of nano-TiO 2 , and enhance the photocatalytic efficiency of nano-TiO 2 particles.In the literature, a few techniques, such as hydrolytic deposition, sol-gel, and chemical precipitation methods, have been reported for the preparation of TiO 2 -n-CaCO 3 .The hydrolysis of TiCl 4 on the surface of CaCO 3 and the resultant composite exhibited the maximum photocatalytic degradation efficiency (95%) of Rhodamine B (10 ppm) upon UV irradiation [21,25,26].The preparation of CaCO 3 -TiO 2 composite particles has been demonstrated through carbonation in a TiO 2 system [27].The preparation of CaCO 3 -TiO 2 composite particles with hydrophobic agglomeration has been detailed [28,29].The CaCO 3 -TiO 2 composite was prepared using the chemical precipitation method by coating the surface of CaCO 3 with TiO 2 using titanium sulfate as the titanium source [30].The resulting product, CaCO 3 -TiO 2 , has been proven to have high ultraviolet absorption capacity.However, the above methods for the preparation of CaCO 3 -TiO 2 are complex, time-consuming, and costly.It should be mentioned that the utility of CaCO 3 -TiO 2 composites for construction material development is scarce.
The photocatalytic CaCO 3 -TiO 2 composites were optimized to achieve high catalytic efficiency for NO x degradation, and aqueous methylene blue (MB) photodegradation efficiencies were hardly reported.In this work, we develop a CaCO 3 -TiO 2 composite with various weight percentages (wt.%) of catalyst-based cement mortar and surface coating on the mortar to enhance the efficiency and also the production of large-scale, cost-effective construction materials.

Preparation of Photocatalyst Included Mortar
Mortar specimens were prepared following the compressive strength test method (KS L 5105) of hydraulic cement mortar.In a typical preparation, cement and standard sand were used in a weight ratio of 1:2.45, and the amount of water was 60% based on the weight percentage of the cement.The mortar specimen size was 100 mm long, 50 mm wide, and 5 mm high.The photocatalyst was applied on the surface of the early-aged mortar (cured in water for 7 days and dried in the air for 2 days) thin plate samples; toward that, approximately 0.5 g of the CT composite photocatalyst was dispersed in 1 g of water and directly applied uniformly on the mortar surface as a coating and dried at 80 • C. For testing the mortars mixed with photocatalysts, in addition, the calculated amount of photocatalyst was mixed with 5, 10, and 15% based on the weight percentage of cement.For the NO x reduction test (KS L ISO 22197-1), water curing at 25 • C was performed for 7 days.The fabricated cement mortar specimen is shown in Figure S1.

Characterization
The phase purity of the composite samples was confirmed using room temperature XRD (Panalytical, United Kingdom) measured with Cu-Kα (λ = 1.5418Å) radiation and a 0.02 • scan step size from (2θ) 10 to 80 • .Field emission scanning electron microscopy (FE-SEM) was analyzed with Hitachi and X-ray photoelectron spectroscopy (XPS), Thermo Fisher, Waltham, MA, USA (NEXSA).The optical properties were analyzed with a UV-Vis spectrophotometer S-3100 (Scinco Co., Ltd., Seoul, Republic of Korea).The photo-catalytic performance of the photocatalysts was investigated by irradiating the MB solution under a UV lamp (20 W, λ = 352 nm) purchased from SANKYO DENKI CO., Ltd., Tokyo, Japan.The concentration of MB was assessed with a UV-Vis spectrophotometer S-3100 (Scinco Co., Ltd.) at 665 nm.The concentrations of NO x were recorded with the chemiluminescence technology of the NO x analyzer-Serinus 40.

Photodegradation of MB
The experiments were performed following the specifications of KS L ISO 10678.Before irradiation, 0.5 g of photocatalyst powder was added to the MB solution prepared with a concentration of 10 mg/L.The bulk MB (200 mL) was prepared in water, and the TiO 2 sample was dispersed with continuous stirring for 10 min.The dispersion was kept in a dark chamber for 1 h to ensure adsorption-desorption equilibrium.Subsequently, the mixed solution was irradiated under UV light and stirred constantly, while the temperature inside the stainless chamber was maintained at 25 • C. At periodic time intervals, a catalyst solution was taken successively from the mixture using an appropriate filter mounted on a syringe to separate photocatalyst particles.The concentration of MB was assessed by recording the absorbance of the solution at 665 nm.

Photodegradation of NO x
NO x removal experiments were carried out according to the KS L ISO 22197-1 standard.The photoactivity of the prepared samples was measured through a photoreactor placed in a stainless box whose dimensions were 620 mm × 430 mm × 285 mm.The photoreactor was 430 mm long, 100 mm wide, and 40 mm high.On the top cover of the stainless box, light sources were installed to induce photoactivity.The size of the sample in the photoreactor was 100 mm long, 50 mm wide, and 5 mm high.The distance from the top surface of the mortar thin plate sample to the optical window of the photoreactor was approximately 10 mm.The prepared mortar sample was introduced into the photoreactor, and then the mass flow controller was used to adjust the flow rate of NO gas, water vapor, and air.The concentrations of NO x were recorded with the chemiluminescence technology of the NO x analyzer-Serinus 40.The photocatalytic experiment was carried out when NO x was stabilized at 1000 ppb for 30 min after reaching the adsorption-desorption equilibrium in dark conditions.A UV lamp emitting UV rays between 310 nm and 400 nm was used to illuminate the photocatalytic mortar sample surface at an intensity of 1000 µW/cm 2 for 6 h.Afterward, the UV light was turned off, and the NO x concentration went back to the stabilized concentration level monitored for 30 min.

Possible Photocatalytic Mechanism of MB
The photocatalytic mechanism of MB dye degradation by the TiO 2 -CaCO 3 composite is depicted in Figure 1.
placed in a stainless box whose dimensions were 620 mm × 430 mm × 285 mm.The photoreactor was 430 mm long, 100 mm wide, and 40 mm high.On the top cover of the stainless box, light sources were installed to induce photoactivity.The size of the sample in the photoreactor was 100 mm long, 50 mm wide, and 5 mm high.The distance from the top surface of the mortar thin plate sample to the optical window of the photoreactor was approximately 10 mm.The prepared mortar sample was introduced into the photoreactor, and then the mass flow controller was used to adjust the flow rate of NO gas, water vapor, and air.The concentrations of NOx were recorded with the chemiluminescence technology of the NOx analyzer-Serinus 40.The photocatalytic experiment was carried out when NOx was stabilized at 1000 ppb for 30 min after reaching the adsorption-desorption equilibrium in dark conditions.A UV lamp emitting UV rays between 310 nm and 400 nm was used to illuminate the photocatalytic mortar sample surface at an intensity of 1000 µW/cm 2 for 6 h.Afterward, the UV light was turned off, and the NOx concentration went back to the stabilized concentration level monitored for 30 min.

Possible Photocatalytic Mechanism of MB
The photocatalytic mechanism of MB dye degradation by the TiO2-CaCO3 composite is depicted in Figure 1.The TiO2 component of TiO2-CaCO3 acts as a photocatalyst to mainly perform the photocatalytic function.Under the influence of UV light irradiation, the high-energy photons activate to create photoinduced electrons (e − ) and holes (h + ) in TiO2.The negatively charged electrons are energized to cross the band gap (Eg) and jump into the CB, leaving behind positively charged holes in the VB.In the presence of CaCO3, the CB electrons migrate towards the surface of CaCO3 (CaCO3 acts as an e-sink).This electron transfer process can prevent the e − -h + pair recombination.Then, the electrons trapped in CaCO3 can react with adsorbed O2 to generate free superoxide radicals (•O2 − ).Simultaneously, photogenerated holes combine with H2O or the adsorbed hydroxyl ion (OH − ) molecules to form hydroxyl radicals (•OH).Finally, the generated free radicals (•OH, •O2 − , h + ) with high oxidizing ability would destroy the MB molecules into low-weight intermediates.These smaller compounds are further degraded to similar molecules, such as CO2 and H2O.
The entire photocatalytic process for MB degradation is summarized in the following equations: The entire photocatalytic process for MB degradation is summarized in the following equations: Intermediates → Degradation products + CO 2 + H 2 O (10)

The Possible Photocatalytic Mechanism of NO x
The mechanism of NO x reduction using pristine and TiO 2 -CaCO 3 composite catalysts reduces the NO x under UV light irradiation.With the exposure to UV light, the catalyst composite becomes activated, resulting in the generation of electron-hole pairs, which is a crucial step in the photocatalytic process.The composite material remains intact and can continually serve as a photocatalyst, facilitating the reduction of NO x and the oxidation of organic contaminants under UV light exposure.Once the NO x molecules are reduced and organic contaminants are oxidized, they may desorb from the composite surface as harmless gases or byproducts [31].This process continues as long as there is an energy source, such as UV light, to activate the TiO 2 -CaCO 3 component of the composite.It offers an eco-friendly approach to reduce NO x emissions and potentially eliminate organic pollutants from the surrounding environment, making it a valuable method for enhancing air quality and reducing the environmental impact of construction materials.
The chemical reactions involved are as follows: Adsorption of NO x : Generation of electron-hole pairs under UV light: Reduction of NO x under UV light: NO/TiO 2 -CaCO 3 + e − → N 2 + O 2 + TiO 2 -CaCO 3 (13) These equations illustrate the essential steps in the photocatalytic NO x reduction process using a TiO 2 -CaCO 3 composite with UV light as the energy source [32,33].

Results and Discussion
The photocatalytic efficiency of the prepared CaCO 3 -n-TiO 2 composites, catalysts with various weight percentages (wt.%) of C and T (CT-1, CT-2, CT-3, CT-4, and CT-5), was analyzed.The photocatalytic experiments were carried out in both aqueous as well as in gaseous conditions.Typically, the photodegradation experiments of MB were carried out in aqueous solution under UV irradiation.Gaseous NO x photodegradations were performed with the prepared composite photocatalysts under UV irradiation.

Photodegradation of MB
The photocatalytic activity of the composite catalysts C, T, CT-1, CT-2, CT-3, CT-4, and CT-5 under UV light irradiation was analyzed.The absorbance changes at 665 nm were monitored over the reaction interval of 15 min up to 3 h during the photocatalytic degradation process and are shown in Figure 2. The percentage of MB dye degradation efficiency was calculated with the following equation: where C 0 is the initial concentration of the MB dye solution, and C is the final concentration of the MB dye absorption after irradiation [34].
degradation process and are shown in Figure 2. The percentage of MB dye degradation efficiency was calculated with the following equation: where C0 is the initial concentration of the MB dye solution, and C is the final concentration of the MB dye absorption after irradiation [34].The photocatalytic degradation of MB by pure T was found to be the slowest as evidenced by the lowest MB-PDE of 44.39% after 3 h (shown in Table 1).The photocatalytic degradation of MB by pure T was found to be the slowest as evidenced by the lowest MB-PDE of 44.39% after 3 h (shown in Table 1).The MB-PDE of C after 3 h is deplorably low (9.40%).The MB-PDE values of the CaCO 3 -n-TiO 2 composite photocatalysts take the order CT-4 > CT-2 > CT-3 > CT-5 > CT-1 > T > C; the degradation efficiency is shown in Figure 2. The trend indicates that there is variation in the MB-PDE values amongst the composite photocatalysts.The CT-4, at 74.5%, exhibits the highest MB-PDE value.It gives a clue that not only the simple synergistic effect of the two components in the composites but also the composition play an important role in deciding the photocatalytic efficiency of the prepared photocatalysts [35][36][37][38].The photodegradation of MB under UV irradiation follows the pseudo-first-order kinetics, and the rate constants derived from the slopes of the linear plots are presented in Figure S1.In a comparison of the pristine samples of C and T, the composite samples of CT-1, CT-2, CT-3, CT-4, and CT-5 had increased rate constants in comparison to their pristine counterparts.
Consequently, this indicates that there can be a hybrid effect that may influence the rate of MB photodegradation [39,40].The rate constant comparison informs that the CT-4 samples showed the highest photodegradation rate constants for MB photodegradation amongst the photocatalysts tested.The rate constant for the CT-4 samples is typically reported to be 0.45 s −1 , which is considerably higher than the rate constant for pure T samples (0.2 s −1 ) as well as the rate constant for C samples (0.03 s −1 ), which indicates that there has been a hybrid effect between the C and T photocatalysts on the composite CT-4 photocatalytic properties [41].

Photodegradation of NO x
The photocatalytic performance of pristine and CaCO 3 -n-TiO 2 composite catalysts with varying weight percentages of CaCO 3 and n-TiO 2 (C and T) was evaluated for NO x removal under UV light at room temperature and is shown in Figure 3.The catalyst exhibited effective photocatalytic activity, leading to a notable decrease in the concentrations of NO x .The NO x removal performance was computed using the following equation: where NO x represents the initial concentration of NO x , and NO x out is the recorded concentration at the end of the photodegradation process [32].
74.5%, exhibits the highest MB-PDE value.It gives a clue that not only the simple synergistic effect of the two components in the composites but also the composition play an important role in deciding the photocatalytic efficiency of the prepared photocatalysts [35][36][37][38].The photodegradation of MB under UV irradiation follows the pseudo-first-order kinetics, and the rate constants derived from the slopes of the linear plots are presented in Figure S1.In a comparison of the pristine samples of C and T, the composite samples of CT-1, CT-2, CT-3, CT-4, and CT-5 had increased rate constants in comparison to their pristine counterparts.Consequently, this indicates that there can be a hybrid effect that may influence the rate of MB photodegradation [39,40].The rate constant comparison informs that the CT-4 samples showed the highest photodegradation rate constants for MB photodegradation amongst the photocatalysts tested.The rate constant for the CT-4 samples is typically reported to be 0.45 s −1 , which is considerably higher than the rate constant for pure T samples (0.2 s −1 ) as well as the rate constant for C samples (0.03 s −1 ), which indicates that there has been a hybrid effect between the C and T photocatalysts on the composite CT-4 photocatalytic properties [41].

Photodegradation of NOx
The photocatalytic performance of pristine and CaCO3-n-TiO2 composite catalysts with varying weight percentages of CaCO3 and n-TiO2 (C and T) was evaluated for NOx removal under UV light at room temperature and is shown in Figure 3.The catalyst exhibited effective photocatalytic activity, leading to a notable decrease in the concentrations of NOx.The NOx removal performance was computed using the following equation: where NOx represents the initial concentration of NOx, and NOx out is the recorded concentration at the end of the photodegradation process [32].Figure 4 shows the pristine and different concentrations of the composite samples of NOx degradation efficiency between the mortar, pristine, and composite photocatalysts.The mortar sample that showed the lowest NOx photodegradation efficiency of 1.6% was the sample mixed with the C sample.In contrast, the CT-4 composite samples exhibited the highest NOx photodegradation efficiency of 60% during 6 h.The observed trends in photocatalytic NOx removal efficiencies suggest a complex interplay between CaCO3 and TiO2 content in the CaCO3-n-TiO2 composite catalysts.The catalyst CT-4 highlights the importance of an optimal balance between CaCO3 and n-TiO2 for the highest NOx removal.This removal efficiency can be attributed to the synergistic effect between CaCO3 and n-TiO2, where the higher percentage of n-TiO2 appeared to enhance the catalytic activity for NOx degradation.The increase in n-TiO2 content seemed to have a marginal effect on the catalyst performance, suggesting a potential threshold or optimal balance between carbon and titanium dioxide content for enhanced photocatalytic activity.

NOx Photodegradation of Catalyst CT-4 Mixed Cement Mortar
The highest efficiency NOx removal catalyst CT-4 mixed cement mortar possesses excellent photocatalytic activity amongst the composites; studies were performed to know the influence of coating of CT-4 over the mortar surface on the photocatalytic NOx removal efficiency.This study aimed to develop environment-friendly construction materials for photocatalytic NOx degradation in cement mortar, and the results are shown in Figures S2 and S3.The photocatalyst CT-4 was mixed with cement mortar at various ratios (0%, 5%, 10%, and 15%) as shown in Table 2. To evaluate the effectiveness of NOx photodegradation, plain mortar (0%) was used as a reference sample, and the average NOx concentrations were analyzed during three Figure 4 shows the pristine and different concentrations of the composite samples of NO x degradation efficiency between the mortar, pristine, and composite photocatalysts.The mortar sample that showed the lowest NO x photodegradation efficiency of 1.6% was the sample mixed with the C sample.In contrast, the CT-4 composite samples exhibited the highest NO x photodegradation efficiency of 60% during 6 h.The observed trends in photocatalytic NO x removal efficiencies suggest a complex interplay between CaCO 3 and TiO 2 content in the CaCO 3 -n-TiO 2 composite catalysts.The catalyst CT-4 highlights the importance of an optimal balance between CaCO 3 and n-TiO 2 for the highest NO x removal.This removal efficiency can be attributed to the synergistic effect between CaCO 3 and n-TiO 2 , where the higher percentage of n-TiO 2 appeared to enhance the catalytic activity for NO x degradation.The increase in n-TiO 2 content seemed to have a marginal effect on the catalyst performance, suggesting a potential threshold or optimal balance between carbon and titanium dioxide content for enhanced photocatalytic activity.

NO x Photodegradation of Catalyst CT-4 Mixed Cement Mortar
The highest efficiency NO x removal catalyst CT-4 mixed cement mortar possesses excellent photocatalytic activity amongst the composites; studies were performed to know the influence of coating of CT-4 over the mortar surface on the photocatalytic NO x removal efficiency.This study aimed to develop environment-friendly construction materials for photocatalytic NO x degradation in cement mortar, and the results are shown in Figures S2 and S3.The photocatalyst CT-4 was mixed with cement mortar at various ratios (0%, 5%, 10%, and 15%) as shown in Table 2. To evaluate the effectiveness of NO x photodegradation, plain mortar (0%) was used as a reference sample, and the average NO x concentrations were analyzed during three segments of NO x degradation as 30 min off state, followed by 30 min on state, and this cycle was performed three times during the 210 min as shown in Figure 5.

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Nanomaterials 2024, 14, 136 9 of 17 segments of NOx degradation as 30 min off state, followed by 30 min on state, and this cycle was performed three times during the 210 min as shown in Figure 5.The NOx degradation was measured for the mortar samples mixed with the CT-4 photocatalyst at different inclusion levels.The results indicated that the NOx removal increased with an increase in the weight percentage of the photocatalyst on the surface.The average NOx concentrations for the mortar mixed with the CT-4 catalyst at 5%, 10%, and 15% inclusions were 895 ppb, 866 ppb, and 825 ppb, respectively.Moreover, the mortar mixed with the CT-4 photocatalyst at 15% exhibited higher NOx photodegradation efficiency than the mortar mixed with 5% and 10%.This could be attributed to the larger component amount of CT-4 catalyst encompassed on the mortar surface during the manufacturing process.It can be observed that the mortar coated with 15% photocatalyst had the highest NOx degradation efficiency, indicating the potential effectiveness of a reduced content of coating of CT-4 catalyst and suggesting that CT-4 can be affected and used as the economical additive with mortar for NOx removal in cement mortar.

UV-Visible Analysis
The UV-DRS spectra of the pristine and CaCO3-n-TiO2 composite exhibited characteristic absorption features in the ultraviolet to visible range and are shown in Figure 6a.The NO x degradation was measured for the mortar samples mixed with the CT-4 photocatalyst at different inclusion levels.The results indicated that the NO x removal increased with an increase in the weight percentage of the photocatalyst on the surface.The average NO x concentrations for the mortar mixed with the CT-4 catalyst at 5%, 10%, and 15% inclusions were 895 ppb, 866 ppb, and 825 ppb, respectively.Moreover, the mortar mixed with the CT-4 photocatalyst at 15% exhibited higher NO x photodegradation efficiency than the mortar mixed with 5% and 10%.This could be attributed to the larger component amount of CT-4 catalyst encompassed on the mortar surface during the manufacturing process.It can be observed that the mortar coated with 15% photocatalyst had the highest NO x degradation efficiency, indicating the potential effectiveness of a reduced content of coating of CT-4 catalyst and suggesting that CT-4 can be affected and used as the economical additive with mortar for NO x removal in cement mortar.

UV-Visible Analysis
The UV-DRS spectra of the pristine and CaCO 3 -n-TiO 2 composite exhibited characteristic absorption features in the ultraviolet to visible range and are shown in Figure 6a.
segments of NOx degradation as 30 min off state, followed by 30 min on state, and this cycle was performed three times during the 210 min as shown in Figure 5.The NOx degradation was measured for the mortar samples mixed with the CT-4 photocatalyst at different inclusion levels.The results indicated that the NOx removal increased with an increase in the weight percentage of the photocatalyst on the surface.The average NOx concentrations for the mortar mixed with the CT-4 catalyst at 5%, 10%, and 15% inclusions were 895 ppb, 866 ppb, and 825 ppb, respectively.Moreover, the mortar mixed with the CT-4 photocatalyst at 15% exhibited higher NOx photodegradation efficiency than the mortar mixed with 5% and 10%.This could be attributed to the larger component amount of CT-4 catalyst encompassed on the mortar surface during the manufacturing process.It can be observed that the mortar coated with 15% photocatalyst had the highest NOx degradation efficiency, indicating the potential effectiveness of a reduced content of coating of CT-4 catalyst and suggesting that CT-4 can be affected and used as the economical additive with mortar for NOx removal in cement mortar.

UV-Visible Analysis
The UV-DRS spectra of the pristine and CaCO3-n-TiO2 composite exhibited characteristic absorption features in the ultraviolet to visible range and are shown in Figure 6a.A wide absorption band in the range between 250 nm and 400 nm was observed.The spectra demonstrated distinctive absorption peaks and shoulder regions, indicative of the electronic transitions within the materials.The band gap energy (Eg) of the catalyst composite was determined from the following relation: where α is the optical absorption coefficient, hν is the photon energy, Eg is the energy band gap, A is the constant, and n = 1 and n = 2 are the direct and indirect band gap semiconductors [34].The optical energy transition of the CaCO 3 -n-TiO 2 composite is directly transition allowed, so n = 1. the spectra analysis revealed distinct absorption features for each composition, while the band gap analysis provided valuable insights into the optical properties of the CaCO 3 -n-TiO 2 composite catalysts with varying C and T weight percentages.The plot of (αhν)2 vs. hν catalyst composite is shown in Figure 6b, which depicts the variation energy band gap concerning composite compositions.The energy band gap of the n-TiO 2 indirect transition was calculated as 3.20 eV, and that of CaCO 3 was calculated as 4.15 eV.The composites of CaCO 3 -n-TiO 2 with various compositions of CT-1, CT-2, CT-3, CT-4, and CT-5 band gap values are 3.21, 3.20, 3.23, 3.17, and 3.19 Ev for the composite catalysts.It shows that the CaCO 3 -n-TiO 2 composite exhibits similar light absorption of n-TiO 2 .This result shows that the similar properties of n-TiO 2 consist of strong photolytic degradation properties [42,43].These findings lay a foundation for further understanding the photocatalytic activities and potential applications of these composite catalyst materials.

Micro-Structure Analysis
The powder X-ray diffraction patterns of the CaCO 3 -n-TiO 2 composites of various compositions (CT-1, CT-2, CT-3, CT-4, and CT-5) are presented in Figure 7.For comparison purposes, the XRD patterns of pure T and C are also displayed.The diffraction peaks are observed at 25.4 • , 37.8 • , 48.1 • , and 54.0 • for T and CaCO 3 -n-TiO 2 composites that are assigned to the (101), (004), (200), and (211) crystal planes of the anatase phase (JCPDS No. 00-071-1166 and JCPDS No. 00-083-0578).The peak shape and position of the CaCO 3 -n-TiO 2 composites are approximately the same as those of pure anatase TiO 2 (T).However, the intensities of the anatase peaks are suppressed.Keeping this information, it is envisaged that TiO 2 exits on the surface of CaCO 3 .The sharp and narrow peaks of CaCO 3 are assigned to the diffraction planes (hkl) planes (012), (104), (110), (113), (202), (018), (116), and (122) crystalizing planes of calcite phase of CaCO 3 [44].There is an increased intensity of characteristic diffraction peaks belonging to C in conjunction with the decreased intensity of TiO 2 diffraction in TiO 2 -CaCO 3 composite samples following the influence of TiO 2 on the surface of CaCO 3 [43].The coexistence of diffraction peaks of the calcite and anatase phases in the CaCO3n-TiO2 composites suggests that the crystalline nature of the pure components is retained in the composites.(Table 3).The crystalline unit cell parameters of T (tetragonal) and C (trigonal) are derived from the major diffraction (101) and (104) peaks of T and C, respectively (Table 3).On perusal of Table 3, it can be inferred that the lattice constants a and c of the anatase phase are increased from 3.752 Å and 9.405 Å to a higher value.
Table 3.The powder XRD parameters of T, C, CT-1, CT-2, CT-3, CT-4, and CT-5 composite catalysts.The coexistence of diffraction peaks of the calcite and anatase phases in the CaCO 3 -n-TiO 2 composites suggests that the crystalline nature of the pure components is retained in the composites.(Table 3).The crystalline unit cell parameters of T (tetragonal) and C (trigonal) are derived from the major diffraction (101) and (104) peaks of T and C, respectively (Table 3).On perusal of Table 3, it can be inferred that the lattice constants a and c of the anatase phase are increased from 3.752 Å and 9.405 Å to a higher value.On the contrary, the lattice constant a of C remains unaltered upon composite formation, and the value of c decreased from pure C (c = 17.050Å) to a lesser value.This information suggests that, during composite formation, the c-axis of T (anatase phase) is elongated, possibly by the insertion of Ca ions.The average crystallite size was obtained using the Scherrer formula: where D is the crystalline size, λ is the wavelength of Cu-kα radiation (1.54056 Å), β is the full-width half maximum of peak intensity, and θ is the peak position.The crystallite sizes of T and C were calculated from the (101) and (104) diffraction peaks of T and C, respectively.The crystallite size of the composites calculated based on (101) diffraction peak of T (T-50.31nm, CT1-53.83,CT2-54.02,CT3-54.83,CT4-54.4 and CT5-58.12)was found to be slightly larger than the size of pristine T. Based on that, it can be inferred that there can be a surface layer of C on the surface of the T.And the size of the composites calculated based on (104) diffraction peak of C showed a decrease as compared to pristine C (Table 3) and an increase as compared to pristine T. The coating of C over T or T over C could be the reason for the changes in the sizes of the composite particles.The morphology of the T, C, CT-1, CT-2, CT-3, CT-4, and CT-5 composites are shown in Figure 8a-g.
The existence of irregularly shaped granular TiO 2 particles of different sizes (with particle sizes of 0.05-0.60µm) can be observed with good dispersity.We consider that smaller particles are agglomerated to be present as larger-sized particles [45].Figure 8g presents the morphology of pure C. It can be observed that C existed rhombohedral phased calcite with a cubical morphology as a major component and hexagonal phased vaterite with spherical morphology.Figure 8a-e present the morphologies of the various composites (CT-1, CT-2, CT-3, CT-4, and CT-5), which suggest that particles with mixed morphologies of both C and T and the sizes of the particles are dependent on the composition of C and T in the composites.On close analysis of Figure 8a-e, the average particle size is higher for CT-1, CT-2, and CT-3, the composites having larger proportions of C in the composite.The average particle sizes of CT-4 and CT-5 (the composites having higher proportions of T) are comparatively smaller.Keeping the morphological and size variations amongst the composites, we envisage coating of C over T for CT-1, CT-2, and CT-3 and coating of T over the surface of C for CT-4 and CT-5.The observed higher photocatalytic efficiency for CT-4, as noticed in Figure 8a-c, can be due to the existence of more T phases on the surface of C and the hybrid effect of C and T on the photocatalytic activity.
the composite.The average particle sizes of CT-4 and CT-5 (the composites having higher proportions of T) are comparatively smaller.Keeping the morphological and size variations amongst the composites, we envisage coating of C over T for CT-1, CT-2, and CT-3 and coating of T over the surface of C for CT-4 and CT-5.The observed higher photocatalytic efficiency for CT-4, as noticed in Figure 8a-c, can be due to the existence of more T phases on the surface of C and the hybrid effect of C and T on the photocatalytic activity.

Elemental and Electronic States
XPS was used for the qualitative surface elemental composition and quantitative states of the various elements.Figure 9 shows the XPS survey spectra for the TiO 2 -CaCO 3 composites (CT-1, CT-2, CT-3, CT-4, and CT-5) and their precursors.

Elemental and Electronic States
XPS was used for the qualitative surface elemental composition and quantitative states of the various elements.Figure 9 shows the XPS survey spectra for the TiO2-CaCO3 composites (CT-1, CT-2, CT-3, CT-4, and CT-5) and their precursors.The survey spectra inform the existence of Ca, Ti, and O as major elements through the respective binding energy peaks.The absence of other elements indicates the purity of the prepared composite samples.The individual core level spectra for the elements T, C, CT-1, CT-2, CT-3, CT-4, and CT-5 are shown in Figure S4.The deconvoluted individual core level spectra for the elements T and the CT-4 composite mixture were depicted in Figure 10a,b.The survey spectra inform the existence of Ca, Ti, and O as major elements throug the respective binding energy peaks.The absence of other elements indicates the purit of the prepared composite samples.The individual core level spectra for the elements T C, CT-1, CT-2, CT-3, CT-4, and CT-5 are shown in Figure S4.The deconvoluted individua core level spectra for the elements T and the CT-4 composite mixture were depicted i Figure 10a,b  The charge correction for the core level peak positions was carried out by C1s (284.eV) as a reference.The core level spectrum of Ti2p of T exhibits binding energy (BE) peak at 458.2 and 464.9 eV, corresponding to the spin-orbit splitting of 458.2 and 464.9 eV rele vant to Ti2p3/2 and Ti2p1/2 electronic states with a BE difference of 6.70 eV.
This indicates the presence of Ti 4+ valance states [28].Further, the BE peak position o Ti2p is shifted towards higher energy for CT-4, informing the probable changes in th electronic state of Ti 4+ (Figure 10b).The result is consistent with the XRD results (  The charge correction for the core level peak positions was carried out by C1s (284.8 eV) as a reference.The core level spectrum of Ti2p of T exhibits binding energy (BE) peaks at 458.2 and 464.9 eV, corresponding to the spin-orbit splitting of 458.2 and 464.9 eV relevant to Ti2p3/2 and Ti2p1/2 electronic states with a BE difference of 6.70 eV.

Intensity
This indicates the presence of Ti 4+ valance states [28].Further, the BE peak position of Ti2p is shifted towards higher energy for CT-4, informing the probable changes in the electronic state of Ti 4+ (Figure 10b).The result is consistent with the XRD results (Table 3), informing the changes in the lattice constants tetragonal phase of TiO 2 .Considering the ionic radius of Ti 4+ and Ca 2+ is 0.064 nm and that of Ti 4+ is 0.106 nm, doping of TiO 2 by Ca ions is expected [46].Figure 10a,b compare the BE peaks of Ca 2p.The BE peaks are noticed for pure C at 347.2 eV and 350.9 eV, corresponding to Ca 2p1/2 and Ca 2p3/2, which are in good agreement with the Ca 2+ oxidation state, respectively [47].Furthermore, there was a small shift observed in the binding energy peaks of Ca 2p for CT-4 (Figure 10b).The absence of satellite peaks and individual core level peak splitting in the composite samples details the elemental and phase purity of the samples.The core level spectra of O1s are approximately 530 eV in all pure and composite samples.Moreover, the case of pure samples (TiO 2 and CaCO 3 ) shows a maximum of 529.5 eV and approximately 531.6 eV.These binding energy maximums correspond to the metal-oxygen and carbonoxygen bonding in the TiO 2 and CaCO 3 structures.In the case of the composite samples, these two O1s core energy peak maximums are retained without appreciable changes.The deconvoluted peaks at 529.5 eV and 531.6 eV in the composite samples describe the success of composite formation on the catalytic products [48].The C1s core level spectrum of CT-4 shows BE peaks at 284.8 eV and 290 eV, informing the bonding in the CaCO 3 structure.From the above observations, we could conclude that CT-4, having changed in the electronic states of Ti 4+ and Ca 2+ due to possible interaction during preparation, could cause enhanced photocatalytic effects as compared to pure T.

Conclusions
In this study, we developed a facile and cost-effective method for the large-scale production of photocatalysts using CaCO 3 -loaded n-TiO 2 cementitious composites.The optimal balance between CaCO 3 and n-TiO 2 influences the catalyst efficiency for degrading MB dye solution and NO x under UV light exposure.Under the influence of UV light irradiation, the high-energy photons activate to create photoinduced electrons (e − ) and holes (h + ) in TiO 2 .As the TiO 2 -CaCO 3 catalyst composite is activated with UV light, electronhole pairs are generated, making a crucial phase in the photocatalytic process.Especially, the higher ratio of n-TiO 2 exhibits heightened catalytic activity, suggesting a synergistic relationship between the two constituents characterized with physiochemical analyses.The observed trends in the photodegradation of NOx underscore a nuanced interplay between the CaCO 3 and TiO 2 content within the CaCO 3 -n-TiO 2 composite catalysts.This efficient pollutant removal is attributed to a synergistic effect between CaCO 3 and n-TiO 2 , where a higher percentage of n-TiO 2 notably enhances the photocatalytic activity.The CT-4 photocatalyst exhibits exceptional degradation performance, making it a promising application for environmentally friendly and cost-effective construction materials.

Figure 1 .
Figure 1.MB degradation possible mechanism of TiO 2 -CaCO 3 composite.The TiO 2 component of TiO 2 -CaCO 3 acts as a photocatalyst to mainly perform the photocatalytic function.Under the influence of UV light irradiation, the high-energy photons activate to create photoinduced electrons (e − ) and holes (h + ) in TiO 2 .The negatively charged electrons are energized to cross the band gap (Eg) and jump into the CB, leaving behind positively charged holes in the VB.In the presence of CaCO 3 , the CB electrons migrate towards the surface of CaCO 3 (CaCO 3 acts as an e-sink).This electron transfer process can prevent the e − -h + pair recombination.Then, the electrons trapped in CaCO 3 can react with adsorbed O 2 to generate free superoxide radicals (•O 2 − ).Simultaneously, photogenerated holes combine with H 2 O or the adsorbed hydroxyl ion (OH − ) molecules to form hydroxyl radicals (•OH).Finally, the generated free radicals (•OH, •O 2 − , h + ) with high oxidizing ability would destroy the MB molecules into low-weight intermediates.These smaller compounds are further degraded to similar molecules, such as CO 2 and H 2 O.The entire photocatalytic process for MB degradation is summarized in the following equations: TiO 2 + hν (UV) → TiO 2 (e − CB + h + VB )(1)

Figure 4
Figure4presents the bar chart indicating the comparison of the NO x removal efficiency (%) of the various composites and pure components (C and T).The results in Figures3 and 4show that CT-4 exhibited excellent NO x photodegradation compared with the other composites and pure components.

Figure 4
Figure 4 presents the bar chart indicating the comparison of the NOx removal efficiency (%) of the various composites and pure components (C and T).The results in Figures 3 and 4 show that CT-4 exhibited excellent NOx photodegradation compared with the other composites and pure components.

Figure 9 .Figure 9 .
Figure 9. XPS survey spectrum of CT-1, CT-2, CT-3, CT-4, and CT-5 composites and pristine components (C and T).The survey spectra inform the existence of Ca, Ti, and O as major elements through the respective binding energy peaks.The absence of other elements indicates the purity of the prepared composite samples.The individual core level spectra for the elements T, C, CT-1, CT-2, CT-3, CT-4, and CT-5 are shown in Figure S4.The deconvoluted individual

Table 2 .
Preparation of mortar and mortar mix with photocatalyst proportion.

Table 2 .
Preparation of mortar and mortar mix with photocatalyst proportion.

Table 3
informing the changes in the lattice constants tetragonal phase of TiO2.Considering th