Compatibility and Photocatalytic Capacity of the Novel Core@shell Nanospheres in Cementitious Composites

Abstract

In this paper, a novel core@shell nanosphere (TiO₂@CoAl-LDH) based on layered double hydroxide (LDH) combined with a nano-TiO₂ semiconductor was synthesized and introduced to cementitious materials via spraying technology and a smearing method. The compatibility with a cementitious matrix and the effects of TiO₂@CoAl-LDH on cement hydration, surface microstructure, and the microscopic mechanical properties of mortar were investigated by AFM, microhardness testing, FESEM, and BET analysis. Meanwhile, the effects of TiO₂@CoAl-LDH introduction methods on the photocatalytic performance and durability of the photocatalyst were systematically evaluated by methylene blue (MB) removal ratio and wear testing. The results show that TiO₂@CoAl-LDH exhibits enhanced compatibility with cementitious matrices and a higher photocatalytic capacity than individual CoAl-LDH and nano-TiO₂. The photocatalytic mortar prepared via spraying technology (CM-C) displays a higher photocatalytic capacity than that prepared via the smearing method (CM-S). Among them, the mortar with two layers of photocatalytic coatings (CM-C2) has the highest MB removal ratio, which reached 95.1% within 120 min of UV-visible light irradiation. While on the other hand, the wear test revealed that the smeared mortar has a higher photocatalytic capacity and better photocatalyst durability than the sprayed mortar. This work is expected to contribute to the development of multifunctional sustainable building materials.

1. Introduction

Cement-based materials, which represent one of the most important elements for buildings, are widely used due to their convenient usability, low cost, and impressive mechanical properties. However, building materials with a largely exposed surface area can irreversibly experience degradation under the impact of deteriorating air quality. An attractive strategy is to exploit the self-cleaning function of large-scale building surfaces by introducing photocatalytic composites into cementitious materials. As we know, the excited electron-hole pairs on semiconductors can effectively remove organic or inorganic pollutants adsorbed on building surfaces under sunlight. Thus, the proper application of these photocatalytic materials with self-cleaning effects will have great practical significance for protecting our buildings. Nanotitanium dioxide (TiO₂) is the most representative semiconductor used in the building and construction industries because of its nontoxicity, as well as its high catalytic capacity. Studies have shown that TiO₂ can degrade NO_x (NO and NO₂) attached to its surface under UV irradiation in cement-based materials. Then, the degradation product, nitrate (NO₃⁻), can be washed away from the building surfaces by rainwater so as to achieve the dual function of air purification and surface self-cleaning [1,2]. The schematic diagram of the reaction process is shown in Figure 1. In addition to mentioning the photocatalytic behavior of TiO₂ semiconductors in cementitious composites, the development of new photocatalytic materials should also consider the following aspects: improving the photocatalytic capacity of catalysts by various modification methods [3,4], strengthening the crucial properties in modified cement-based materials, such as mechanical and physical surface properties [5,6], and maintaining the highly efficient and stable photocatalytic activity of the photocatalysts in cement-based materials under complex environments [7,8,9].

According to some reports on cement-based materials that incorporate TiO₂, the introduction of TiO₂ nanoparticles can change the subsequent properties, including the cement hydration rate and crystal orientation and the crystallinity of the hydration products, pore structures, and the distribution [10,11]. There are various introduction methods, like mixing, spraying, smearing, and sol-gel coating, for applying TiO₂ particles in cementitious materials [12,13,14]. It should be noted that the introduction methods should be reasonably selected since different introduction methods directly affect the photocatalytic activity (e.g., the number and quality of available photocatalytic sites) and surface properties (e.g., the surface roughness and hydrophilicity).

Layered double hydroxides (LDHs), also known as anionic clay, are usually expressed as [M^II_1-x M^III_x(OH)₂](Aⁿ⁻)^x/n mH₂O, where [M^II_1-x M^III_x(OH)₂] represents a positively charged layer, and (Aⁿ⁻)^x/n and mH₂O represent the interlayer anions and water molecules, respectively [15,16]. Due to its excellent visible light response-ability and acid-alkali resistance, LDH with a specific layered structure has attracted widespread research attention as an efficient ultraviolet-visible light photocatalyst [17,18,19]. Among the numerous metal ions, cobalt is considered an ideal LDH nonferrous metal due to its stable photocatalytic activity. It has been confirmed that the existence of the composite hierarchical structure of TiO₂-LDH can accelerate the coupling and transfer of electrons at the interconnection interface, thus improving the separation efficiency of photogenerated electron-hole pairs [20,21]. In addition, the high surface area of LDHs can facilitate the adsorption and diffusion of environmental pollutants on the surface, accelerate the transfer of the degradation products, and promote the photocatalytic process. Meanwhile, previous research has found a similarity in molecular structures between LDHs and cement hydration products, and thus LDHs exhibit good compatibility with cement-based materials [22]. In this context, the combination of semiconductors with a certain LDH can effectively improve their dispersion performance and compatibility with cementitious materials. Moreover, a good photocatalytic coating should possess excellent properties, such as photocatalytic capacity and a strong, cohesive force with the carrier surface, which are considered important indexes for evaluating the selfcleaning and durability properties of the functional coatings [13].

This study aims to prepare a new TiO₂@CoAl-LDH core@shell nanosphere via an in situ growth method and study its compatibility and photocatalytic activity in cement-based materials. In order to assess the effect of TiO₂@CoAl-LDH nanospheres on the properties of cementitious materials, the mineral morphology, microhardness, and porous structure of paste containing TiO₂@CoAl-LDH were studied in the first part of this paper. Further study was carried out on the selfcleaning performance of the mortars prepared via spraying technology (thin coating) and the smearing method (paste layer). The effects of the TiO₂@CoAl-LDH introduction methods and spraying times on photocatalytic capacity were systematically evaluated. The potential relationship between the surface properties (i.e., surface roughness and surface hydrophilicity) and photocatalytic capacity of the mortars was investigated. In addition, the stability of TiO₂@CoAl-LDH nanospheres under external interference was explored via wear testing, which is crucial for long-term applications.

2. Results and Discussion

2.1. The Effect of TiO₂@CoAl-LDH on Phase Development in Hardened Cement Paste

The mineral morphology of cement pastes with and without TiO₂@CoAl-LDH nanospheres at the ages of 3d, 7d, 14d, and 28d are shown in Figure 2. As can be seen from Figure 2a,e, after hydration for 3 days, similarly shaped calcium silicate hydrate (C-S-H) flocculent precipitates can be observed in both the cement paste samples. Compared with the reference paste, the paste incorporated with 2% TiO₂@CoAl-LDH nanospheres has more widely distributed and denser C-S-H flocculent precipitation and a small amount of needle-like, high-sulfur calcium sulphoaluminate hydrate (AFt), and petal-like calcium monosulfoaluminate hydrate (AFm) were also formed. By comparing Figure 2b,e, after curing for 7 days, it was found that the hydration products of the reference paste sample (CP-R), such as flake calcium hydroxide (CH) and needle-like AFt, had similar mineral structures and appearance morphology to those of the CP-2% sample cured for 3 days. This observed phenomenon indicates that the existence of TiO₂@CoAl-LDH has promoted the hydration of the cement. In fact, due to the high specific surface area, the TiO₂@CoAl-LDH nanospheres provide additional nucleation sites for cement hydration, thereby accelerating the early hydration rate; a similar conclusion was recorded in our previous investigation [23]. Meanwhile, a large quantity of fine and short needle-like AFt was observed in the CP-2% paste samples after curing for 7 days, as shown in Figure 2f.

In Figure 2c,g, with the extension of curing age, the quantity of CH sheets, C-S-H gel, and needle-like AFt crystals in both cement pastes increased significantly, especially for the CP-2% paste samples. After continuous hydration for 28 days, as seen in Figure 2d,h, the CP-R paste sample exhibited more internal pores than CP-2%, which were filled by needle-like ettringite crystals (0.5–1 μm in thickness and 5μm in length) and C-S-H gel clusters. In contrast, the fibrous crystalline aggregates with good crystallinity formed by the C-S-H floccules and needle-like AFt were clearly observed in the CP-2% paste samples, uniformly distributed and intertwined, forming a dense network structure. Overall, the addition of TiO₂@CoAl-LDH nanospheres not only promotes the hydration of cement but also has a significant impact on the morphology and distribution of the hydration products.

2.2. The Effect of Tio₂@Coal-LDH on the Microscopic Strength of Cementitious Composites

Figure 3 presents the microhardness test results for the paste samples with and without TiO₂@CoAl-LDH nanospheres cured for 3 days, 7 days, 14 days and 28 days. It can be observed that the microhardness values of both paste samples have similar growth curves as a function of curing age. Compared to the values obtained for the CP-R paste samples, an increase from 27.8 Mpa at 3 days to 66.9 Mpa at 28 days in the microhardness of the CP-2% paste samples is remarkable. Interestingly, the microhardness values of the CP-R paste samples cured for 7 days are similar to those of the CP-2% paste samples only cured for 3 days, while at 28 days, the microhardness value of the CP-2% paste samples is about 1.45 times of that CP-R paste samples. This result further validates the conclusions obtained from the above section.

According to previous studies, the mechanical properties of cementitious composites largely depend on their hydration rate and degree [24,25,26], but the microstructure differences caused by hydration also need to be noted. As shown in Figure 2, compared with the pure cement paste, the interior of the cement paste with 2% TiO₂@CoAl-LDH presents a more uniform and dense fibrous surface morphology. This means that the introduction of 2%TiO₂@CoAl-LDH nanospheres has refined the microstructure of the cement paste to a certain extent and made its appearance more compact. In addition, those precipitated nanoneedle-like hydrates can more effectively fill the submicron pores and cracks in the paste and thus reinforce the paste. In other words, they are beneficial for enhancing the relevant mechanical properties of the cement paste.

2.3. The Effect of TiO₂@CoAl-LDH on Surface Microscopic Structural Properties of Cementitious Composites

Figure 4 shows the specific surface area and pore size distribution of the CP-R and CP-2% paste samples after curing for 28 days. As seen in Figure 4a, the nitrogen adsorption-desorption isotherms of both paste samples have a typical H3 hysteresis loop when the relative pressures are between 0.4 and 1, indicating the existence of a mesoporous structure [27,28]. In comparison with the CP-R paste samples (27.7 m²/g), the CP-2% samples (39.8 m²/g), which contain TiO₂@CoAl-LDH nanospheres, have a higher specific surface area. On another note, as shown in Figure 4b, the pores of the CP-2% paste sample have a more obvious bimodal distribution than CP-R, suggesting that diverse pore diameters are formed during cement hydration. Actually, the formation of this more pronounced bimodal structure in the CP-2% paste sample originates from the irregular honeycomb pores formed by the stacking of the strip hydrates with TiO₂@CoAl-LDH nanospheres [29]. Obviously, in the case of the CP-2% samples, the increase in pore volume value indicates that the addition of the TiO₂@CoAl-LDH nanospheres promotes the development of macropores (>50 nm) and mesopores (2 nm to 50 nm). It has been reported that the increase in these pore structures can provide more photocatalytic active sites and improve the light utilization ratio by increasing the light transmission channel. Therefore, it can be inferred that the introduction of TiO₂@CoAl-LDH nanospheres is conducive to the improvement of surface area and pore distribution for cementitious composites, which provides the possibility for a high photocatalytic capacity.

Based on the above analysis, in terms of the morphology, mechanical properties, and pore structure of the cementitious composites, it can be concluded that TiO₂@CoAl-LDH nanospheres play an active role in the hydration process of cement. However, taking into account the practical application of paste materials in engineering (such as surface decoration materials for building exterior walls), the CP-2% paste samples were applied as mortar decoration layers for further study. In order to assess their photocatalytic selfcleaning properties, additional MB degradation tests were conducted in the follow-up work.

Similar to the paste samples, the mechanical properties and related physical surface properties of different mortar samples were characterized after curing for 28 days, and the results are described in

Table 1. Performance indexes of mortar samples with and without TiO₂@CoAl-LDH nanospheres after curing for 28 days.

Mortar System	Ra(μm)	Hv (Pa)	θ (°)
CM-R	4.41	216.1	55.7 ± 2.4
CM-S	3.22	271.9	28.4 ± 2.1
CM-C1	4.18	221.9	51.1 ± 1.1
CM-C2	3.74	217.3	44.6 ± 1.8
CM-C3	3.63	219.8	39.4 ± 0.9

. As can be seen, the CM-R mortar samples exhibit the highest surface roughness (Ra) and contact angle (θ) due to the nonuniformity of their surface structure. After surface treatment, i.e., spraying and smearing, a significant reduction in both surface roughness and contact angle was observed, indicating that their surface hydrophilicity increased, especially when smearing the mortar (CM-S). Furthermore, the similarity in microhardness values among CM-C1, CM-C2, and CM-C3 suggests that the photocatalytic coating prepared via spraying technology has no or little influence on the mechanical property of mortar surfaces, while the mortar samples (CM-S) prepared by the smearing method were enhanced. This result supports the results obtained in Section 2.2, related to mechanical performance. Nevertheless, it can be found that regardless of the spraying technology and/or smearing method, the introduction of TiO₂@CoAl-LDH nanospheres confers surface hydrophilicity to the mortars, with an enhanced mechanical property.

2.4. Photocatalytic Activity of the Cementitious Composites Containing Tio₂@Coal-LDH

Figure 5 presents the MB removal ratio curves obtained by the powder materials under constant UV-visible light. After 60 min irradiation, the MB removal ratios of the 25 mg TiO₂@CoAl-LDH (mass ratio of TiO₂ is 0.085, and the TiO₂ content is calculated to be 2.13 mg) and 10 mg TiO₂ powder sample reached a stable plateau state of 91.8% and 61.4%, respectively, which implies that the removal ratio cannot increase further. In contrast, a slight degradation was observed for the 15 mg CoAl-LDH powder samples, which is only 10.3%. In addition, it is noteworthy that the content of TiO₂ in the TiO₂@CoAl-LDH sample was reduced by 78.7% when comparing 2.13 mg to 10 mg (of TiO₂) in the reaction systems, but the MB removal ratio was about 1.5 times that of the pure TiO₂ sample under the same condition. Therefore, it can be inferred that a significant improvement in photocatalytic capacity is ascribed to the synergistic effect between the TiO₂ particles and the CoAl-LDH nanosheets. This is especially the case when considering the low photocatalytic capacity of the pure CoAl-LDH samples.

The MB removal ratio curves of the mortar samples are shown in Figure 6. It is expected that, for all mortars prepared with spraying technology, the photocatalytic capacity can be enhanced with an increase in spraying time. However, the CM-C3 mortar samples, with three layers of coating, displayed a different photocatalytic behavior. As can be seen, the MB removal ratio of CM-C3 is lower than that of CM-C2 after 120 min UV-Visible light irradiation. Under certain assumptions, this can be construed as three sprays increasing the aggregation probability of the TiO₂@CoAl-LDH nanospheres on the face of the mortar, thereby reducing the utilization rate of the UV-visible light. Previous research has shown that the accumulation of excessive nanoparticles could increase the crystal size and reduce its specific surface area, thus hindering the diffusion of reactants on the surface of the mortar [30,31]. In addition, the results of similar experiments confirmed that the photocatalytic capacity didn’t increase with the increase in spraying time, but there was an optimal value [32]. These results confirmed the findings on why the mortar samples with the most TiO₂@CoAl-LDH photocatalytic coatings could not achieve the highest photocatalytic capacity.

When compared to spraying technology, the mortar prepared by the smearing method shows lower photocatalytic capacity, likely due to the covering effect of the cement paste. It can be expected that the covering effect dramatically reduces the number of available active sites and, thus, the light absorption efficiency. However, it was interesting to note that the CM-S mortar samples exhibited a short-term high photocatalytic capacity at the initial stage (0–25 min). This phenomenon can be attributed to its large surface roughness and minimum contact angle (hydrophilicity), as shown in Table 1. In fact, the surface of mortar with an appropriate roughness can have a large contact area with water or pollutants and maintain an appropriate flux and retention rate [33], which could promote the reaction process between the pollutants and the photocatalysts. On the other hand, strong hydrophilicity makes the hydroxyl group content of the corresponding mortar surface increase remarkably during the photo-induced process, which further accelerates the photocatalytic decomposition rate of the pollutants [34]. With the progress of a photocatalytic reaction, the decomposition of organic pollutants promotes the water droplets to spread to the whole surface of the mortar. When the surface hydrophilicity of all mortar samples reaches the same or similar state, the number of photocatalytic sites becomes the main factor affecting photocatalytic capacity. Consequently, the photocatalytic capacity of CM-S mortar decreased significantly (after 25 min) due to its small number of active sites. In conclusion, the mortar samples prepared by spraying technology have a higher photocatalytic capacity than that of the mortars prepared by smearing methods. Besides, both the proper spraying times and surface properties play a certain positive role in improving photocatalytic performance.

In order to investigate the long-term durability of the surface coatings, a photocatalytic MB degradation test was carried out on the worn mortar samples, and the results are given in Figure 7 and

Table 2. MB removal ratio of mortar samples before and after wearing.

Mortar System	After 120 min Irradiation
	Before Wearing (%)	After Wearing (%)	Decrement (%)
CM-S	65.6 ± 2.6%	70.8 ± 2.1%	−7.9 ± 2.3%
CM-C1	69.8 ± 3.1%	32.1 ± 2.7%	54.0 ± 2.9%
CM-C2	95.1 ± 1.7%	52.9 ± 1.3%	44.4 ± 1.5%
CM-C3	90.0 ± 2.8%	77.4 ± 2.7%	14.0 ± 2.7%

. It can be observed that compared to the sample before wearing, the removal ratio of all the sprayed mortars (after wear testing) was significantly reduced, especially for the CM-C1 samples, which decreased from 69.8% to 32.1%. This indicates that wear has a great influence on the surface coating and, thus, its photocatalytic performance. It can be expected that, under the action of continuous wearing, the physically deposited photocatalytic coating is easily peeled off from the mortar substrate, which greatly reduces the number of photocatalytically active sites, resulting in a low photocatalytic capacity. This mechanism is well understood, and similar conclusions can be found in other research [35].

However, it is noteworthy that, in the case of the smeared mortar, the removal ratio increases slightly after wearing, from the initial 65.6% to 70.8%. This result seems to be different from what we expected. Figure 8 shows the surface morphology and the elemental distribution of Ca and Ti (of the smeared mortar) before and after wearing. It can be observed that, after wear testing, the surface structure changes from flat to rough with more micro-nano pores, and the mapping signal strength of the Ti element on the surface is slightly enhanced. Therefore, it is speculated that this interesting phenomenon is relevant to the change in the number of available photocatalytically active sites caused by the change in the surface pore structure before and after wearing.

2.5. Proposed Mechanisms

The photocatalytic MB degradation before and after wearing indicates well that the introduction of TiO₂@CoAl-LDH nanospheres into cementitious composites can produce a stable system involving surface mechanical properties and the distribution of pores and hydrates, even if this creates low but acceptable photocatalytic capacity. A proposed mechanism for enhancing the photocatalytic capacity of smeared mortar is as follows: on the one hand, the TiO₂@CoAl-LDH nanosphere has a high specific surface area and volume ratio, which can accelerate the formation of the main cement hydration products (e.g., needle-like AFt, CH, and C-S-H), thereby enhancing the overall performance of the cement mortar. This can be regarded as an important indicator of the good compatibility between the photocatalyst and the cementitious substrate [5,36]. On the other hand, the stacking interaction between the TiO₂@CoAl-LDH nanospheres and needle-like hydration products can form many irregular honeycomb pores, as shown in Figure 9. The existence of these structures can provide more photocatalytically active sites and thus improve the utilization of light by increasing the light transmission channels, as shown in Figure 9. In addition, due to the accelerating effect on cement hydration, the pores on the surface of the mortar become uniform, and the relative volume of the pores and macropores in the sample increases, which was confirmed by the BET results. As shown in Figure 10, these pores are mainly in a connected or nonconnected state. Under the action of wearing, the number of photocatalytically active sites attached to the paste surface decreases with the loss of the surface structure, but the transition from a nonconnected to a connected state increases the possibility for more photocatalytic particles being activated inside. As a result, the photocatalytic capacity of the smeared mortar is improved.

3. Materials and Methods

3.1. Materials

The 42.5-grade ordinary Portland cement (OPC, Fujian Cement Co., Ltd) was used for the paste and mortar sample preparation. The chemical compositions of the OPC were measured by X-ray fluorescence (XRF), and are shown in

Table 3. The main chemical composition of cement.

Oxides	CaO	SiO2	Al2O3	Fe2O3	MgO	SO3	TiO2	Others
Wt.%	56.26	26.97	4.49	4.08	3.22	3.47	0.37	1.14

. The particle size distribution and XRD pattern are given in Figure 11. Chemical reagents, such as Titanium tetrabutoxide (TBT), aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O), ammonium fluoride (NH₄F), ammonium hydroxide, cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), urea, and di-ammonium hydrogen citrate were obtained from Sinopharm, China.

3.2. Sample Preparation

3.2.1. Preparation of Core@Shell Nanospheres and Suspension

The TiO₂ nanosphere was synthesized according to a modified protocol from Wang et al. [37,38]. Typically, ammonium hydroxide (0.2 mol) and deionized water (0.1 mol) were first added into 250 mL of ethanol and stirred well to obtain the well-dissolved solution. Then, tetrabutyl titanate (2.5 mL) and ethanol (12.5 mL) were added dropwise to the above solution. After thorough mixing, the mixture was transferred into a Teflon-liner, wrapped in a stainless-steel reactor, and heated at 180 °C for 48 h. After cooling to room temperature, the white flocculent precipitate was washed three times with deionized water and subsequently with anhydrous ethanol. Then, the final precipitate was dried overnight under 60 °C.

The TiO₂@CoAl-LDH core@shell nanospheres were prepared by the following procedure, as reported elsewhere [23]. Typically, Co(NO₃)₂·6H₂O (6 mmol), Al(NO₃)₃·9H₂O (2 mmol), urea (20 mmol), and NH₄F(8 mmol) were firstly dissolved in 70 mL of deionized water to prepare the precursor solution. Then, 0.15 g of TiO₂ nanoparticles were uniformly dispersed in the precursor solution by ultrasonication for 2 h. The resulting mixture was poured into a Teflon-lined stainless-steel autoclave and reacted at 100 °C for 10 h. The obtained pink precipitation was subsequently washed with water and anhydrous ethanol three times and finally dried at 60 °C for 12 h.

The TiO₂@CoAl-LDH suspension was prepared as follows. A certain mass of TiO₂@CoAl-LDH nanospheres (2 g) was uniformly dispersed into 100 mL deionized water. In order to prevent possible agglomeration, di-ammonium hydrogen citrate was used as a dispersant and ultrasonically dispersed on a cell crusher for half an hour.

The morphology and structural details of the prepared TiO₂ and TiO₂@CoAl-LDH nanospheres were analyzed by scanning electron microscope (SEM) and transmission electron microscopy (TEM). It can be seen from Figure 12a that the pristine TiO₂ nanospheres display a round shape with an average particle size of 270 nm. After the in situ growth of the CoAl-LDH nanosheets, the perpendicular intercalated structure and flower-like morphology of the nanosheets can be observed with a TiO₂ core (as shown in Figure 12b). The corresponding TEM images (Figure 12c,d) show a core@shell structure with a CoAl-LDH nanosheet thickness of about 5 nm. The high-resolution (HR) TEM images show lattice fringes corresponding to the crystal plane, with a spacing of about 0.27 nm and 0.35 nm, which can be attributed to the (012) plane of the CoAl-LDH phase and the (101) plane of the anatase TiO₂ in Figure 12f, respectively. In addition, the energy dispersive X-Ray spectroscopy (EDX) mapping analysis of the TiO₂@CoAl-LDH nanospheres is shown in Figure 12e, where Co, Al, and Ti are uniformly distributed in the whole nanospheres. Therefore, by grafting CoAl-LDH nanosheets onto the surface of the TiO₂ nanospheres, the TiO₂@CoAl-LDH nanospheres with core@shell heterostructures were successfully obtained.

BET surface area of TiO₂ and TiO₂@CoAl-LDH were calculated by the Barrett-Joyner-Halenda (BJH) model and desorption isotherms, while micropore-specific surface area and outer surface area (mesopores and macropores) were estimated by t-Plot methodology. As shown in

Table 4. Surface areas of TiO₂ and TiO₂@CoAl-LDH samples.

Materials	Surface Area (m2/g)
	BET	t-Plot Micropore	t-Plot External
TiO2	36.82	5.36	42.61
TiO2@CoAl-LDH	230.67	15.41	235.84

, the specific surface area of the TiO₂@CoAl-LDH nanospheres is 230.67 m²/g, which is about 6.26 times that of the pristine TiO₂ nanoparticles.

3.2.2. Preparation of the Cementitious Composites

The cement paste and mortar specimens were prepared according to Chinese Standard GB/T 17671. For the evaluation of the compatibility between the cement paste and TiO₂@CoAl-LDH core@shell nanospheres, cement pastes with and without 2 wt.% TiO₂@CoAl-LDH (by weight of cement) nanospheres were fabricated with a constant water-cement ratio (w/c = 0.4). The paste specimens were cast in short cylinders (ø30 mm × 15 mm) and then moved to the standard curing room (temperature and relative humidity control at 23 ± 2 °C and 98%, respectively), curing for 3, 7, 14, and 28 days. These paste specimens were labeled as CP-2% and CP-R, respectively. Meanwhile, the cement mortars had the mass ratio of the cement, water, and sand, equaling 1:0.5:3. As designed, the mortar specimens were cast into another type of short cylinder (ø50 mm × 20 mm), and cured for 28 days under the same curing conditions as the paste specimens.

In order to evaluate the influence of the different introduction methods of the TiO₂@CoAl-LDH nanospheres on photocatalytic activity, spraying technology and a smearing method were utilized, respectively, to prepare mortar specimens. The procedures are as follows: for preparing mortars by spraying technology, the mortar was fixed on a 45° bracket. Then, the prepared stable suspension was sprayed on the mortar by adjusting the nozzle to 50 cm, perpendicular to the surface. After each spray, the mortar specimens were dried at 60 °C for 30 min. Finally, the mortars with one to three spraying coatings were obtained and labeled as CMC-1, CMC-2, and CMC-3. For preparing mortars via the smearing method, a fresh cement paste containing 2 wt.% TiO₂@CoAl-LDH was firstly prepared and put onto the surface of a previously wetted mortar substrate and then spread evenly with a nylon thread. After drying for 60 min, the mortar specimens with a paste layer were obtained and labeled as CM-S. The reference mortars without TiO₂@CoAl-LDH were also prepared for comparison purposes, labeled as CM-R.

3.3. Characterization and Measurement

3.3.1. Surface Properties and Microstructural Examination

The contact angle (CA) was determined by an OCA 20 Optical contact angle measuring instrument (DataPhysics Instruments GmbH, Filderstadt, Germany). Typically, a certain volume of liquid (5 μL) was slowly dropped onto the surface of the mortar by micro syringe, and then the change in the contact angle was observed by optical microscope. To ensure the reliability of the test, the final results of the contact angle value (θ) for each sample were taken from the average value of five random measuring points. The surface roughness of the mortar samples was detected by scanning probe microscopy in atomic force microscopy (5500M Hitachi, Tokyo, Japan) with a Si tip cantilever. The Ra parameter represents the average roughness value obtained by using a linear probe with a length of 4 mm. The morphologies and hydrated minerals of the paste samples were analyzed using a field scanning electron microscope (Nova Nano SEM 230, FEI Corporation, Hillsboro, OR, USA) at a high acceleration voltage of 15 kV. The specific surface area and micro-nano hierarchical porous structure of the paste samples were analyzed by using an automatic specific surface and porosity analyzer (ASAP 2020, Micromeritics Instruments Inc, Norcross, GA, USA). Before testing, the paste samples were normally degassed at 60 °C for 24 h.

3.3.2. Microscopic Strength

According to previous studies [39], microscopic hardness is commonly used to investigate the surface mechanical properties of cementitious composites because there is a positive correlation between microhardness and the surface mechanical properties. In this study, the microhardness of the mortar specimen was measured by a Vickers microhardness tester (THSV-1-800-AXY, Tuojing Instrument Technology Corporation, Hainan, China) with a fixed load of 1kg. Before the test, all mortar specimens were grounded and then polished with a metallographic polishing machine (Unipol-830, Hefei Kejing Material Technology Corporation, Anhui, China) until a glossy and smooth surface was achieved. In addition, 30 tests were conducted in different regions on each mortar surface to ensure the reliability of the measurements. The microhardness value was calculated according to the following formula:

$$Hv=\frac{1}{30}{{\displaystyle \sum }}_{i=1}^{30}1.8544\times \frac{9.8P}{{\left(d_i\times {10}^{-3}\right)}^2}$$

(1)

where $P$ represents the constant load value applied (i.e., 1 kg); $d_i$ represents the diagonal length of indentation (mm); $Hv$ represents the average microhardness value (Pa).

3.3.3. Photocatalytic Selfcleaning Performance

The photocatalytic removal of methylene blue (MB) is one of the simplest and most convenient tests to investigate the selfcleaning performance of photocatalytic materials. In recent years, several reports have come up with a new test device suitable for the photocatalytic degradation of dyes by cementitious composites [5,40], as shown in Figure 13. Specifically speaking, in order to ensure the dye made full contact with the photocatalytic surface, 20 ppm MB solution was loaded into the bottomless glass tube (ø40 × 35 mm) on the mortar sample. Then, the mortar sample was immersed in the MB solution with the same concentration for 24 h to quickly reach a constant saturation state. Finally, the adsorbed saturated mortar samples were transferred to another fresh MB solution for photocatalytic reaction under constant irradiation light (PLS-SXE300D xenon lamp; intensity of UV–visible light spectra was 0.922 mW/cm²). The distance between the lamp and the specimen was set as the same (40 cm).

The MB removal by each mortar sample was investigated by sampling the glass tube within a given 15 min interval for up to 120 min. The MB concentration was measured by monitoring the major absorption peak at λ = 650 nm using a UV spectrophotometer (UV-2450, Shimazu). The MB removal ratio is calculated as follows:

$$\delta =\frac{C_0-C_t}{C_0}\times 100\%$$

(2)

where $\delta$ represents removal ratio of MB (%); $C_0$ expresses the original concentration of the solution in the adsorption equilibrium state (ppm); $C_t$ represents the concentration of the solution at any time t (ppm).

Meanwhile, the difference in photocatalytic activity between the monomers (TiO₂ and CoAl-LDH) and the composite (TiO₂@CoAl-LDH) was also assessed by MB removal testing. However, considering the influence of some external factors (such as masking effects) on photocatalytic activity during the preparation of cement-based materials, the corresponding photocatalytic powder materials were also subjected to the test. The photocatalytic removal experiment was carried out in a CEL-APR100H photochemical temperature-controlled integrated reactor under the same experimental conditions. A total of 10 mg TiO₂ powder, 15 mg CoAl-LDH powder, and 25 mg TiO₂@CoAl-LDH composite powder were dispersed into 100 mL MB solution (20 ppm), respectively. Then, the solid-liquid mixture was quickly stirred for 45 min in a sealed dark environment to achieve adsorption/desorption equilibrium. The MB removal of each powder sample was investigated by sampling from the reactor within a given 30 min interval for up to 120 min. Finally, MB removal ratios of all samples were also calculated using formula 2.

3.3.4. Durability of the Photocatalytic Coatings under the Wear Test

The durability assessment of the photocatalytic coatings applied to the mortars was carried out after wear testing. For this purpose, the samples were placed on a reciprocating-type wear tester with a fixed 75 N constant load at 100 rpm/min and each sample was worn for 5 min. Then, after wearing, the samples were cleaned with ethanol to remove the wear debris generated during the test before being subjected to the other assigned experiments.

4. Conclusions

In this study, a novel TiO₂@CoAl-LDH nanosphere with a core@shell structure was successfully synthesized via a combination of hydrothermal and in situ growth methods. The TiO₂@CoAl-LDH nanospheres were introduced into cementitious materials by either spraying technology or a smearing method. The effects of the TiO₂@CoAl-LDH nanospheres on the microstructure, micromechanical properties, and photocatalytic performance of the cementitious materials were investigated. Meanwhile, the effects of nanospheres introduction methods on the photocatalytic performance and durability of the photocatalyst were systematically evaluated. The conclusions can be drawn as follows:

(1) The introduction of TiO₂@CoAl-LDH core@shell nanospheres into cementitious materials promotes the cement hydration process and the development of pore structures, thereby improving microscopic mechanical properties.

(2) Due to the synergistic effect of nano-TiO₂ and CoAl-LDH nanosheets, the TiO₂@CoAl-LDH core@shell nanospheres exhibit enhanced compatibility with cementitious matrices and a higher photocatalytic capacity than that of individual CoAl-LDH and nano-TiO₂.

(3) The TiO₂@CoAl-LDH nanospheres introduction methods affect the number of photocatalytically active sites on the mortar surface. The mortar prepared via spraying technology (CM-C) has a higher photocatalytic capacity than that prepared by the smearing method (CM-S). Among them, the mortar with two layers (CM-C2) of the photocatalytic coating has the highest MB removal ratio, which reaches 95.1 within 120 min of UV-visible light irradiation.

(4) The wear tests showed that the photocatalytically active sites on the surface of the sprayed mortar are significantly reduced by continuous wearing, resulting in a significant decrease in photocatalytic capacity. In contrast, the unique porous structure on the surface of the smeared mortar increases the possibility of more photocatalysts being activated inside, which is conducive to an increase in photocatalytic capacity.