Enhanced Photocatalytic Activity in Photocatalytic Concrete: Synthesis, Characterization, and Comprehensive Performance Assessment of Nano-TiO₂-Modified Recycled Aggregates

Abstract

This study presents an exhaustive exploration into the development and rigorous evaluation of nano-TiO₂-modified recycled aggregates (NT@RAs) as an environmentally sustainable substitute for natural aggregates in concrete applications. A methodical framework was devised for the synthesis and thorough characterization of NT@RAs, emphasizing the optimization of nano-TiO₂ loading onto the RA surface and within its intricate porous structure. The investigation encompassed three distinct types of recycled aggregates: recycled glass sands (RGSs), recycled clay brick sands (RCBSs), and recycled concrete sands (RCSs). Of particular interest, NT@RGS, with its properties of an inherently smooth surface texture and low water absorption, was found to exert a favorable influence on the rheological behavior of concrete, manifested in reduced yield stress, thereby underscoring the potential for fine-tuning mix designs to enhance workability. As the substitution levels of NT@RGS and NT@RCBS escalated, an initial decrement in compressive strength was discernible, which subsequently reversed to strength restoration at optimized substitution ratios. This phenomenon is attributed to the synergistic interplay among NT@RA components. Remarkably, NT@RA-incorporated concrete demonstrated unparalleled self-cleaning abilities, surpassing the performance of concrete with direct nano-TiO₂ powder incorporation. This comprehensive research contributes significantly to the advancement in sustainable, high-performance photocatalytic construction materials within the realm of concrete technology. It underscores the potential for enhancing not only the rheological and mechanical properties but also the environmental responsiveness of concrete through the innovative utilization of NT@RAs.

1. Introduction

In the era of rapid urbanization and industrialization, the construction industry stands as a cornerstone of economic growth, yet it is inextricably linked to an alarming environmental challenge: the escalating generation of construction and demolition waste (CDW) [1,2]. The sheer magnitude of CDW, estimated to constitute approximately 30–40% of all solid waste globally, poses significant management difficulties, threatening landfills’ capacity and posing a risk to the natural environment through leachate contamination and greenhouse gas emissions [3,4,5]. Consequently, the development of sustainable strategies for CDW management has emerged as a pressing global agenda, aimed at minimizing waste generation, promoting circular economy principles, and mitigating the environmental impacts associated with its disposal [6].

One promising avenue towards addressing this challenge lies in the valorization of CDW through its conversion into recycled aggregates (RAs), a process that not only alleviates the burden on landfills but also addresses the parallel issue of dwindling natural aggregate resources [7]. RAs, obtained primarily through crushing and screening CDW, have been increasingly incorporated into concrete production as a substitute for natural aggregates, offering a dual benefit of waste reduction and resource conservation [8]. However, the widespread adoption of RAs in construction materials has been hindered by their inferior physical and mechanical properties compared to natural aggregates, particularly their higher porosity [9], lower density [10], and greater water absorption capacity [11]. These limitations necessitate innovative approaches to enhance the performance of RA-based materials and facilitate their high-value utilization.

Studies [12,13,14] delve into the realm of surface engineering and nanotechnology, exploring the potential of nanoscale titanium dioxide (nano-TiO₂) as a functional modifier to transform RAs into a novel class of materials—photocatalytic recycled aggregates (PC-RAs). N. Avinash Reddy et al. [15] examined the photocatalytic performance of TiO₂-coated cement mortars under sunlight, comparing intermixed and surface-coated variants. It confirmed that TiO₂-coated cement enhances pollutant degradation and self-cleaning properties, making it ideal for urban surfaces. Teng yaun et al. [16] evaluated the g-C₃N₄/TiO₂ composite’s efficiency in pollutant degradation, using recycled concrete powder (RCP) as a carrier. The results showed that RCP significantly improves the sustainability of concrete production while maintaining high photocatalytic efficiency, aided by nano-TiO₂ and RCP’s high porosity [17]. By anchoring nano-TiO₂ particles onto the surface and within the porous structure of RAs, we aim to create a hybrid material that combines the structural benefits of RAs with the environmental remediation capabilities of nano-TiO₂, ultimately yielding photocatalytic recycled concrete (PC-RC) with enhanced functional performance.

The concept of using nano-TiO₂ for environmental remediation is well established, with numerous studies [18,19,20,21] demonstrating its efficacy in degrading various pollutants, including volatile organic compounds (VOCs), nitrogen oxides (NOx), and dyes. However, the direct application of nano-TiO₂ particles in large-scale environmental systems is often limited by their high cost, agglomeration tendency, and difficulty in recovery. Integrating nano-TiO₂ into RAs, on the other hand, offers a cost-effective and scalable solution, as the aggregate serves as a stable support matrix for the nanoparticles, enhancing their dispersion and durability while facilitating their reuse in construction materials. Moreover, the porous nature of RAs presents a unique advantage for nano-TiO₂ loading, as it provides ample surface area and internal voids for the nanoparticles to adhere and infiltrate, resulting in a more intimate and uniform distribution [22,23,24]. This configuration not only maximizes the exposure of nano-TiO₂ to incident light but also facilitates the transport of pollutants to the active photocatalytic sites, enhancing the overall photocatalytic efficiency [25,26,27].

This research endeavors to delve into the realm of sustainable construction materials by devising an innovative approach for the fabrication of nano-TiO₂-modified recycled aggregates (PC-RAs) and comprehensively evaluating their potential as replacements for natural aggregates in concrete. The overarching objective is to establish a rigorous protocol for the synthesis and characterization of PC-RAs, with emphasis on optimizing the incorporation of nano-TiO₂ onto the RA surface and within its porous matrix. Furthermore, this study investigates the rheological properties of PC-RC, examining the impact of varying PC-RA concentrations on the plasticity and flow behavior of the fresh concrete mixture. The mechanical performance of PC-RC is also evaluated, focusing on assessing its structural integrity and load-bearing capacity. To gain insights into the microstructure, advanced imaging methodologies are utilized to characterize the microstructural features of PC-RC, elucidating the interactions between nano-TiO₂ and the RA matrix and their implications on the overall microstructure. Additionally, the photocatalytic efficacy of PC-RC under simulated solar radiation is examined, quantifying its capacity to degrade model pollutants and evaluating its performance in terms of environmental remediation.

2. Results and Discussion

2.1. Rheological Properties of Photocatalytic Recycled Mortar

Figure 1 illustrates the rheological properties of freshly mixed photocatalytic mortar when NT@RAs replace natural river sand as fine aggregates at a 100% substitution rate. Here, NT@RAs are composed of NT-RCBS or NT-RCA combined with NT-RGS at specific ratios (1:0, 3:1, 1:1). By performing a linear regression analysis on shear rates and their corresponding shear stress values, linear equations corresponding to different NT@RA combinations can be obtained. The coefficient of the first-order term in this equation represents the plastic viscosity value, while the constant term indicates the yield stress value.

For mortar incorporating NT-RCBS, as shown in Figure 1 (left), an increase in the substitution rate of NT-RGS leads to a gradual decrease in mortar yield stress. When the combination ratios are 3:1 and 1:1, the yield stress decreases by 17.93% and 35.41%, respectively, compared to the 1:0 ratio. The plastic viscosity decreases by 15.35% and 28.59%, respectively, relative to the 1:0 ratio. Since the primary factors influencing rheological properties include aggregate material, particle size, and morphology [28,29,30], the smoother surface of NT-RGS (see Figure 2) results in less frictional loss against the cement paste during flow, positively impacting the rheological properties of the recycled mortar. The extremely low water absorption of NT-RGS helps reduce water consumption with lubricating effects in the paste, thereby mitigating competition for free water between it and the cementitious materials. This characteristic implies that NT-RGS does not excessively absorb water in the paste, maintaining effective water utilization. This contributes to sustaining the working performance of the mortar while also preventing potential reductions in compressive strength and durability due to water competition. This compensatory effect is corroborated by Nacim Khelil et al.’s research [31,3233], demonstrating that combining smoother dune sand and rougher crushed sand as fine aggregates for mortar preparation effectively enhances concrete flowability.

As depicted in Figure 1 (right), the variation pattern of the paste yield stress with an increasing NT-RGS substitution rate in mortar incorporating NT-RCA is akin to that observed in mortar with RCBS addition. When the combination ratios are 3:1 and 1:1, the yield stress decreases by 18.93% and 35.07%, respectively, relative to the 1:0 ratio. Correspondingly, the plastic viscosity reduction rates decrease by 14.85% and 28.62%, respectively. Based on the preceding analysis, the rough surface of NT-RCBS may exert an adverse effect on the rheological properties of mortar. However, this issue can be effectively mitigated through its combined use with NT-RGS. Due to its extremely low water absorption, NT-RGS facilitates the retention of water within the mortar system, reducing water consumption. This, in turn, partially compensates for the potential water loss and increased paste viscosity that might arise from the rough surface of NT-RCA. Therefore, the combined utilization of NT-RCBS or NT-RCA with NT-RGS not only enhances the working performance of photocatalytic recycled mortar but also optimizes its rheological characteristics. Such optimization is instrumental in boosting overall construction efficiency and quality. This finding underscores the importance of considering aggregate surface properties and their interactions in the development of high-performance recycled mortars, emphasizing the potential of tailored aggregate combinations to address specific rheological challenges.

2.2. Mechanical Properties of Photocatalytic Recycled Mortar

Compressive strength, as a crucial indicator for evaluating the mechanical performance of cement-based materials in practical applications, is the focus of this section. Here, this study investigates the influence of various combinations of NT@RAs on the 28-day compressive strength of photocatalytic recycled mortar. By analyzing the effects of NT@RAs under different overall substitution rates on the compressive strength of mortar, this study aims to provide a scientific basis for optimizing the mix ratio of photocatalytic recycled mortar. The results are presented in Figure 3.

Photocatalytic recycled mortar with two types of fine aggregates exhibits higher compressive strength compared to unmodified recycled mortar. This enhancement can be partially attributed to the addition of ultrafine metakaolin, which plays a vital role in effectively inhibiting the alkali–aggregate reaction prone to occurring after the incorporation of photocatalytic recycled glass sand, while also compensating for the negative impact of the smooth surface of RGS on mortar strength. As the content of photocatalytic recycled red brick sand and glass sand increases, the compressive strength of the mortar gradually decreases from 83.3 MPa to around 50 MPa, with a maximum reduction of 45.38%. However, when the ratio of clay brick sand to glass sand is 3:1, a slight increase in compressive strength is observed as the overall substitution rate increases from 25% to 50%. This may be because, with a higher overall substitution rate, there is an increased amount of hard photocatalytic recycled glass sand and porous, rough recycled clay brick sand that binds more tightly with the cement matrix. During the process of compressive cracking, the friction force resisting detachment from the matrix increases, surpassing the reduction in compressive strength caused by the decrease in river sand. As the overall substitution rate further increases (75%, 100%), the compressive strength of the mortar decreases significantly (57 MPa→37.2 MPa), with a reduction exceeding 30%. This may be due to the excessive overall substitution rate of recycled fine aggregates, which increases the amount of recycled glass sand. While its high hardness contributes to compressive strength, it cannot compensate for the weak bond with the cement matrix caused by its relatively smooth surface. On the other hand, when NT is loaded onto RCBS, the formed aggregates may reduce the effective bonding area between old and new mortar to some extent, leading to a decrease in the density of recycled mortar. This density reduction implies an increase in air content, i.e., porosity, in the matrix, which may adversely affect the strength and durability of the mortar [34].

Although NT-RCBS has lower hardness than NT-RGS, its rough outer surface contributes to enhancing the interbonding with the surrounding paste, which is beneficial for improving the strength of the interfacial transition zone. The rough surface can increase the mechanical bonding force between old and new paste, thereby enhancing the bonding strength. Meanwhile, due to its smooth surface characteristics, NT-RGS can improve the rheological properties of mortar, making it easier to shape. Moreover, its higher hardness compared to NT-RCBS also contributes to the enhancement in mortar strength. Therefore, appropriately increasing the content of NT-RGS can leverage its low water absorption and smooth surface characteristics to improve the rheological properties of mortar, while also enhancing the mechanical properties of mortar through its higher hardness. As a result, when the ratio of clay brick sand to glass sand is higher than 1:1, and the overall substitution rate increases from 50% to 75%, the decrease in compressive strength is minimal. This may be because NT-RCBS compensates for its lower hardness through stronger mechanical bonding force with the surrounding paste, while NT-RGS compensates for the weakened bonding performance due to its smooth surface through its higher hardness.

When recycled clay brick sand and recycled glass sand are mixed in equal proportions and used as a whole to replace a recycled fine aggregate, as shown in Figure 4a, the compressive strength of the photocatalytic recycled mortar increases with the increase in the overall substitution rate, yet it remains lower than that of the mortar prepared with river sand. For the mortar where recycled clay brick sand completely substitutes river sand at a 100% rate, its strength decreases by 37.82% compared to pure river sand mortar, but increases by 23.03% relative to the original recycled mortar. For the mortar prepared by incorporating a recycled concrete fine aggregate and recycled glass sand in equal proportions, as illustrated in Figure 4b, the trend is analogous to that of the recycled red brick sand combination: compared to pure river sand mortar, the strength of the mortar where a photocatalytic recycled concrete fine aggregate fully replaces river sand at a 100% rate decreases by 38.90%, yet it increases by 12.61% when compared to pure recycled mortar. The reason why the compressive strength of recycled mortar is universally lower than that of mortar prepared with river sand lies in the fact that, due to their porous structures, NT-RCBS (nano-TiO₂-coated recycled concrete brick sand) and NT-RCA (nano-TiO₂-coated recycled concrete aggregate) generally exhibit lower hardness than river sand. Furthermore, the incorporation of RGS (recycled glass sand) to some extent weakens the interfacial bonding between old and new paste, thereby reducing the compressive strength of the mortar.

Additionally, owing to the filling effect of nanoparticles, the combination of a photocatalytic recycled concrete fine aggregate and photocatalytic recycled glass sand exhibits a smaller decrease in compressive strength across different composite substitution rates compared to the combination of recycled red brick sand, as depicted in Figure 4b. It is observed that the recycled concrete fine aggregate treated at a concentration of 3% exhibits a more dispersed and uniform distribution of nanoparticle agglomeration sites on its surface compared to recycled red brick sand. This results in relatively less air content when bonding with the cement matrix, allowing the filling effect of nanoparticles to be fully leveraged. When the overall substitution rate exceeds 75%, the inhibitory effect of the filling effect of nanoparticles on the decrease in compressive strength of recycled mortar becomes insignificant. This may be attributed to the fact that when the total amount of nanoparticles exceeds a certain threshold, their dispersion within the cement-based material deteriorates, leading to concentrated agglomeration and subsequently, poor density.

2.3. Microstructure of the Photocatalytic Recycled Mortar

To investigate the impact of photocatalysts and their incorporation methods on the cement hydration process in mortar, scanning electron microscope (SEM) images of various samples hardened for 28 days were observed. Two control groups were established for these samples: one was prepared by substituting 100% of river sand with an unmodified recycled fine aggregate, with other conditions remaining consistent; the other involved directly mixing x grams (where x represents the total effective loading of NT on the recycled fine aggregate used in the experimental group) of powdered nano-TiO₂ during mortar mixing. The experimental group consisted of mortar specimens prepared by substituting river sand with a recycled concrete fine aggregate treated with a 3% NT aqueous dispersion at different proportions.

The results indicated that, although nano-TiO₂ is not a commonly used admixture in cement-based materials, the presence of nano-sized TiO₂ particles in the mortar leads to an increase in the number of nucleation sites, accelerating cement hydration [35]. Mortar samples prepared using an unmodified recycled fine aggregate exhibited pronounced honeycomb-like characteristics under microscopic examination, with a widespread distribution of platelet-shaped calcium hydroxide (CH). The morphology of the C-S-H gel was similar to the type II calcium silicate hydrate described in the research by Taylor et al., as illustrated in Figure 5a [36]. In the recycled mortar samples directly incorporated with powdered nano-TiO₂, a significant amount of acicular ettringite was observed exposed within the loose hydration products at the fracture surface and interfacial transition zone, as depicted in Figure 5b. When the substitution rate of the photocatalytic recycled concrete fine aggregate reached 75%, the prepared mortar samples exhibited a morphology distinctly different from the two control groups, with a more uniform distribution of hydration products and insignificant density variations on the fracture surface, as shown in Figure 5c. Furthermore, when the substitution rate of NT-RCA (nano-TiO₂-coated recycled concrete aggregate) reached 100%, the recycled mortar specimens exhibited contrasting results. Despite the highest concentration of TiO₂ at this dosage, the samples exhibited the lowest compressive strength. The reason for this adverse effect can be attributed to the poor dispersion of high-concentration nanoparticles in the mortar, leading to agglomeration and subsequently, poor density of the mortar, as evident in Figure 5d. This finding underscores the importance of considering the dispersion and dosage of nano-TiO₂ in the development of photocatalytic concrete materials.

2.4. Self-Cleaning Effect of Photocatalytic Recycled Mortar

To investigate the comparative efficiency of the photocatalytic degradation of methyl blue in mortar surfaces prepared via distinct incorporation strategies, a control group was established, where 3% (by mass of cement) powdered nano-titanium dioxide (nano-TiO₂) was directly mixed with cement and aggregates to produce the photocatalytic recycled mortar. After curing in a standard chamber for 28 days, the samples underwent hydration termination and drying processes, followed by exposure to visible light for 2 h. The methyl blue degradation results for the control group, along with those for the RCBS (recycled clay brick sand)-based and RCA (recycled concrete aggregate)-based photocatalytic recycled mortars, are depicted in Figure 6. The outcomes reveal significant variations in the photocatalytic performance among the different samples.

The methyl blue degradation rates for all samples after 60 min were below 50%, indicating that the adsorption–degradation equilibrium had not yet been achieved at this stage. Notably, when nano-TiO₂ powder was directly incorporated, the degradation rate was less than 20%. This phenomenon can be attributed primarily to the dominance of methyl blue adsorption and natural degradation processes within the photocatalytic mortar, as opposed to the enhanced degradation rates observed with the composite addition of various photocatalytic recycled fine aggregates. Photocatalysis is a surface-mediated chemical process, wherein intimate contact between the photocatalyst and reactants, coupled with illumination activation, is crucial. It was contended that increased porosity favors improved photocatalytic activity. Furthermore, the incorporation of recycled glass sand enhances light transmittance within the mortar matrix, further boosting the photocatalytic degradation efficiency.

The photocatalytic degradation of methylene blue (MB) by titanium dioxide (TiO₂) represents a highly efficient and environmentally benign process rooted in the unique optical and electronic properties of TiO₂ [37]. Upon exposure to ultraviolet (UV) light, specifically in the range of 300–400 nm, TiO₂ undergoes a photon-induced excitation, whereby electrons are promoted from the valence band to the conduction band, leaving behind positively charged holes (h⁺) in the valence band [38]. This charge separation initiates a cascade of redox reactions on the TiO₂ surface, where the holes react with adsorbed water molecules to generate hydroxyl radicals (•OH), while the excited electrons reduce oxygen molecules to form superoxide radicals (O₂⁻•) or, in the presence of water, hydrogen peroxide (H₂O₂), which can further yield hydroxyl radicals [39]. These hydroxyl radicals, being among the most potent oxidizing agents, non-selectively attack the MB molecules, initiating a degradation process that breaks down the organic structure into smaller, less toxic intermediates, ultimately resulting in the formation of carbon dioxide (CO₂), water (H₂O), and other inorganic compounds. The process is self-sustaining as the electrons and holes eventually recombine, restoring TiO₂ to its original state, ready to absorb more UV light and initiate another cycle of photocatalytic degradation.

It is thereby evident that the mere incorporation of nano-TiO₂ powder, albeit possessing photocatalytic properties, does not sufficiently leverage the full potential of photocatalytic degradation within the mortar system. The incorporation of photocatalytic recycled aggregates, which offer a larger surface area for the photocatalytic reaction and improved light penetration, represents a more effective strategy for enhancing the self-cleaning performance of recycled mortar.

3. Experimental Details

3.1. Materials

The primary cementitious material utilized in this study is Portland cement, specifically grade PO 52.5 as defined in the national standard GB175-2023 (common Portland cement) for Common Portland Cement. This cement grade possesses a strength class of 52.5 and an apparent density of 3100 kg/m³, with main chemical compositions in

Table 1. Chemical composition of cement (wt.%).

Composition	SiO2	Al2O3	CaO	Fe2O3	MgO	K2O	Na2O	SO3	LOI a
Content	23.22	4.64	65.65	0.03	0.88	0.08	0.04	3.37	1.99

Note: a LOI indicates the loss of ignition.

. As a mineral admixture, metakaolin (MK), procured from a market, is employed. The recycled fine aggregates (RAs) under investigation comprise recycled concrete sands (RCSs), recycled clay brick sand (RCBS), and recycled glass sand (RGS). These aggregates are sourced from the crushing and screening processes of recovered concrete, clay bricks, and glass waste generated from construction and demolition waste (C&DW). To ensure consistency in the properties of the RAs, all samples used in the experiments are from the same production batch.

Table 2. Basic performance indices of RAs.

Aggregate	Size/mm	AD/kg·m−3	WA/%	CV/%	PC/%
RCA	0.6–4.75	2387	7	13.4	2.8
RCBS	0.6–4.75	1800	17.5	17.9	4.7
RGS	0.6–4.75	2500	-	-	-

Note: AD—apparent density, WA—water absorption, CV—crushing value, PC—powder content.

outlines the fundamental performance indicators of these various RAs, confirming their compliance with the technical requirements stipulated in the standard GB/T 25177-2010: recycled coarse aggregate for concrete. The natural aggregate employed serves as a control and is sourced from natural river sand. The photocatalyst used is commercially available Evonik Industries’ P25 nano-titanium dioxide, a powder formulation renowned for its performance. Its average particle size is around 21 nm.

3.2. Sample Preparation

Preparation of photocatalytic recycled aggregates (TiO₂@RA)

Photocatalytic recycled aggregates (hereinafter referred to as TiO₂@RAs) are fabricated through the coating of nano-titania (NT) onto the surface of RAs. Initially, the RAs are immersed in a 0.1 mol/L NaOH solution for 24 h to introduce hydroxyl groups onto their surfaces. Following this step, the RAs are rinsed thoroughly with deionized water four times and subsequently dried at 105 °C for 12 h to remove residual moisture. Subsequently, a dispersing agent is added to a measured amount of NT hydrosol or powder, which is then subjected to ultrasonication at 275 watts (W) for 30 min to achieve a well-dispersed NT suspension. This step ensures a uniform distribution of NT particles, which is crucial for effective coating onto the RAs.

Thereafter, the alkaline-treated RAs are introduced into the dispersed NT hydrosol or aqueous dispersion system under magnetic stirring. The mixture is allowed to soak for 30 min, during which time the NT particles adhere to the RAs’ surfaces, forming a coating layer. Finally, the NT-coated RAs are separated from the suspension and further dried under vacuum at 65 °C for 24 h. This drying process consolidates the NT layer onto the RAs, resulting in the formation of TiO₂@RAs. The obtained TiO₂@RAs exhibit enhanced properties due to the presence of the NT coating, which not only modifies their surface chemistry but also endows them with photocatalytic capabilities, opening up new avenues for their utilization in various fields.

Preparation of photocatalytic concrete containing TiO₂@RA

Photocatalytic concrete, measuring 40 × 40 × 40 mm, was fabricated in this experimental study. The detailed mix proportions for these specimens are presented in

Table 3. Mix. proportion of photocatalytic concrete containing nano-TiO₂-treated RAs.

No.	w/b	Binding		Aggregates (%)			WR
		MK	C	RS	NT@RAs	Ratio (by wt%)
1	0.45	0.3	0.7	100	0	-	0.13%
2				75	25	100:0	0.14%
3						75:25	0.20%
4						50:50	0.21%
5				50	50	100:0	0.17%
6						75:25	0.19%
7						50:50	0.21%
8				25	75	100:0	0.16%
9						75:25	0.20%
10						50:50	0.22%
11				0	100	100:0	0.18%
12						75:25	0.22%
13						50:50	0.25%

Note: MK—metakaolin, C—cement, RS—river sands, Ratio—the weight ratio between NT@RCBS or NT@RCS and NT@RGS, WR—water reducer.

. The water-to-binder ratio was determined to be 0.45, whilst the sand-to-binder ratio is 2.5. The aggregate composition was strategically designed, with one mixture utilizing a combination of recycled crushed brick sand (RCBS) and recycled glass sand (RGS), while the other employed a blend of a recycled concrete aggregate (RCA) and RGS. To ensure optimal workability, an appropriate amount of a water-reducing admixture was incorporated, resulting in an initial flowability of the freshly mixed mortar ranging from 140 to 160 mm.

The entire mixing procedure was rigorously conducted in accordance with the specifications outlined in GBT17671-2021, utilizing a Hobart mixer to ensure homogeneity and consistency. Upon the completion of mixing, the freshly prepared mortar was promptly poured into molds, and the surface was subsequently covered with a layer of plastic film to minimize moisture evaporation. After curing at room temperature for 24 h, the specimens were demolded and transferred to a standard curing chamber maintained at a temperature of 20 ± 2 °C and a relative humidity exceeding 95%, where they were stored until reaching the target age for subsequent testing.

3.3. Testing

The self-cleaning properties of the photocatalytic recycled concrete are intimately linked to the optical characteristics of their surfaces. To quantify these properties, a UV-VIS spectrophotometer (Color Spectrum Optics) was employed to conduct colorimetric tests. The colorimeter stands as a pivotal instrument, enabling precise measurements of color intensity and composition. This precision device incorporates a controlled light source, a monochromator or filter system for wavelength selection, a sample chamber designed for minimal light interference, and a sensitive detector that converts optical signals into electrical outputs for processing. The colorimeter’s microprocessor or computer interface then processes these signals, calculating and displaying color parameters such as absorbance, reflectance, or transmittance values. The advantages of using a colorimeter are numerous, including its ability to perform a quantitative analysis, facilitate non-destructive testing, offer ease of use through intuitive interfaces, and demonstrate versatility across various sample types and sizes. By thoroughly outlining the colorimeter’s components, functionality, applications, and benefits, the characterization techniques’ section becomes more robust, enhancing the scientific rigor and reproducibility of the experimental findings. Prior to the self-cleaning assessment, the top surfaces of the concrete underwent a polishing process using a series of SiC (silicon carbide) sandpapers with varying grit sizes (P250, P400, P600, P1200, and P2500). Following polishing, the surfaces were meticulously rinsed with deionized water and subsequently purged with a high-pressure air blower to eliminate any residual debris.

Each sample’s top surface was then stained by uniformly spraying a 0.1 mg/L solution of methylene blue, subsequently left in a dark environment at approximately 20 °C to simulate natural nighttime drying. To simulate the photocatalytic process, the samples were exposed to a UV lamp with an intensity of 10 ± 0.05 W/m² for varying durations (0, 15, 45, 105, 225, 345, 1365, 1785, and 2800 min), during which time the colorimetric changes indicative of methylene blue degradation were continuously monitored [31]. For each sample, nine evenly distributed points on the methylene blue-stained top surface were measured four times, and the average colorimetric indices were calculated.

Subsequently, the spectrophotometer was utilized to directly measure the color change parameters (e.g., $L^{*}, a^{*}, b^{*}$) at each test point, as per established protocols. To visually assess the photocatalytic degradation rate of methylene blue, the color difference (ΔE) before and after irradiation was calculated using Equation (1), and the rate of change was derived from Equation (2).

$$∆E=\sqrt{{\left(L_t^{*}-L_0^{*}\right)}^2+{\left(a_t^{*}-a_0^{*}\right)}^2+{\left(b_t^{*}-b_0^{*}\right)}^2}$$

(1)

$$R_{∆E}={\displaystyle \frac{\left({∆E}_0-{∆E}_t\right)}{{∆E}_0}}\times 100\%$$

(2)

where $L_0^{*}$, $a_0^{*}$, and $b_0^{*}$ represent the initial colorimetric coordinates ($L^{*}, a^{*}, b^{*}$) at time 0 prior to irradiation, whereas $L_t^{*}$, $a_t^{*}$, and $b_t^{*}$ denote their respective values after exposure to radiation for t hours. These distinct alphabetic notations encapsulate distinct color dimensions: $L^{*}$ depicts the luminance ranging from white to black, $a^{*}$ signifies the transition from red to green hues, and $b^{*}$ represents the shift from blue to yellow tones. Furthermore, ${∆E}_0$ quantifies the color difference before and after the mortar surface, initially unexposed to irradiation, becomes contaminated with methylene blue. Conversely, ${∆E}_t$ represents the average color variation between the color measured post-irradiation and the original, uncontaminated surface color, prior to any methylene blue exposure.

Besides the spectrophotometer, the rotary viscometer (Lamy rheology instruments RM100, Champagne au Mont d’Or, France) was used to measure the rheological behavior of samples, whilst the ESEM (Quanta FEG 250, FEI, Hillsboro OR, USA) was used to observe the morphology and microstructure.

4. Conclusions

In this paper, a detailed investigation has been conducted by formulating RAs into NT@RAs and employing them as fine aggregates in photocatalytic concrete. The findings reveal a favorable synergistic effect exhibited by different types of NT@RAs within the photocatalytic concrete. The key conclusions encompass the following:

(1)

NT@RGS, with its relatively smooth surface and minimal water absorption properties, facilitates reduced internal friction within the mortar, significantly enhancing the rheological properties of fresh mortar. As the replacement ratio of NT@RGS increases, the yield stress of the mortar paste gradually decreases. When the ratio of NT@RGS to RCBS (recycled clay brick sand) is 1:3 or 1:1, the yield stress of the mortar decreases by 17.93% and 35.41%, respectively, compared to 0% replacement. Similarly, at a ratio of NT@RGS to RCA (recycled concrete aggregate) of 1:3 or 1:1, the yield stress reduction is 18.93% and 35.07%, respectively, from the baseline.

(2)

While the incorporation of NT@RGS and NT@RCBS leads to a decline in the compressive strength of the mortar, with the maximum reduction reaching 45.38%, an intriguing trend emerges when the ratio of RCBS to RGS is 3:1. Under this condition, as the overall replacement ratio increases from 25% to 50%, a slight increase in compressive strength is observed. This might be attributed to the fact that a higher overall replacement ratio results in a greater proportion of harder RGS and porous, rough RCBS within the same volume, which binds more tightly with the cementitious matrix. This enhances the frictional resistance against detachment from the matrix during compressive failure, outweighing the detrimental effects of reduced soundness (SS) on the compressive strength of the recycled concrete.

(3)

An evaluation of the self-cleaning performance of photocatalytic recycled mortars prepared with varying NT incorporation methods indicates that none of the specimens achieved adsorption–degradation equilibrium within 7 days, with degradation rates remaining below 50%. Notably, the direct incorporation of powdered NT samples yielded the lowest degradation rate, less than 20%. Conversely, mortar specimens prepared using impregnated recycled fine aggregates, specifically at a ratio of NT@RCA:NT@RGS = 1:1 and a 100% substitution of river sand, demonstrated a substantial enhancement in photocatalytic self-cleaning efficiency, achieving a rate of 51.7%.