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Article

Assessment of Photocatalytic Nano-TiO2 Mortars’ Behavior When Exposed to Simulated Indoor Conditions of Glazed Buildings

by
Roberta Picanço Casarin
1,*,
Jéssica Deise Bersch
1,*,
Joana Maia
2,
Angela Borges Masuero
1 and
Denise Carpena Coitinho Dal Molin
1
1
Programa de Pós-Graduação em Engenharia Civil: Construção e Infraestrutura (PPGCI), Núcleo Orientado para a Inovação da Edificação (NORIE), Universidade Federal do Rio Grande do Sul (UFRGS), Av. Osvaldo Aranha, 99, 7th Floor, Porto Alegre 90035-190, Brazil
2
CONSTRUCT-LFC, Faculty of Engineering (FEUP), University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal
*
Authors to whom correspondence should be addressed.
Buildings 2023, 13(9), 2250; https://doi.org/10.3390/buildings13092250
Submission received: 27 July 2023 / Revised: 29 August 2023 / Accepted: 2 September 2023 / Published: 5 September 2023
(This article belongs to the Special Issue Eco-Innovative Envelope Systems – New Approaches to Sustainable Buildings)
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Abstract

:
The application of nano-TiO2 as a photocatalytic agent in buildings’ internal surfaces has recently attracted attention to mitigate microorganism growth, soiling, and contamination in indoor environments. This work aimed at comparing the Rhodamine B (RhB) dye degradation efficiency of three different mortar compositions subjected to simulated internal radiation, in which nano-TiO2 (10 wt% of binder mass) was dispersed by ultrasonic and mechanical methods. Mortar specimens were produced with white Portland cement, hydrated lime, sand, and water in different volume proportions of 1:1:6 (cement:lime:sand), 1:3 (cement:sand), and 1:4 (cement:sand). The first stage of the research evaluated samples exposed to the natural outdoor environment and proved the efficiency of specimens’ photoactivity when covered by a glass layer. The second and principal phase of the study simulated indoor conditions in glazed buildings through artificial weathering in which the composition of 1:1:6 was mechanically dispersed and exhibited the highest global color change (ΔE) values for RhB staining. The main finding of the study was that the mortars exposed to simulated indoor conditions presented high ΔE grades, classified as easily perceived by the human eye. This demonstrates the photocatalytic efficiency in an internal building environment that receives radiation through a glass surface.

1. Introduction

The durability of buildings and related materials is one of the main challenges in the civil construction sector. Regarding their lifespan, natural degradation is led by climate agents such as water and temperature [1] and is increased by pollution factors [2]. In this context, one of the main components affected by pollution is the envelope, which is normally coated with mortar—widely used as rendering for masonry, a type of construction present in more than 70% of the world’s registered buildings [3].
The primary sources of pollutant particles that might adhere to building envelopes include motor vehicles, industries, and power plants—which are also the main contributors to air pollution [4]. Among the common air pollutants that affect building materials are nitrogen dioxide, ozone, carbon dioxide, chlorides, sulfur dioxide, nitrates, and volatile organic compounds (VOCs) [5]. Soot may deposit on building surfaces, soiling them by darkening their color [6]. Thus, pollution, as one of the major problems in urban spaces [7], directly affects buildings’ components. However, the negative effects are not restricted to the façades since they extend to people’s health and buildings’ indoor air quality as well [8]; Sivanantham et al. [9] and Abdel-Salam [10], for example, developed studies specifically concerning indoor exposure to air pollutants in buildings.
Moreover, further than pollution, biofilms may also deteriorate cementitious surfaces and affect building occupants’ health [11]. In the façades of historic buildings, for instance, algae, cyanobacteria, bacteria, and fungi, the main microorganisms composing biofilms, can lead to discoloration and degradation [12]. Regarding indoor air quality, viruses, bacteria, and fungi can be considered the major contaminants, and different microorganisms may affect humans’ health at different concentration levels [13].
The challenge of health and aesthetic issues regarding the relation of buildings with air pollution and microorganisms justifies the research for solutions. Photocatalytic materials, such as titanium dioxide (TiO2), are among the promising alternatives regarding their capacity to degrade particulate pollutants while attributing self-cleaning properties to surfaces [14]. The use of TiO2, mainly in its anatase form, can further take advantage of its ability to decompose organic materials by acting upon biological pigments and biofilms [15].
TiO2 is a semiconductor that works through heterogeneous photocatalytic processes, being one of the most used photoinduced catalysts due to its strong oxidative ability. Under UV (ultraviolet) irradiation with a wavelength lower than 380 nm, an electron is transferred from the valence band of the TiO2 to its conduction band, resulting in a hole in the valence band [16]. Electron-hole pairs on the surface of the semiconductor particles react with water and adsorbed oxygen, resulting in reactive species like O2−, OH·, and HO2· and the mineralization of pollutants [17]. Holes and electrons may also react directly with organic and inorganic compounds adsorbed on the surface; conversely, they may recombine, making it a challenge to inhibit recombination [16].
Mortar renderings with TiO2 fit within the urgent pursuit for a more sustainable built environment [18]. In this context, Jin et al. [19] studied photocatalytic mortars with a proportion of 1:2:4 (water:cement:sand) and a 2.5, 5, and 10 wt% of nano-TiO2 concentration in relation to the cement weight. They verified that TiO2-containing mortars efficiently convert nitrogen oxides (NOx) by exposure to solar radiation. Higher NOx conversion was obtained through higher photocatalyst rates and ultraviolet A (UVA) radiation intensity or lower NOx gas flow rates and relative humidity [19]. Lucas et al. [8], developing mortars with 0 to 5 wt% nano-TiO2, identified that the most suitable composites combined an optimum amount of photocatalyst, specific for each mortar composition, with an adequate pore size distribution and total porosity, enabling the pollutants to access the internal matrix structure.
De la Rosa et al. [6] studied TiO2-containing mortars regarding the deposit of organic compounds, like soot, on building surfaces; by exposure to UVA irradiation, TiO2 catalyzed the alteration of the molecular composition of soot, enhancing its decomposition. Diamanti et al. [20] studied the durability of TiO2-added mortars considering the potential for their application in building envelopes by exposure to an urban environment for two years; even though aging affected the photoactivity of the surfaces because of soiling, nearly 70% of the photocatalytic efficiency could be recovered with alternated cycles of visible ultraviolet (UV–Vis) irradiation and rinsing, as tested through dye degradation. Research on the antimicrobial potential of TiO2 within the construction industry is significantly lower than on air purification and self-cleaning properties [21]. Based on the results obtained with the study of TiO2-based self-compacting glass mortars, Guo et al. [22] concluded that the bacteria-inactivation processes catalyzed by the TiO2 are more complex than the NO removal, and the performance of the nanomaterial in the two cases may be different.
Concerning the economic feasibility of the photocatalytic mortars produced, a mortar thickness of 3 mm was reported in the work of Jin et al. [19] as providing optimal cost in relation to photocatalytic efficiency. TiO2 addition in cement-based building materials is reported as providing relatively low cost when compared with other photocatalysts [23] or maintenance works [24]. According to Liao et al. [25], the cost associated with cleaning a building’s facade is high due to factors such as the need for equipment, labor expenses, and the duration of the maintenance work: a scenario for which photocatalytic coatings are highly applicable. Furthermore, the use of recycled materials in TiO2-added mortars may compensate for the costs of the photocatalytic addition, as shown in a cost analysis of a photocatalytic mortar produced with recycled clay brick sands, recycled glass, and a 5 wt% of nano-TiO2 addition (related to the cement mass), which resulted in an 80% cost reduction [26].
Pursuing the improvement of indoor air quality through passive measures, Giosuè et al. [27] studied multifunctional innovative mortars, including the addition of three types of nano-TiO2; the best-behaving composition led to the NOx removal of up to 30% under UV irradiation and, although lower (around 18% and 5% for different compositions), the photoactivity was also verified under visible radiation. Bhagyamma et al. [28] investigated the self-cleaning behavior by dying and dipping 1:3 (cement:sand, in weight) mortar specimens containing 0% to 5% of nano-sized TiO2 in methylene blue; self-cleaning was more efficient with higher TiO2 dosages and by exposure to sunlight (outdoor environment) when compared to LED lights simulating an indoor environment. However, from the studies, although it was lower, a photoinduced activity on TiO2 may be obtained from its addition to mortars in indoor environments.
The application of TiO2 in building materials can also improve the aesthetic appearance of architectural façades [29], which are structural or non-structural elements created to provide finishing for exterior or interior surfaces [30]. In the work of Guo et al. [29], a transparent coating based on TiO2 was applied to architectural mortars and showed efficiency in NOx removal and self-cleaning (by the removal of Rhodamine B). These results show the possible feasibility of applying photocatalytic mortars to create architectural mortars for internal building environments.
Regarding the production of TiO2-containing mortars, the dispersion procedure of the nanomaterial affects the mortar properties in the fresh and hardened states [31]. Furthermore, agglomeration of nano-TiO2 due to its large specific surface area and strong van der Waals’ force also hinders photocatalytic properties [32], which may highlight the importance of an adequate dispersion. Yousefi et al. [33] stated that agglomeration could be avoided and photocatalytic properties increased by dispersing nano-TiO2 in saturated lime water with an ultrasonication treatment before mixing with cement. Atta-ur-Rehman et al. [34] submitted TiO2 nanoparticles to ultrasonication for at least 45 min using a sonic probe seeking deagglomeration. Dantas et al. [31] studied different mortars with nano-TiO2 by mechanical stirring with standard and high (10,000 rpm with Cowles-type propellant) energies of dispersion; better dispersion conditions led to enhanced microstructural homogeneity. In another study, Dantas et al. [24] applied a high dispersion energy (2000 rpm with Cowles-type propellant for 5 min) to pre-disperse TiO2 in water before adding it to cement matrixes. Pathak and Vesmawala [35] obtained better dispersion of nano-TiO2 using polycarboxylate as a chemical dispersing agent. González-Sánchez et al. [36] also verified increased self-cleaning ability and NO degradation by adding superplasticizers to air lime mortars. Therefore, several methods to avoid agglomeration of TiO2 nanoparticles are presented in the literature; however, the proper dispersion of the photocatalyst is still a challenge to solve in seeking its best performance [37].
In this context, the present study investigates the performance of different TiO2-added mortars’ compositions to be used in indoor environments of buildings with a glazed envelope. The importance and significance of the study are justified considering the widespread presence of this type of building in urban environments and the long time spent by the users inside, highlighting the need for adequate indoor air quality. The novelty of this work relies on the assessment and interrelation of three important factors in the photoactivity: the indoor exposure, the different compositions of rendering mortars, and the dispersion techniques. To the authors’ knowledge, there are no reports of previous studies addressing the interconnection of those factors, and photocatalytic mortars for internal building environments were evaluated in just a few recent studies [27,28,38]. Hence, the study undertakes a research gap in the techniques for the development of TiO2-based coatings for internal building surfaces.
Concerning the ultraviolet radiation emitted by the sun, the wavelengths corresponding to the UVA portion of the light spectrum can pass through most ordinary glasses; on the other hand, wavelengths regarding ultraviolet B (UVB) are absorbed [39]. Studies exposing TiO2 to UVA radiation are more common than those referring to UVB and ultraviolet C (UVC) [40], possibly due to the consolidated knowledge of the excitement of the nanoparticles by UV wavelengths between 320 nm and 400 nm [41]. Pozo-Antonio et al. [40] studied Si-based consolidants with nano-TiO2 under exposure to UVA or UVB radiations, considering the wavelength limits of 340 nm to 400 nm and 270 nm to 420 nm, respectively, with the prominent peaks at 365 nm and 310 nm; UVB radiation was identified as the most effective to degrade soot for concentrations of 0.5 and 1 wt% of TiO2, except for 3 wt%. Differences are, therefore, expected by considering an intermediate glass layer between sunrays and the mortar samples.
To achieve the paper’s goal, mortars with different proportions of materials were studied, and different TiO2 dispersion methods were applied, looking for possible influences on the resulting photocatalytic activity. The photoactivity was evaluated through Rhodamine B (RhB) degradation and monitored with chromatic coordinates from the CIELab color space (or CIE L*a*b*, from CIE: Commission Internationale de l’Eclairage; in which the letters L*, a*, and b* represent each of the three values of the color space). Further than the self-cleaning ability, RhB may suitably simulate particulate pollutants due to its similar structure, for example, to polycyclic aromatic hydrocarbons, which result from combustion processes [14].
Firstly, a research step was carried out by exposing specimens to the external environment of Porto Alegre, Brazil, to compare the dye degradation performance of TiO2-containing mortars over which sunlight shone directly in relation to specimens receiving sunlight after passing through a glass slide, simulating an indoor environment. After proving the photocatalytic efficiency even with the presence of the glass, a subsequent stage in the research followed, in which the performance of the different photocatalytic mortars, concerning materials’ proportions and TiO2-dispersion methods, was compared through accelerated aging, aiming to understand their behavior in indoor environments of glazed buildings’ envelopes.

2. Materials and Methods

2.1. Materials

2.1.1. White Portland Cement

The structural white Portland cement CPB-40 (WHITE CEM I 52.5R EN 197-1) produced in Secil—Companhia Geral de Cal e Cimento, S.A (imported from Supremo Cimentos, S.A., de Jaraguá do Sul, Brazil) had a 28 day compressive strength superior to 52.5 MPa, according to the European standard EN 197-1 [42]. The material also followed the limits of 1 h for initial setting time, 5 mm of hot expansibility and whiteness of 78%, according to the manufacturer. Additional tests resulted in a unit weight of 1007.73 kg/m3, determined based on the Brazilian regulation ABNT NBR 16972 [43], and a specific gravity of 2950 kg/m3, obtained following ABNT NBR 16605 [44]. Laser granulometry performed in PSA 1090 L Anton Paar equipment with a granulometric range of 40 nm–500 µm resulted in a mean size of 15.78 µm, with a D10, D50, and D90 of 2.35 µm, 12.76 µm, and 31.47 µm, respectively. Figure 1 shows the mass loss of the white Portland cement obtained through thermogravimetry in Mettler Toledo equipment with a temperature range of 25 °C–1000 °C and a heating rate of 20 °C/min. The highest occurrence of mass loss, 8.0%, can be observed between 600 °C and 750 °C, probably corresponding to the decomposition of calcite (CaCO3) in the composition of the non-hydrated cement [45].

2.1.2. Hydrated Lime

The hydrated lime used was specified as type CH-I, which must have more than 90% of total oxides (CaOt + MgOt), according to ABNT NBR 7175 [46], and was produced from Cerro Branco Indústria e Comércio de Cal LTDA (commercialized by Dagoberto Barcellos S.A., Caçapava do Sul, Brazil). The material presented a carbonic anhydride level ≤ 7% and plasticity ≥ 110%. In relation to particle diameters, the material had a mean size of 17.06 µm, with a D10, D50, and D90 of, respectively, 2.37 µm, 13.24 µm, and 35.39 µm. Additional tests resulted in a unit weight of 573.26 kg/m3 [43] and a specific gravity of 2305 kg/m3 [44]. According to the thermogravimetry results, presented in Figure 2, most of the hydrated lime weight loss occurred between 300 °C and 500 °C and between 600 °C and 750 °C. Among others, portlandite (Ca(OH)2) or brucite (Mg(OH)2) dehydroxylation possibly explains the mass loss around 460 °C and 420 °C, respectively, and the residual presence of calcite (CaCO3) is possibly responsible for the mass loss after reaching 600 °C [45].

2.1.3. Aggregate

River quartz sand was used as the aggregate in a fine portion, presenting a unit weight of 1519.87 kg/m3 [43] and a density of 2057 kg/m3, determined following ABNT NBR 16916 [47]. Figure 3 shows the particle size distributions of the aggregate, which had a fineness modulus of 0.93 and a maximum diameter of 0.60 mm.

2.1.4. Nano-Titanium Dioxide

Nanopowder TiO2 (P25, Degussa, Evonik Industries, Essen, Germany) was added to the mortars, presenting a specific surface area of 50 ± 15 m2/g, average particle diameter of 21 nm, pH of 3.5–4.5, HCl content ≤ 0.30% and TiO2-content (based on ignited material) ≥99.5%, and an anatase/rutile proportion of 80/20. Additionally, the material has a tamped density of 130 g/L and a specific gravity of 4.18 g/cm3, according to the manufacturer. An X-ray diffraction (XRD) on the nano-TiO2 (Figure 4) showed the presence of the two crystallographic phases of the material, with a prevalence of anatase crystal peaks, which is in accordance with the proportions reported by the manufacturer.

2.2. Production of Mortars and Characterization in the Fresh and Hardened States

The photocatalytic mortars were produced with three different compositions, including 1:1:6 (cement:lime:sand, in volume), 1:3 (cement:sand, in volume), and 1:4 (cement:sand, in volume). Similarly to Jin et al. [19], to evaluate the photoactivity, the TiO2-containing mortar specimens were applied as a top-layer with 3 mm over a 10 mm height bottom-layer substrate, prepared with industrial ready-to-mix mortar with a specific gravity of 1.50 kg/cm3 and fresh state density of 1.80 kg/cm3, 14 days earlier than the top coating.
Within the production of the mortars, 10 wt% of nano-TiO2, over the binder mass (cement or cement and lime), was added to half of the specimens prepared, and the photocatalyst was previously dispersed in deionized water (in the proportion of 10 wt% TiO2 in relation to the water). The percentage of 10 wt% of nano-TiO2 was chosen based on other works that evaluated mortars with the same proportion [19] or a very close percentage, such as 8 wt% [48] or 9 wt% [34]. The dispersion was performed in two ways. The first was using an ultrasonic bath for 30 min, following the period of time from other studies [49], in digital ultrasonic bath equipment (10 L, frequency of 40 kHz, SolidSteel, Piracicaba, Brazil). The second dispersing method was carried out using a mechanical mixer at 1330 rpm for 1 min.
Two samples with 47.5 mm × 100 mm for each mortar proportion and exposure condition were prepared using the white Portland cement, the hydrated lime, the quartz sand, the nano-TiO2, and potable water, following the Brazilian standard ABNT NBR 16541 [50] for mortar mixture preparation for tests. Furthermore, a set of six prismatic specimens with 40 mm × 40 mm × 160 mm for each studied mortar combination was prepared for characterization tests in the hardened state at 28 days.
The amount of water was adjusted to obtain similar results of 240 mm ± 20 mm in the flow-table test, performed following ABNT NBR 13276 [51], resulting in the water/binder proportions presented in Table 1. In Table 1, identification acronyms are shown for each combination of mortar composition, dispersion method (in which UD refers to ultrasonic dispersion and MD to mechanical dispersion), and addition of TiO2 (in which Ti stands for nano-TiO2 addition). Phases 1 and 2 concern the stages of the research: Phase 1 refers to the testing of photocatalytic efficiency with the presence of glass in the outdoor environment, and Phase 2 is the accelerated weathering study in the simulated indoor environment.
Tests for determining the specific gravity and the air-entrained content of the mortars in the fresh state were executed following ABNT NBR 13278 [52]. After 28 days of cure in a controlled environment, with a temperature of 28 °C ± 1.5 °C and air relative humidity of 80% ± 5%, the prismatic specimens were tested to obtain values for specific gravity in the hardened state, elasticity modulus, and water absorption by capillarity, according to the standards from ABNT NBR 13280 [53], 15630 [54], and 9779 [55], respectively. Figure 5 illustrates the phases and tests followed during the production of mortars and characterization in the fresh and hardened states.

2.3. Staining with RhB and Aging Procedures

After 28 days of curing, the 47.5 mm × 100 mm specimens with and without TiO2 were dyed using an RhB solution prepared with deionized water at a concentration of 0.5 g/L. The staining was carried out by submerging the specimens in the RhB solution for 24 h (Figure 6), followed by 24 h of drying in laboratory ambient conditions (temperature of 23 °C ± 1 °C and relative humidity of 70% ± 10%).
After drying, the first stage of the research was carried out by exposing dyed specimens (produced with 30 min ultrasonic-bath dispersion of TiO2) to the external environment, aiming to compare the behavior of the mortars in external conditions (like if they were applied in a façade) with internal conditions (simulating their application in the indoor of a glazed building). The specimens were exposed in Porto Alegre, with geographic coordinates of 30°01′58.5″ S and 51°13′18.2″ W, for 5 days in July. The short period of time was set to verify the quick photoactivation of the samples in the outdoor environment.
The simulation of the outdoor conditions in the external environment considered the incidence of direct sunlight, as illustrated in Figure 7a, with samples oriented towards 343° north at an inclination angle of 45°. The indoor condition was simulated in the external environment by positioning the specimens inside a 400 mm depth chamber covered by a standard 6 mm glass and placed facing the same orientation as the outdoor specimens, as shown in Figure 7b. Due to the shadow resulting from the sides of the chamber, a limitation should be highlighted concerning the sunray incidence over the specimens: probably, the duration of the solar incidence was lower for the indoor simulated environment than for the outdoor. However, as the main objective from the first research stage was to prove the TiO2 efficiency even with the presence of the glass, this more conservative approach does not negatively affect the conclusions.
For the second stage of the research, aiming at understanding the effects of nano-TiO2 dispersion techniques, the same mortar compositions were produced using two different dispersion methods: the mechanical and the ultrasonic bath. The samples were exposed to a simulated indoor condition performed by artificial accelerated weathering in a Suntest XXL+ chamber (ATLAS Material Testing Solutions). The exposure conditions followed Cycle Number 1 for general coatings, as specified by ASTM D6695 (Standard Practice for Xenon-Arc Exposures of Paint and Related Coatings) [56]; therefore, specimens were exposed to cycles of 102 min of light, regarding irradiance of 0.35 ± 0.02 W/(m2·nm) at 340 nm, at 50 ± 5% RH, followed by 18 min of light and water spray. The black panel temperature (BPT) was 63 ± 2 °C. For this test, the samples were positioned 10 cm below a standard 6 mm glass to simulate indoor walls in glazed buildings, as illustrated in Figure 8.
Color coordinates from the CIELab color space were measured with a Konica Minolta CM-2500d spectrophotometer for the specimens exposed in the external environment and the accelerated weathering chamber. Ten different points on each sample were selected for studying the color parameters at all evaluating times. For the first stage of the research, measures were taken before and after the staining of the specimens and after the 5 days of radiation exposure. Then, for the second research phase, color coordinates were obtained after 160 min, 280 min, and 440 min of accelerated exposure.
Color variation in time (ΔE) was calculated following Equation (1), in which L* represents the brightness, a* the color hues from red to green, and b* the color hues from blue to yellow. ΔE represents the spatial difference between two points, corresponding to the color at a time of interest t and the initial color at t = 0 [57]. According to ASTM C1501 [58], the human eye may perceive changes between 1.0 and 3.0 in ΔE values, which can be considered acceptable; on the other hand, ΔE values above 3.0 indicate readily perceptible changes in color for the human eye.
ΔE = √((Lt* − L0*)2 + (at* − a0*)2 + (bt* − b0*)2)
Photographic records of the samples for each measurement time were also obtained using a 12-megapixels digital camera with a resolution of 4000 pixels × 3000 pixels in a light-controlled environment. After obtaining the color measurements over time, analysis and discussions were carried out. To support the arguments and conclusions, an analysis of variance (ANOVA) was performed for the results, considering a significance level of 0.05. Figure 9 depicts the aging phases and tests followed to study the photocatalytic behavior of the produced specimens.

3. Results and Discussion

3.1. Characterization of the Mortars

The results for the mortars’ characterization in the fresh state are shown in Table 2, and for the 28 day set of experiments in Table 3 and Table 4. Regarding the flow-table test, the range of consistency index (Table 2) was intentionally set as 240 mm ± 20 mm, a usual workability found for rendering mortars [4,31], and for which water quantities were adjusted.
Overall, in the three types of compositions with TiO2 addition, the fresh state density was higher than the control samples without the photocatalyst. This behavior was also observed in another study by Jabali et al. [4]: when TiO2 content increased, the fresh density values of mortars were also higher. This assertion finds support in the research of Naganna et al. [48], who concluded that TiO2 nanoparticles serve as effective nano-fillers, enhancing the concrete’s resistance to water permeability. The average density also presented higher values for the three combinations produced with TiO2 mechanical dispersion, which might indicate a better efficiency of this method over the ultrasonic bath in packing the fine portion of the materials. Furthermore, for each composition, the average air-entrained content was higher for the mortars without nano-TiO2 addition. A slight decrease in air entrainment of mortars with P25 TiO2 addition was also reported in the work of Dantas et al. [59] (when compared to a reference mortar). The reduction in air-entrained content can be explained by a possible improvement in particle packing and reduction in porosity due to the addition of the nanomaterial, as found in other studies [34,60].
For the hardened state properties (Table 3), a slight decrease in bulk density was observed for the photocatalytic mortars formulated with proportions 1:1:6 and 1:3 compared to the control samples without TiO2, indicating some differences from what was immediately observed in the fresh state. Another study presented similar results for lime-based mortars, in which 28 day TiO2-added mortars had density values of 1200 kg/m3 [27]. A behavior of higher density, such as the one presented by the photocatalytic mortars in the proportion 1:4, was also observed by Noorvand et al. [61] through scanning electron microscopy (SEM) after 7 and 28 days, and by Alwan et al. [62], who added a 0.5, 1, and 1.5 wt% of nano-TiO2 in self-compacting mortars. The high bulk density values were mainly attributed to the porosity reduction and creation of bindings [61] with the photocatalyst.
The water absorption by capillarity (Table 3) was significantly reduced for the formulation with a higher quantity of cement (1:3) and increased for the compositions using lime. These findings align with Maia et al. [63], who observed capillary pores in lime-based mortars, resulting in higher capillary absorption coefficients than in cement-based formulations. Comparatively, a study that evaluated nano-TiO2-added mortars in binder:sand ratios of 1:1 and 1:2 presented a higher water absorption by immersion for the composition with less sand [64].
Although increases in the dynamic modulus of elasticity could be expected with the addition of the nanometric photocatalyst due to the closure of existing pores in the mortar [65], the results (Table 4) were lower for all compositions with TiO2 addition. However, the reductions in the dynamic modulus of elasticity in the present paper were in agreement with some reductions in bulk density, indicating higher porosities leading to a lower modulus [31]. Furthermore, reductions in the modulus of elasticity could be expected considering the higher water/binder proportions in the TiO2-containing mortars [66,67], which might also have influenced the porous structure of the specimens. Dantas et al. [31] found dynamic modulus of elasticity results between 7.78 GPa and 10.80 GPa in white cement mortars with TiO2 addition, which were also generally lower than their reference mortar result. In addition, higher values were found for the studied formulations with higher amounts of cement (1:3 and 1:4), similar to the behavior found by Pozo-António and Dionísio [68] in a study that also evaluated mortars prepared with different compositions of cement and lime [31]. Apparently, the different hardened state properties were affected in particular ways regarding the addition of nano-TiO2 and the varied water/binder ratios.

3.2. First Stage of Color Change Analysis: External Exposure

The initial phase of tests evaluating color change was performed to demonstrate the photocatalytic activity by exposure to the external environment, simulating indoor and outdoor conditions, and comparing the mortar samples with TiO2 addition to the reference ones of each composition without the photocatalyst. The results for the value L, which represents lightness and has a color range from 0 (black) to 100 (white), are shown in Figure 10. The measurements were taken from natural color (28 days after preparation), after the staining process, and after 5 days of exposure to the urban environment with sunlight radiation.
Based on Figure 10a, the compositions with TiO2 exposed to the external conditions representing outdoor environments (receiving direct sunlight radiation) had higher final lightness values when compared to the control groups, as expected and similarly to other studies [4,6,15,68], except for photocatalytic mortars produced with a 1:4 cement:sand ratio. Thus, although RhB may experience a dye-sensitized photoreaction [69], the increased results when in the presence of the photocatalyst probably demonstrate the occurrence of the catalytic process.
While there was no significant difference in L values among the specimens with TiO2 addition and the reference ones, it is noteworthy that most of the specimens in the simulated indoor exposure displayed higher average L values after 5 days than the measurements after staining with RhB (Figure 10b). This suggests a potential self-cleaning effect in indoor conditions, which was further analyzed by the average global color change (ΔE) results (Figure 11).
From the results for global color change (ΔE) shown in Figure 11, external exposure simulating outdoor conditions resulted in greater ΔE values than the simulated indoor conditions, which was expected due to the direct incidence of sunlight and, thus, UVA radiation [70,71] and the following quick efficiency of the photocatalytic reactions. Although the ΔE results were reduced using glass in the indoor condition, the values were still above three, representing readily perceptible changes in color for the human eye [58].
Focusing on the photocatalytic mortars exposed to the simulated indoor building conditions (Figure 11b), all of the cases of higher changes in color occurred to the TiO2-added mortars when compared to the control groups—for the three mortar coating compositions tested. Furthermore, the statistical analysis by ANOVA (Table 5) confirmed that by changing the type of exposure (outdoor or simulated indoor conditions), a significant difference in ΔE values is observed, as well as by modifying the mortar composition (1:1:6, 1:3, or 1:4) and the TiO2 addition (0 or 10%). Other authors [72,73] have also observed that incorporating nano pigments in coatings reduced the color changes after environment exposure.
Overall, for both exposure cases, the highest ΔE values were exhibited by the photocatalytic composition 1:3, while the 1:1:6 ratio displayed the lowest values. Diamanti et al. [20] also found that photocatalytic mortars with a higher amount of cement (in a cement:sand ratio of 1:2 and 5% of TiO2 addition) better maintained lightness and solar reflectance during the external exposure time, in comparison to analogous mortar without the TiO2 addition.

3.3. Second Stage of Color Change Analysis: Artificial Weathering

The second stage of analysis focused on evaluating the RhB dye degradation efficiency of the photocatalytic mortars produced with the two different dispersion techniques (ultrasonic and mechanical) and exposed to artificial accelerated weathering under a simulated glazed building condition, regarding the same three proportions from the previous phase. Photographic records of the samples are shown in Figure 12, from which visual differences in pink-red shades are clearly observed when comparing non-photocatalytic mortars to the other two situations. Similar visual results for RhB de-soiling in external environment conditions were found in Guo et al. [70].
An analysis of the L average values showed, for all cases, that lightness tended to return to its initial values or even overlapped them (Figure 13), indicating the coating mortars’ complete efficiency in restoring black-white shades of colors. There are reports of ΔL values closely returning to their initial measures, but, to the authors’ knowledge, only in external photocatalytic surfaces [15,18,70]. Moreover, the average ΔE values were higher than 25 for all of the cases (Figure 14), representing differences easily seen by the human eye [58]. Similar results were found by Bhagyamma et al. [28] when assessing the self-cleaning performance by dying and dipping with methylene blue 1:3 (cement:sand, in weight) mortar specimens containing 0% to 5% of nano-TiO2; mortars submitted to LED light in indoor environments were efficient, although less than those exposed to sunlight.
ΔE values were tested for significance by statistical ANOVA analysis (Table 6), and the results indicated that different color changes were verified by varying either the composition or the TiO2-dispersion techniques. The combinations of composition × dispersion also significantly influenced the ΔE values. Additionally, the mortar with cement and lime in its composition (1:1:6) with TiO2 mechanical dispersion exhibited the highest ΔE. The better efficiency of the composition with lime might be explained by chemical activation of the mortar with the lime material: Yousefi et al. [33] showed that the dispersion of nano-TiO2 in saturated lime water followed by the addition of the resulting suspension to cement powder led to a better distribution of nano-TiO2 in the cement and CaTiO3 formation; an increase in the photocatalytic properties under UV radiation and visible light could, thus, be identified.
To assess the significance of the results, a Tukey’s multiple (pair-wise) comparison test was performed (Table 7) and did not show any difference among many of the average values. The mortar composition 1:1:6 exhibited average ΔE values with a statistical difference depending on the method of dispersion: ultrasonic or mechanical. In contrast, in the other pair of compositions, the dispersion method’s effect was not significant in producing different ΔE values. Likewise, the mortar formulations 1:3 and 1:4 dispersed either by mechanical or ultrasonic methods had similar ΔE values between them and with the composition 1:1:6 dispersed by an ultrasonic bath (Figure 15).
Previous works have also reported a high photocatalytic efficiency for lime-based and lime-cement-based TiO2-added mortars. De la Rosa et al. [6] studied lime-based mortars with 2.5% and 5% of TiO2, which were efficient in the photodegradation of a diesel soot layer deposited on the specimens’ surfaces. Gemelli et al. [15] evaluated a mortar composed of cement, slaked air lime, and river sand in the proportion 1:1:7, which received the application of TiO2 integrated into a SiO2 matrix, creating a durable coating that prolonged the antifouling performance. Differently from the present study’s findings, another study [68] achieved the lowest ΔE for lime-based mortars, while the cement-based mortars resulted in the highest values.
The results obtained for dye degradation reinforce the ability of the TiO2 regarding the self-cleaning behavior attributed to the photocatalytic surfaces. Furthermore, following Krishnan et al. [14], which evaluated the particulate degradation by TiO2-containing silicate coatings with RhB representing atmospheric pollutants, the mortars from the present study may also suggest the TiO2 performance facing urban environments contaminants. Concerning especially the antimicrobial interface of the studied photocatalyst, future works considering doping and heterojunctions could further modify and enhance the active response toward indoor light sources [18,74].

4. Conclusions

This paper evaluated the RhB dye degradation efficiency of photocatalytic mortars produced with different compositions (1:1:6, 1:3, and 1:4, cement:lime:sand or cement:sand, in volume) and nano-TiO2 dispersion methods (ultrasonic bath and mechanical). Two main stages were performed to understand the performance of TiO2-added mortars in indoor environments of glazed buildings by analyzing color changes (ΔE).
In the first stage of the research, which comprised the exposure of specimens to the external urban environment, the simulated indoor condition, receiving solar radiation through window glass, led to higher ΔE values for all mortar compositions with TiO2, regarding the change from RhB color. The best-performing case was for the composition 1:3, with 10 wt% of TiO2 addition.
When assessing the ΔE from RhB staining in simulated indoor glazed building conditions by artificial weathering, the impact of changing the dispersion method was only significant for the composition 1:1:6, in which the mechanically dispersed formulation exhibited the highest values. For the compositions 1:3 and 1:4, mechanical and ultrasonic dispersions produced ΔE average results without a statistically significant difference.
Although the mortar specimens exposed to outdoor conditions in the external environment presented the greatest ΔE results, all of the formulations exposed to both simulated indoor conditions (in the first and second stages of the research) also presented high ΔE grades, classified as easily perceived by the human eye, indicating the self-cleaning and possibly depolluting efficiency in internal building environments receiving radiation through a glass. Further research is suggested to include microstructural analysis such as SEM with energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP), as well as antimicrobial performance. Mainly regarding indoor air quality concerns and passive improvement measures, the potential of using TiO2 in interior environments should be highlighted and thoroughly investigated.

Author Contributions

Conceptualization, R.P.C. and J.D.B.; methodology, R.P.C.; investigation, R.P.C.; resources, A.B.M. and D.C.C.D.M.; data curation, R.P.C.; writing—original draft preparation, R.P.C. and J.D.B.; writing—review and editing, R.P.C., J.D.B., J.M., A.B.M. and D.C.C.D.M.; visualization, R.P.C.; supervision, J.M., A.B.M. and D.C.C.D.M.; funding acquisition, J.M., A.B.M. and D.C.C.D.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), grant number 142161/2019-5; and CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), grant number 88887.702359/2022-00; and by Base Funding—UIDB/04708/2020 of the CONSTRUCT (Instituto de I&D em Estruturas e Construções) funded by national funds through the FCT/MCTES (PIDDAC).

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors appreciate the support of LAMTAC (Laboratório de Materiais e Tecnologia do Ambiente Construído), LINCE (Laboratório de Inovação em Cimentos Ecoeficientes), PPGCI (Programa de Pós-Graduação em Engenharia Civil: Construção e Infraestrutura), UFRGS (Universidade Federal do Rio Grande do Sul), and FEUP-UPorto (Faculty of Engineering of the University of Porto).

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Mass loss of the white Portland cement obtained through thermogravimetry.
Figure 1. Mass loss of the white Portland cement obtained through thermogravimetry.
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Figure 2. Mass loss of the hydrated lime obtained through thermogravimetry.
Figure 2. Mass loss of the hydrated lime obtained through thermogravimetry.
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Figure 3. Average aggregate particle size distribution.
Figure 3. Average aggregate particle size distribution.
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Figure 4. XRD results of nano-TiO2 P25.
Figure 4. XRD results of nano-TiO2 P25.
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Figure 5. Production of mortars and characterization in the fresh and hardened states.
Figure 5. Production of mortars and characterization in the fresh and hardened states.
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Figure 6. Staining of the mortars with Rhodamine B (RhB): (a) Specimens before staining; (b) Specimens submerged in 0.5 g/L RhB solution for 24 h.
Figure 6. Staining of the mortars with Rhodamine B (RhB): (a) Specimens before staining; (b) Specimens submerged in 0.5 g/L RhB solution for 24 h.
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Figure 7. Exposure of dyed specimens to the external environment simulating: (a) Outdoor conditions; (b) Indoor conditions.
Figure 7. Exposure of dyed specimens to the external environment simulating: (a) Outdoor conditions; (b) Indoor conditions.
Buildings 13 02250 g007
Buildings 13 02250 g008
Figure 8. Specimens within the accelerated weathering chamber for evaluating dye degradation under simulated indoor conditions on glazed buildings: (a) Specimens positioned in the machine; (b) Specimens positioned below the glass.
Figure 8. Specimens within the accelerated weathering chamber for evaluating dye degradation under simulated indoor conditions on glazed buildings: (a) Specimens positioned in the machine; (b) Specimens positioned below the glass.
Buildings 13 02250 g008
Buildings 13 02250 g009
Figure 9. Aging phases and tests to study the photocatalytic behavior of the mortar specimens.
Figure 9. Aging phases and tests to study the photocatalytic behavior of the mortar specimens.
Buildings 13 02250 g009
Buildings 13 02250 g010
Figure 10. Lightness (L) average values for the specimens of the three mortar compositions in their original natural color, after the RhB staining process and after 5 days of exposure to the urban environment in: (a) outdoor conditions, receiving direct sunlight radiation; (b) simulated indoor exposure, receiving sunlight radiation through window glass.
Figure 10. Lightness (L) average values for the specimens of the three mortar compositions in their original natural color, after the RhB staining process and after 5 days of exposure to the urban environment in: (a) outdoor conditions, receiving direct sunlight radiation; (b) simulated indoor exposure, receiving sunlight radiation through window glass.
Buildings 13 02250 g010
Buildings 13 02250 g011
Figure 11. Average values for global color change (ΔE) after 5 days of exposure to the urban environment in: (a) outdoor conditions, receiving direct sunlight radiation; (b) simulated indoor exposure, receiving sunlight radiation through window glass.
Figure 11. Average values for global color change (ΔE) after 5 days of exposure to the urban environment in: (a) outdoor conditions, receiving direct sunlight radiation; (b) simulated indoor exposure, receiving sunlight radiation through window glass.
Buildings 13 02250 g011
Buildings 13 02250 g012
Figure 12. Photographic records in three of the measurement times for all of the compositions of mortars exposed to artificial accelerated weathering, simulating indoor conditions in glazed buildings.
Figure 12. Photographic records in three of the measurement times for all of the compositions of mortars exposed to artificial accelerated weathering, simulating indoor conditions in glazed buildings.
Buildings 13 02250 g012
Buildings 13 02250 g013
Figure 13. Lightness (L) average values throughout time for all of the compositions of mortars and dispersion methods used in their production, regarding exposure to artificial accelerated weathering, simulating indoor conditions in glazed buildings.
Figure 13. Lightness (L) average values throughout time for all of the compositions of mortars and dispersion methods used in their production, regarding exposure to artificial accelerated weathering, simulating indoor conditions in glazed buildings.
Buildings 13 02250 g013
Buildings 13 02250 g014
Figure 14. Average global color change (ΔE) values and the correspondent spreads for each composition and dispersion method used to produce mortars exposed to artificial accelerated weathering, simulating indoor conditions of glazed buildings.
Figure 14. Average global color change (ΔE) values and the correspondent spreads for each composition and dispersion method used to produce mortars exposed to artificial accelerated weathering, simulating indoor conditions of glazed buildings.
Buildings 13 02250 g014
Buildings 13 02250 g015
Figure 15. Representation of Tukey’s multiple (pair-wise) comparison test for the compositions 1:1:6 and 1:4 with the two types of dispersion; the green line represents average values without a statistically significant difference.
Figure 15. Representation of Tukey’s multiple (pair-wise) comparison test for the compositions 1:1:6 and 1:4 with the two types of dispersion; the green line represents average values without a statistically significant difference.
Buildings 13 02250 g015
Table 1. Identification acronyms and water/binder proportions of the mortars.
Table 1. Identification acronyms and water/binder proportions of the mortars.
IdentificationMortar CompositionDispersion Method–PhaseAddition of TiO2Water/Binder Proportion
1:1:61:1:6 (cement:lime:sand)Ultrasonic–1No2.21
1:31:3 (cement:sand)Ultrasonic–1No1.30
1:41:4 (cement:sand)Ultrasonic–1No1.43
1:1:6–Ti1:1:6 (cement:lime:sand)Ultrasonic–1Yes2.40
1:3–Ti1:3 (cement:sand)Ultrasonic–1Yes1.43
1:4–Ti1:4 (cement:sand)Ultrasonic–1Yes1.50
1:1:6–UD1:1:6 (cement:lime:sand)Ultrasonic–2Yes2.40
1:3–UD1:3 (cement:sand)Ultrasonic–2Yes1.43
1:4–UD1:4 (cement:sand)Ultrasonic–2Yes1.50
1:1:6–MD1:1:6 (cement:lime:sand)Mechanical–2Yes2.40
1:3–MD1:3 (cement:sand)Mechanical–2Yes1.43
1:4–MD1:4 (cement:sand)Mechanical–2Yes1.50
Table 2. Consistency index, density, and air-entrained content (fresh state) for the different mortar compositions.
Table 2. Consistency index, density, and air-entrained content (fresh state) for the different mortar compositions.
IdentificationConsistency IndexAverage Density (kg/m3)Average Air-Entrained Content (%)
Average (mm)Standard Deviation (SD) (mm)Coefficient of Variation (CV)
1:1:62463.781.54%1736.2113.25
1:1:6-UD2372.521.06%1748.3312.47
1:1:6-MD2281.730.76%1942.3812.00
1:32262.641.17%1767.0815.21
1:3-UD2541.530.60%1796.2211.89
1:3-MD2263.611.60%2026.8414.51
1:42564.361.70%1753.0313.50
1:4-UD2403.461.44%1763.1513.24
1:4-MD2555.031.97%1996.0013.08
Table 3. Bulk density and water absorption by capillarity in the hardened state.
Table 3. Bulk density and water absorption by capillarity in the hardened state.
IdentificationBulk Density—Hardened StateWater Absorption by Capillarity
Average (kg/m3)Standard Deviation (SD) (kg/m3)Coefficient of Variation (CV)Average
(g/cm2)		Standard Deviation (SD) (g/cm2)Coefficient of Variation (CV)
1:1:61708.5430.641.79%18.250.341.84%
1:1:6-UD1660.796.760.41%20.470.643.12%
1:1:6-MD1671.364.790.29%22.830.793.45%
1:31872.4412.790.68%5.290.519.56%
1:3-UD1767.1321.161.20%8.300.597.08%
1:3-MD1823.128.060.44%8.760.536.06%
1:41718.6527.141.58%11.100.918.18%
1:4-UD1722.3923.401.36%12.880.161.28%
1:4-MD1776.383.990.22%15.811.086.86%
Table 4. Dynamic modulus of elasticity after 28 days—hardened state.
Table 4. Dynamic modulus of elasticity after 28 days—hardened state.
IdentificationDynamic Modulus of Elasticity
Average (GPa)Standard Deviation (SD) (GPa)Coefficient of Variation (CV)
1:1:64.630.398.42%
1:1:6-UD3.380.020.59%
1:1:6-MD2.480.072.65%
1:316.280.975.96%
1:3-UD8.880.141.58%
1:3-MD9.060.161.78%
1:49.760.212.15%
1:4-UD6.720.091.34%
1:4-MD5.950.182.97%
Table 5. Results from statistical analysis of variance (ANOVA) for global color change (ΔE) of the three compositions of mortars, reference, and TiO2-added, within the two types of conditions simulated during exposure to the urban environment, after 5 days.
Table 5. Results from statistical analysis of variance (ANOVA) for global color change (ΔE) of the three compositions of mortars, reference, and TiO2-added, within the two types of conditions simulated during exposure to the urban environment, after 5 days.
Effects and Effects’
Interactions		Square SumDegrees of FreedomSquare AverageF TestProbabilitySignificative Influence?
Composition (A)149.612.0074.8018.930.00Yes
TiO2 addition (%) (B)792.881.00792.88200.610.00Yes
Type of exposure (C)1422.591.001422.59359.930.00Yes
A × B208.642.00104.3226.390.00Yes
A × C21.972.0010.982.780.07No
B × C608.521.00608.52153.960.00Yes
Error426.86108.003.95---
Table 6. Statistical analysis of variance (ANOVA) for global color change (ΔE) for the three compositions of mortars and the two types of dispersion methods submitted to artificial weathering for 440 min.
Table 6. Statistical analysis of variance (ANOVA) for global color change (ΔE) for the three compositions of mortars and the two types of dispersion methods submitted to artificial weathering for 440 min.
Effects and Effects InteractionsSquare SumDegrees of FreedomSquare AverageF TestProbabilitySignificative Influence?
Composition (A)55,366.70155,366.702571.5750.000000Yes
Dispersion (B)150.00275.003.4840.039835Yes
A × B135.901135.906.3120.015920Yes
Error263.19343042131.59671526.112171390.00467902-
Table 7. Tukey’s test for the coating mortars’ global color variation (ΔE) considering the independent variables composition and dispersion method. NS: not significant; S: significant.
Table 7. Tukey’s test for the coating mortars’ global color variation (ΔE) considering the independent variables composition and dispersion method. NS: not significant; S: significant.
{}CompositionDispersion{1}{2}{3}{4}{5}{6}
11:1:6Ultrasonic SNSNSSS
21:1:6MechanicalS NSNSNSNS
31:3UltrasonicNSNS NSNSNS
41:3MechanicalNSNSNS NSNS
51:4UltrasonicSNSNSNS NS
61:4MechanicalSNSNSNSNS
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MDPI and ACS Style

Casarin, R.P.; Bersch, J.D.; Maia, J.; Masuero, A.B.; Dal Molin, D.C.C. Assessment of Photocatalytic Nano-TiO2 Mortars’ Behavior When Exposed to Simulated Indoor Conditions of Glazed Buildings. Buildings 2023, 13, 2250. https://doi.org/10.3390/buildings13092250

AMA Style

Casarin RP, Bersch JD, Maia J, Masuero AB, Dal Molin DCC. Assessment of Photocatalytic Nano-TiO2 Mortars’ Behavior When Exposed to Simulated Indoor Conditions of Glazed Buildings. Buildings. 2023; 13(9):2250. https://doi.org/10.3390/buildings13092250

Chicago/Turabian Style

Casarin, Roberta Picanço, Jéssica Deise Bersch, Joana Maia, Angela Borges Masuero, and Denise Carpena Coitinho Dal Molin. 2023. "Assessment of Photocatalytic Nano-TiO2 Mortars’ Behavior When Exposed to Simulated Indoor Conditions of Glazed Buildings" Buildings 13, no. 9: 2250. https://doi.org/10.3390/buildings13092250

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