Photocatalytic Cement Mortar with Durable Self-Cleaning Performance

Abstract

Nano-TiO₂-modified mortars are fabricated by introducing TiO₂ nanoparticles to the conventional mortar mix with designed mixing and curing procedures. It was found that additional TiO₂ nanoparticles can accelerate hydration and improve the air void distribution in the mortar matrix. The experiments also showed that 0.5 wt.% and 1 wt.% TiO₂-modified mortar has a comparable mechanical strength to traditional cement mortar. The abrasion resistance is improved with nanoparticles at 0.5 wt.% TiO₂ concentration. The photocatalytic performance of photocatalytic mortar was confirmed by a methylene blue decomposition test. Finally, a multi-physics computational model was constructed to assess the effects of photocatalytic mortar coated on building in air quality improvements in the neighboring area. The benefits are affected by different nano-TiO₂ concentrations, as well as wind conditions in the neighborhood. Overall, this study shows that properly designed nano-TiO₂-modified mortar is promising to achieve multifunctional performance in terms of mechanical strength and durability as well as autogenous self-cleaning of surrounding environment.

1. Introduction

Concrete is one of the most commonly used construction materials. There are a significant number of efforts and strategies to improve the performance and functionality of concrete materials for transportation applications [1]. With the rapid development of nanoscience and nanotechnology, the use of nanoparticles, such as nano silica fume and titanium dioxide, have received particular interest as potential additives in cement-based materials. Titanium dioxide (TiO₂) is a well-known photocatalyst with various applications, especially in environmental and energy-related areas [2,3]. Once activated by ultraviolet, TiO₂ has a strong oxidizing ability to break down organic pollutants present both in air and water, thus it has been widely used as a self-cleaning material. Nano-sized TiO₂ possesses a higher specific surface area than micro-sized particles, and it significantly increases the effective contact area between TiO₂ and pollutants and further facilitates the photocatalytic activity of TiO₂.

TiO₂ photocatalyst has been applied as an additive to many materials for self-cleaning performance [4,5,6,7,8]. TiO₂ either forms a thin film on the substrate or is homogeneously distributed throughout the matrix. For instance, researchers have loaded nano-TiO₂ on the surface of polymer nanofiber to maximize the photocatalytic activity of TiO₂ [9,10,11,12]. Nanofiber has the advantage of a high surface area, and TiO₂-covered nanofiber can greatly increase the contact area between TiO₂ and pollutants. Moreover, TiO₂ thin film not only provides a self-cleaning function, but also has the benefits of long-term stability and corrosion resistance [13,14,15,16]. A. Folli et al. [17] also studied how the size of TiO₂ aggregates influence the photocatalytic performance of TiO₂-containing cementitious materials.

Except for self-cleaning performance, additional TiO₂ can also influence the mechanical properties of concrete material. Reports have been published and showed the positive effects of nano Fe₂O₃ and SiO₂ particles on improving the mechanical properties of the hydrated materials, and the hydration durations decreased with the addition of nanoparticles [18,19,20,21,22]. Various technologies, such as differential thermal analysis, helium inflow, X-ray diffraction, scanning electron microscopy, nuclear magnetic resonance, etc., have been used to study the influences of nanoparticles on hydration characteristics and microstructures of cement-based materials [23,24,25].

In this study, nano-TiO₂-modified mortar was fabricated by introducing TiO₂ nanoparticles into Portland cement mortar, making TiO₂ particles well-distributed in the mortar matrix. In this way, nano-TiO₂ are embedded into the mortar matrix and certain parts can always be exposed, even after the surface layer is worn off by traffic or other abrasive loads. The mechanical properties, including the compressive strength, splitting strength, and abrasive resistance of modified mortars, were measured and compared with traditional mortar. The photocatalytic performance of modified mortar samples after seven years of aging was measured to investigate the long-term photocatalytic activity of nano-TiO₂ in modified mortar. The results demonstrate the positive influence of TiO₂ on the hydration and mechanical properties of mortar, as well as on the development of self-cleaning capabilities. Finally, to evaluate the feasibility of nano-TiO₂-modified cementitious materials coated on building surface, a holistic multi-physics model which couples air flow and chemical reaction was built. Considering a tradeoff between air cleaning efficiency and material cost, a sensitivity study was conducted to find out the optimal TiO₂ coverage under different wind speeds. This study demonstrates the potential of integrating photocatalysts into building materials to achieve multifunctional performance, a novel strategy that bridges catalyst with construction materials.

2. Results and Discussions

2.1. Monitoring of Hydration-Induced Thermal Process

TDR measures the dielectric constant and electrical conductivity of materials, and therefore the monitored TDR signals directly reflect the change in the amount of the free water content (that changes the dielectric constant) and the development of microstructure (i.e., that changes the electrical conductivity) in the Portland cement mortar. Figure 1 compares the TDR signals at different curing times. The changes of TDR signals over time indicate the change in the mortar samples as the hydration process evolves. The free moisture content in the mortar decreases with hydration over time for all three types of mortar samples. The signals are similar at the beginning as shown in Figure 1a and start to show some differences over time. Comparing TDR signals of the plain mortar and two nano-TiO₂ modified mortars at the same hydration stage, the cement mortar with 0.5 wt.% nano-TiO₂ has a higher hydration rate than the other two. This implies that the addition of nano-TiO₂ accelerated the cement hydration. Such observation is consistent with other documented studies.

2.2. Microstructures of Nano-TiO₂-Modified Mortars

The first subfigure of Figure 2 shows the surface morphologies with SEM of mortar sample modified with 1 wt.% nano-TiO₂. The other subfigures show the distributions of a few elements across the sample using the EDX technique. The contours show that the mortar contains various chemical elements, including calcium, silicon, aluminum, iron and oxygen, which are consistent with the mineral and oxide compositions of Portland cement and sand. The major components of Portland cement are CaO, SiO₂, Al₂O_3, and Fe₂O₃, and sand is made of SiO₂ present in quartz form. The elemental distribution image of titanium, solely sourced by TiO₂, implies that the TiO₂ particles are reasonably well distributed on the cement mortar. However, agglomerates were formed in some areas. The size of the agglomerates reaches as much as tens of micrometers. The agglomeration of colloid nanoparticles cn spontaneously occur mainly due to a high specific surface area [26].

2.3. Mechanical Strength Analysis

The mechanical properties of TiO₂-modified mortars, including compressive strength, tensile strength, and abrasive resistance, were analyzed. The compressive strength measured at various curing ages is presented in Figure 3. The results represent the average values from tests conducted on three duplicate specimens. Mortar containing 1 wt.% nano-TiO₂ exhibited a slightly higher compressive strength than plain mortar at all curing times. Additionally, 0.5 wt.% nano-TiO₂ mortar demonstrated the highest compressive strength compared to the other two mortars during the early hydration stage. However, beyond this point, there was no significant difference in compressive strengths between 0.5 wt.% nano-TiO₂-modified mortar and plain mortar.

Splitting tests were conducted to measure the tensile strength of the mortar specimens, with the results summarized in Figure 4. Similar trends were observed from the splitting tests as in the compression tests. TiO₂-modified mortars exhibited higher splitting strength compared to plain mortar at all curing times. Mortar containing 0.5 wt.% nano-TiO₂ mortar showed the highest splitting strength at 3 and 7 days. However, after 7 days, the splitting strengths of all three specimens became comparable.

The reduction in splitting and compressive strengths at higher nanoparticle concentration may be attributed to the formation of weak zones in the nano-TiO₂-modified mortar. These weak zones, caused by the agglomeration of nano-TiO₂ particles, act as structural defects that can facilitate the initiation of damage.

Figure 5 summarizes the results of the abrasion test, showing the abrasion percentages of three mortar specimens at different hydration stages. The nano-TiO₂-modified mortars demonstrated better abrasive resistance compared to traditional cement mortar, and the modified mortar treated with 0.5 wt.% TiO₂ showed a slightly better abrasion resistance over 1 wt.% TiO₂. The reduced effectiveness at a higher nano-TiO₂ concentration may result from such issues as non-uniform dispersion and particle conglomeration. This finding highlights the importance of selecting an appropriate proportion of nano-TiO₂ and employing effective dispersion techniques to maximize the benefits of nano-TiO₂ in enhancing the abrasion performance of cement mortar.

2.4. Photocatalytic Performance Measurements

The photocatalytic performance of modified mortars was measured. The mortars used in the experiment were initially fabricated seven years earlier. The purpose of this measurement is to investigate the long-term photocatalytic capability of nano-TiO₂ in modified mortar. Figure 6 and Figure 7 show the color change of methylene blue on mortar specimens under the irradiation of the sunlight. The same amount of methylene blue solution was dropped on the side surface of the mortar specimens. Once the methylene blue solution was dripped onto the mortar surface, the liquid was found to be easily spread out on TiO₂-modified mortar. It is possible because the hydrophilic TiO₂ particles are exposed to the mortar surface.

The photocatalytic activity of TiO₂ in modified mortars was measured with two diluted methylene blue solutions. Methylene blue is treated as a stain in practical applications. Both experiments show that methylene blue on modified mortar is lighter in color than that on plain mortar after the same period of illumination time under the sunlight. This indicates that TiO₂ on mortar surface is capable of decomposing methylene blue molecules. Regardless of stains with a high or a low concentration, modified mortar has the ability to break down the contaminates, performing a spontaneous self-cleaning role.

The methylene blue decomposition test was also conducted on mortars with surface grounded to emulate the abrasion effects. The mortar specimens that were ground with sandpaper had rough surfaces full of scratches, as shown in Figure 8. After dripping with the methylene blue solution, the liquid quickly spread out due to the deep scratches serving as flow channels. After one day’s irradiation by sunlight, the blue color on two TiO₂-modified mortar specimens faded more than that on the plain mortar specimen. This implies that modified mortars maintain good photocatalytic activity after the abrasion wear, as the TiO₂ embedded inside the matrix is exposed. Introducing TiO₂ particles into the mortar mix maintains the self-cleaning function in both mortars with and without a wear loss. This is an advantage as compared with applying the TiO₂ as a coating layer, particularly when it is subjected to abrasive service loads.

2.5. Air Cleaning Field Test Modeling

The air-cleaning capability of the nano-TiO₂-modified exterior building surface was further evaluated using multi-physics modeling. The ambient air quality improvement near the nano-TiO₂-coated building was analyzed with different TiO₂ coverages and different wind speeds. To create an environment with some degree of air pollution, the initial concentration of NO₂ was set to be 0.3 ppmv, which is three times higher than the EPA standard of 0.1 ppmv [27]. The TiO₂ coverage ranged from a relatively low dosage, 5 g/m², to a very high dosage, 100 g/m². The wind speeds at the inlet varied from calm air, 1 m/s, to a strong breeze, 10 m/s. Figure 9 shows the modeling results of the central cut planes along x–z plane with a medium TiO₂ coverage of 40 g/m² under different wind speeds. The upper diagrams reveal the wind velocity fields near the nano-TiO₂-coated building as the inlet velocities are set as 1, 2, 5, and 10 m/s. The lower diagrams show the NO₂ concentration distributions with different wind speeds in the stationary phase. The white contour lines represent the boundaries where NO₂ concentration equals 0.1 ppmv. It is clearly observed from the contours that, with the same TiO₂ coverage of 40 g/m², the nano-TiO₂-coated building can provide a healthy air environment for about 40 m and 20 m under wind speeds of 1 m/s and 2 m/s, respectively. However, under moderate and strong breezes, the NO₂ concentration is still higher than the EPA standards, and thus, 40 g/m² nano-TiO₂ coated building cannot provide good protection for residents’ health.

To provide a good reference for the TiO₂ coverage in different wind regions, the air quality improvement with combinations of TiO₂ coverages and wind speeds was assessed. It is worth noticing that a higher light intensity that activates TiO₂ can promote the air-cleaning efficiency. In this modeling, the decomposition rate corresponds to the sunlight UV intensity during partial cloudy days in Central Europe. In an actual application, the air-cleaning efficiency could be improved during sunny days. The NO₂ contaminant levels at a height of 1.5 m on the cut plane along the x–z plane (the red cut line as shown in Figure 10a) are shown in Figure 10. This cut line corresponds to the vertical height of a person’s nose from the ground. The air quality at this altitude is crucial to residents’ health, since most people breathe at this height. Figure 10b–e plots the NO₂ concentration change at z = 1.5 m along x axis under different wind speeds. In the calm air region where the wind speed is about 1 m/s, a small TiO₂ coverage of 10 g/m² can reduce more than half of the contaminant near the nano-TiO₂-coated house. A higher dosage, i.e., larger than 20 g/m², can maintain the NO₂ contaminant below the EPA standard. In the light breeze with a speed of 2 m/s, a medium coverage of 40 g/m² is required to achieve a lower NO₂ concentration than the EPA standard. As the wind speed increases to 5 m/s, a very high TiO₂ coverage, i.e., about 100 g/m², is needed to maintain good air quality near the house. As a strong breeze blows through the neighborhood and more contaminants are brought into the neighboring area, even very high TiO₂ coverage is not able to effectively reduce the NO₂ contaminant below the EPA standard. In summary, neighboring areas with faster wind speeds require higher TiO₂ coverage outside the nano-TiO₂-coated house.

3. Materials and Methods

3.1. Specimen Preparations

The mortar mix includes the Type I Portland cement and fine sand. The diameters of fine sand range from 30 μm to 1 mm. The nano-TiO₂ particles used were purchased from NanoAmor, Inc., Thousand Oaks, CA, USA. The detailed technical information of nano-TiO₂ is listed in

Table 1. Technical information about the nano-TiO₂ [27].

Purity	APS	SSA	Color	Morphology	Bulk Density	True Density
>99%	30–40 nm	30 m2 g−1	white	spherical	0.4 g cm−3	3.94 g cm−3

, where APS represents average particle size, and SSA is specific surface area.

Three types of mortars were prepared to evaluate the performance of nano-TiO₂-modified mortar, that is, plain mortar (PM), cement mortar with 0.5 wt.% nano-TiO₂ (Ti 0.5) and cement mortar with 1 wt.% nano-TiO₂ (Ti 1). Note that 0.5 wt.% and 1 wt.% indicate the weight ratio of nano-TiO₂ to cement.

Table 2. Weight proportions of three mortars.

Specimen	Water (g)	Cement (g)	Sand (g)	Nano-TiO2 (g)	Water-Reducer (g)	Total (g)
PM	65	130	320	0	0.39	515.39
Ti 0.5	65	130	320	0.65	0.52	516.17
Ti 1	65	130	320	1.3	0.65	516.95

shows the weight proportions of each component for these three types of mortars.

The mixing process began by adding nano-TiO₂ particles into water and blending for 2 min using an ultrasonic dispersion tank. The TiO₂-water suspension was then stirred at a high speed of 120 rpm for 3 min. The high-speed procedure was applied since high energy is required in order to achieve a uniform distribution of nanoparticles in water. Next, Portland cement was added to the mixture and blended at a medium speed of approximately 80 rpm for 1 min. Sand was gradually incorporated and mixed at the same medium speed. A superplasticizer was then introduced and mixed at a high mixing speed of 120 rpm for 30 s. The mixture was allowed to rest for 1 min before being blended again for 1 min at high speed [28]. The plain mortar was prepared without the addition of nano-TiO₂. Mortar specimens with a diameter of 2 inches and a height of 4 inches were produced using standard 2 by 4 inches molds upon accomplishing the mixing procedures. A total of 42 specimens were cast for each design recipe to ensure there were three duplicate specimens for each type of test.

3.2. Specimen Characterization and Experimental Procedure

Microstructures of nano-TiO₂-modified mortar were determined by a scanning electron microscope (SEM) equipped with energy-dispersive X-ray analysis (EDX). The hydration of cement mortar was monitored by time domain reflectometry (TDR). TDR is a guided wave electromagnetic wave technology that can be used to study hydration behaviors of materials. TDR signals are direct indicators of the amount of free water and the conductivity of cement mortar [29,30]. The compressive strength, tensile strength, and abrasive resistance of mortars were evaluated using a compression test, splitting test, and an abrasion test, respectively. The abrasion tests were performed on mortar specimens at various curing ages. The standard 60# sandpaper was applied in the abrasion tests. During the tests, the specimens were subjected to a surcharge force of 15 lbs. The rotation speed was set at 150 rpm, with a test duration of 1 min. The specimens were weighed before and after the abrasion test to calculate the abrasion rates.

The long-term photocatalytic functionality of the modified mortar was characterized by the methylene blue decomposition method under the sunlight. The specimens were conserved for over seven years prior to conducting photocatalytic tests. With the presence of activated TiO₂, methylene blue as a dye material can be completely decomposed and it turns the blue color to transparent [31].

$$C_{16}{H}_{18}{N}_{3}SCl+25{\displaystyle \frac{1}{2}}{O}_2\stackrel{Ti{O}_2, hv \ge 3.2 \mathrm{e}\mathrm{V}}{\to }HCl+H_{2}S{O}_4+3HN{O}_3+16C{O}_2+6H_{2}O$$

(1)

The methylene blue aqueous solution (1.5 g/100 mL) was purchased from Sigma-Aldrich (St. Louis, MO, USA), and it was further diluted 100 times (15 mg/100 mL) or 1000 times (1.5 mg/100 mL) for the experiments. Diluted methylene blue solutions of 0.25 mL were dropped on the surface of both the plain mortar and the modified mortar, which were then placed outside under the illumination of the sunlight. The color change of the methylene blue stain was recorded and compared after several days’ illumination. Another methylene blue decomposition test was conducted on mortars with the same procedures after surface grinding, which emulates the abrasive wear in practice. The specimens were ground using #80 sandpaper.

3.3. Modeling Method

One practical application of the nano-TiO₂ modified cement mortar was proposed as the exterior building surface for the air-cleaning purpose [32]. Here, a three-dimensional multi-physics model was built to evaluate the air-cleaning capability of the nano-TiO₂-modified cement mortar in the dimensions of an actual house. The model consisted of two parts: a cubic house (15 m in length, 15 m in width, and 10 m in height) and an air domain (100 m in length, 30 m in width, and 20 m in height), as shown in Figure 1. The house was 25 m away from the left side of the air domain. This model assumes that the exterior surfaces of the house are incorporated with the nano-TiO₂-modified cementitious materials, which can decompose the surrounding air contaminant. The air contaminant is blown through the domain by the wind. The wind blows in from the left entrance, where a wind speed was given, and flows out from the right outlet, where the absolute pressure was set to be 1 atm. Since both the geometry and physics are symmetrical along the x–z plane, and thus the size of the model as shown in Figure 11 was reduced to one-half by using symmetry condition without losing any information.

The model was solved in two steps. In the first step, the wind velocity field in the air domain was solved by using a laminar flow module in COMSOL5.0. The normal inflow velocity at the inlet was given with different values ranging from 1 m/s to 10 m/s to simulate quiet days and windy days. In the second step, the transport of a diluted species module and a chemical reaction module in COMSOL were coupled to simulate the transport and decomposition of the air contaminant. In the model, NO₂ was used as the air contaminant that can be decomposed by the TiO₂-based material and turns into harmless soluble nitrate salts after the decomposition. The initial concentration of NO₂ from the inlet was set at 0.3 ppmv, which is higher than the EPA standard of 0.1 ppmv [33], in order to simulate a neighboring area with some degrees of the air pollution. NO₂ migrates as the velocity field simulated in the first step. As NO₂ reaches the exterior surfaces of the house, it can be decomposed at a decomposition rate, and this photocatalytic decomposition surface reaction follows a pseudo-first-order kinetics [34,35,36].

$$r=kC$$

(2)

where C is the concentration of NO₂ on the surface of nano-TiO₂ modified cementitious materials and k is the surface decomposition rate constant. The k value varies with the TiO₂ coverage on the house surfaces.

Table 3. The surface decomposition rate constant k of NO₂ with different TiO₂ coverages.

TiO2 Coverage (g/m2)	Rate Constant k (m/s)	UV Intensity (W/m2)	Source
50	4.02 × 10−3	10 (corresponds to the sunlight UV intensity on partial cloudy days in the Middle Europe)	[35]
5	4.02 × 10−4		Extrapolate from [35]
10	8.04 × 10−4
20	1.61 × 10−3
40	3.22 × 10−3
100	8.04 × 10−3

lists the k value of NO₂ with different TiO₂ coverages, and the k value was extrapolated from the experimental measurement [31].

4. Conclusions

This study examines the mechanical properties and photocatalytic self-cleaning performance of mortars modified by the introduction of TiO₂ nanoparticles. SEM and EDX measurements confirmed that nano-TiO₂ particles were well distributed in the mortar matrix. The mechanical properties, including compressive strength, splitting strength, and abrasive resistance, were slightly improved in both 0.5 wt.% and 1 wt.% nano-TiO₂-modified mortars compared to the plain cement mortar, with the 0.5 wt.% nano-TiO₂-modified mortar achieving higher mechanical performance possibly due to the agglomeration of nanoparticles at a higher percentage. The long-term self-cleaning performance was evaluated using the methylene blue decomposition method. Both modified mortar specimens with 0.5 wt.% and 1 wt.% TiO₂ exhibited good photocatalytic activity under the irradiation of the sunlight. Modified mortars after the abrasion still maintained good photocatalytic performance. These results indicate that nano-TiO₂-modified mortars have good durability in the self-cleaning function. Using multi-physics computational simulations, nano-TiO₂-modified cementitious materials coated on the building exterior surface demonstrate a considerable air quality improvement in the neighborhood. A TiO₂ coverage of 20 to 100 g/m² can maintain the NO₂ contaminant level lower than the EPA standard; neighboring areas with a faster wind speed require a higher content of TiO₂ in the surface coating. The study demonstrates that incorporating photocatalysts into building materials (i.e., nano-TiO₂ modification of mortar) not only improves the mechanical performance of mortar, but also achieves sustained environmental improvements.