1. Introduction
Globally, there is a growing concern about air pollution due to rapid industrialization and urbanization. One of the leading causes of air pollution is particulate matter (PM), which is typically categorized into general particulate matter (PM
10), with a diameter of 10 μm or less, and fine particulate matter (PM
2.5), with a diameter of 2.5 μm or less [
1]. Due to their extremely small size, these particles can infiltrate the respiratory system, including the lungs and blood vessels, potentially leading to various respiratory diseases, such as asthma and bronchitis. Furthermore, precursor pollutants, like nitrogen oxides (NO
x) and sulfur oxides (SO
x), in the atmosphere, when reacting with moisture, ozone, and ammonia, can form particles that are notoriously difficult to remove [
2]. Therefore, technologies that are capable of preemptively removing precursor pollutants are in demand for fine dust reduction. Photocatalysts are known to be effective in removing NO
x, a precursor pollutant that constitutes a significant portion in urban and road environments and can contribute to the formation of fine dust particles [
3,
4]. Photocatalysts are catalytic materials that facilitate the decomposition reactions of precursor pollutants by irradiating them with light, such as ultraviolet radiation.
Among these photocatalysts, titanium dioxide (TiO
2) is a material known for its relatively low cost, high chemical stability, and excellent decomposition performance. It finds applications in various fields, such as semiconductor materials, paints, cosmetics, and more. Particularly in densely populated urban areas, various sources, including vehicles, contribute to the emission of various NO
x pollutants [
5]. However, due to the presence of tall buildings in urban centers, dispersing these emissions can be challenging. Therefore, the application of TiO
2 on road surfaces and building exteriors can present a valuable strategy for mitigating fine dust pollution [
6,
7,
8].
As a result, research on applying TiO
2 photocatalysts to cement, a construction material, is being conducted by various researchers. For instance, Lackhoff et al. [
9] conducted an experimental study where a TiO
2 photocatalyst was added to cement mixtures. They reported that cement with an added TiO
2 photocatalyst could decompose atmospheric pollutants on the surface of structures, making it applicable for the removal of pollutants generated under various environments. Furthermore, Jayapalan et al. [
10] investigated the influence of a TiO
2 photocatalyst on the initial hydration of cement. They considered variables such as the amount of TiO
2 added to the cement and the particle size of TiO
2. Their experiments revealed that an increase in the TiO
2 addition rate accelerated the cement hydration rate, and smaller TiO
2 particles enhanced the hydration reaction with cement [
11,
12]. Additionally, they suggested that the promoted hydration reaction due to the addition of TiO
2 could be utilized to accelerate the strength development of cement pastes. Most of the studies conducted thus far have primarily been experimental studies aimed at applying TiO
2 as a construction material. However, while cement-based construction materials incorporating TiO
2 offer high durability and cost-effectiveness, addressing the fundamental issues associated with high energy consumption, carbon emissions, and dust emissions during cement production remains challenging.
To address these issues, Kwon et al. [
13] conducted research on using eco-friendly construction materials such as zeolite and activated red clay as substitutes for cement. Zeolite is an aluminosilicate mineral with a three-dimensional mesh structure formed through the sharing of oxygen atoms between SiO
4 and AlO
4 tetrahedra [
14,
15,
16]. This material has been known to contain numerous spaces occupied by water molecules and exchangeable cations [
17,
18,
19]. The presence of exchangeable cations in zeolite allows for easy ion exchange, and their size and position influence various physicochemical properties [
20,
21]. The major components of zeolite include SiO
2, Al
2O
3, and Fe
2O
3, which exhibit pozzolanic properties [
22,
23,
24]. Additionally, this material has been valued for its excellent adsorption and catalytic properties, making it primarily used for deodorization and desiccation [
25]. Perraki et al. [
26] confirmed that incorporating zeolite into cement had little impact on its physical and mechanical properties, even when mixed at up to 20%. Activated red clay, on the other hand, shares similar components, such as SiO
2 and Al
2O
3, with conventional concrete admixtures, making it exhibit pozzolanic characteristics [
27]. Go et al. [
28] reported that activated red clay reacts with Ca(OH)
2 to produce 3CaO∙2SiO
2∙3H
2O (C-S-H) and 2CaO∙Al
2O
3∙SiO
2∙8H
2O (C-A-S-H) phases, making it suitable as a cement substitute. Furthermore, they highlighted the positive effects of activated red clay on strength development, odor removal, and heat retention [
29].
It has been proven by several researchers that activated red clay and zeolite can be used as cement substitutes. As mentioned above, activated red clay and zeolite have excellent adsorption performances, so it has been expected that the application efficiency will increase when applying a TiO
2 photocatalyst to mortars with activated red clay and zeolite added [
30,
31]. However, there are limited studies to evaluate the NO
x reduction performance of mortars containing zeolite and activated red clay with TiO
2-coated photocatalysts. In order to apply new materials to the construction field, it is important to secure sufficient experimental results. Therefore, in this study, we experimentally assessed the physical properties and NO
x reduction performance of mortars using zeolite and activated red clay as cement substitutes.
4. Conclusions
In this study, NOx reduction experiments were conducted on mortars incorporating zeolite and activated red clay, leading to the following conclusions:
The flexural strengths of mortars with zeolite and activated red clay substitutions showed that the flexural strength of activated red clay was superior to that of zeolite. As the substitution ratio of zeolite and activated red clay increased, the reduction in strength compared to conventional cement concrete also increased. It was observed that mortars with zeolite substitutions exhibited a 10% to 20% lower flexural strength compared to those with activated red clay substitutions.
Compressive strength exhibited a similar trend to flexural strength. The substitution of zeolite and activated red clay resulted in a reduction in compressive strength ranging from 2.29 MPa to 15.89 MPa compared to the standard specimens. Moreover, specimens with a cement–fine aggregate ratio of 1:3 showed a greater reduction in their strength.
The absorption rate increased as the substitution ratio of zeolite and activated red clay increased in absorption experiments. Zeolite exhibited approximately 5.13% to 16.27% higher absorption rates compared to activated red clay.
The NOx reduction performance was improved by replacing cement with zeolite and activated red clay by up to 12.6% and 13.2%, respectively. Furthermore, activated red clay demonstrated better NOx reduction effects compared to those achieved with zeolite. It will be necessary to identify its cause through future research, such as micropore analyses.
Through this study, it was confirmed that the NOx reduction efficiency of a TiO2 photocatalyst can be increased by adding zeolite and activated red clay. Therefore, it is expected that using zeolite and activated red clay as a cement substitute and applying TiO2 photocatalysts to construction materials that use mortar, such as interlocking blocks and sidewalk blocks, will be effective in reducing roadside NOx in the future.