Coal bottom ash (CBA) is a by-product generated in the coal furnaces of thermal power plants. It has adverse effects on the environment and requires additional landfill storage. However, the physical appearance of CBA is similar to that of sand, with particle sizes ranging from fine to coarse aggregates. Hence, many previous studies have focused on its application instead of sand for conventional concrete and structural fill materials considering natural sand depletion. Roller compacted concrete (RCC) is a zero-slump concrete with better compressive strength than conventional concrete because of the aggregate interlock achieved due to the compactness. It uses a low cement content but requires a large amount of sand. However, the applicability of CBA to RCC is limited. The properties of CBA may vary depending on the location. Therefore, this study aims to evaluate the effect of CBA as a sand replacement in RCC in terms of strength and long-term performance by conducting an empirical experiment under given conditions. Initially, a comprehensive literature review of various concrete strengths using CBA as sand replacement is provided. Thereafter, details related to the experiments on the strength and durability of the given CBA are presented. The results show that the RCC strength improves when CBA increases owing to the good gradation and low percentage of calcium oxide. Furthermore, the samples displayed acceptable freeze–thaw resistance values, but the scaling resistance values were small, owing to the high-water absorption. Therefore, CBA can be used in RCC as sand for improved structural performance, while additional research should be conducted to improve its long-term performance.

Nitrogen oxide (NOx) is a significant precursor of particulate matter (PM), particularly in high-traffic areas. Accordingly, this study aimed to reduce the presence of nitrogen oxide (NOx) along roadsides. Titanium dioxide (TiO₂) is used as a photocatalyst to remove NOx through a chemical reaction. Typically, concrete and TiO₂ are mixed to create TiO₂ concrete. However, air pollutants or UV rays cannot be allowed to come into contact with a significant amount of TiO₂. Thus, the TiO₂ surface penetration method was used to fix TiO₂ to the surface of the concrete. In this method, surface penetrants and TiO₂ are combined and sprayed onto the concrete surface, enabling the possibility of NOx reduction using relatively less TiO₂. When the fixation method is applied to vertical concrete structures, however, a peeling issue arises. To address this, a pressurized TiO₂ fixation method was applied to vertical concrete structures. This method uses external force to penetrate and fix TiO₂ to a specific depth. In the instance of the pressurized TiO₂ fixation method, which was tried for the first time, penetration depth was used to ensure long-term durability as well as NOx removal efficiency. To investigate TiO₂ distribution characteristics, the penetration depth and mass ratio of TiO₂ particles in TiO₂ concrete were measured using a scanning electron microscope (SEM/EDX). In addition, NOx removal efficiencies were evaluated using the NOx analyzing system (ISO 22197-1 standard). The experimental results showed that NOx removal efficiency increased with an increasing TiO₂ mass ratio. When the TiO₂ fixation method was used, the NOX removal efficiency was 32% when the TiO₂ mass ratio at the surface was 50%, and the efficiency was 61% when the TiO₂ mass ratio was 70%. This is attributed to the increase in NOx removal efficiency as TiO₂ content at the concrete surface increases. This study analyzed and forecasted the NOx removal efficiency of TiO₂ concrete based on the mass ratio of TiO₂ on the surface. As the TiO₂ mass ratio increased, the NOx removal efficiency improved, and it was determined that the surface TiO₂ mass ratio significantly influences the NOx removal efficiency. Consequently, this study developed an equation to estimate NOx removal efficiency, making it possible to determine a suitable maintenance interval for TiO₂ concrete.