Environmentally friendly concrete paving blocks for photodegradation of air pollutants

Incorporation of nano-photocatalysts into cementitious materials is an important development for large scale applications. The aim of this project is to make the "cross over" of solid waste recycling and photocatalytic technology feasible and to apply such technology in urban air pollution treatment. A systematic study on assessing the pollutant degradation effectiveness by concrete surface layers that incorporate TiO2 and recycled glass cullets was conducted. Recycled glass cullets, derived from crushed waste beverage bottles, were used to replace sand in preparing the concrete surface layers. The photocatalytic activity of the samples was determined by photocatalytic NOx conversion test in the laboratory. Factors which may affect the pollutant removal performance of the concrete layers, such as glass color, aggregate size and curing age, were investigated. The results showed a significant enhancement of the photocatalytic activity due to the use of glass cullets as aggregates in the concrete layers. The samples fabricated with clear glass cullets exhibited threefold of NOx removal efficiency compared to the samples fabricated with river sand. The light transmittance property of glass was postulated to account for the efficiency improvement, which was confirmed by a separate simulation study. It was also found that the influence of the size of glass cullets was not evident because the shape and size of the aggregates may be changed by the high compaction pressure during the fabrication process. In addition, the photocatalytic activity of the concrete surface layers decreased with curing age, showing a loss of 20% NOx removal after 56 days curing.;The effectiveness of using photocatalytic paving blocks to remove common air pollutants such as NOx has been well demonstrated at laboratory level. To evaluate the performance of the photocatalytic paving blocks used in complex ambient air conditions, two field trials were conducted. One was carried out at the campus of The Hong Kong Polytechnic University, and another one was carried out at three primary schools in different districts of Hong Kong. It was found that the NOx removal efficiency of the paving blocks decreased with time. Washing the block surface with detergent or polishing the surface can regenerate the pollution removal ability. The statistic analysis of the monitoring data showed that 7.2 % NOx was reduced at ground level compared with the NOx concentration in breathing zone in one year monitoring period. Compared with the air quality monitoring method, the sampling box method is more accurate in demonstrating the NOx removal performance. It is suggested that the photocatalytic blocks should be washed periodically and applied in heavily polluted areas to maximize their pollutant removal function. (Abstract shortened by UMI.)...