Air cleaning concrete paving blocks made from recycled construction and demolition waste

Abstract

Economic success has made Hong Kong a worldwide commercial and financial centre as well as being the gateway to China. But similar to other developed countries this path to success has put pressure on the environment. As a result air quality has suffered. The numerous tall buildings, particularly those in the urban area, hinder and prevent the dispersion of air pollutants generated by a high concentration of vehicles at the street level. It is apparent that there is a need to remove pollutants, such as nitrogen oxides (NOx), derived from vehicular emission. Therefore, a way of removing such pollutants needs to be sought. In additional, a huge quantity of construction and demolition wastes (C&DW) is produced every day in Hong Kong, representing a large fraction of the solid waste stream. The extensive building and infrastructure development projects as well as redevelopment of old districts have led to an increase in C&DW generation in the last decade. This has caused the disposal of the wastes a severe social and environmental problem in the territory. Hence, it is necessary to find ways to overcome the growing concern of air pollution as well as the difficulty of reusing recycle aggregates. This study aimed to analyze the effectiveness of incorporating air cleaning agents, such as titanium dioxide (TiO₂) into the concrete paving blocks which were produced by local waste materials. Factors which would affect the performance of the blocks were studied including the porosity of the blocks, the type of waste materials used within the mix design (such as recycled aggregate, furnace bottom ash and recycled glass) and the percentage of TiO₂within the mix design. The effectiveness of different types of TiO₂was compared. To achieve the aim concrete blocks were produced in the laboratory and tested for their physical properties as well as their ability to remove NO. The blocks were prepared in two layers with a base layer and a surface layer. TiO₂was only incorporated in the surface layer. The results indicated that the physical properties of the blocks were al satisfactory in meeting international standards with the compressive strength reaching 60MPa. The measurement of NO photodegradation was carried out using a reactor with the initial NO concentration of 1000ppb. The prepared specimens were placed within a sealed reactor of dimensions 700x400x130 mm and once the targeted concentration level was reached the removal of NO began when the block was irradiated by 2 UV-A lights providing an intensity of 10Wm-2 at the specimen surface. Results showed that the photodegradation of NO is related to the porosity of the blocks. Generally, the higher the porosity of the block, the higher the NO removal ability. It is concluded that the choice, size and content of aggregate material used in the mix design were all important factors. In addition, crushed recycled glass was used as an aggregate in the blocks and was found to benefit the ability to remove NO due to its light transmitting characteristic. It was possible that light could be carried to a greater depth by refraction activating the TiO₂ on the surface as well as within the blocks. Besides the type of aggregate used, the choice of TiO₂was equally important. Three types of TiO₂were tested in this study: (1) P-25 from Degussa Company which is a combination of anatase and rutile forms, selected due to its high purity and accurate specifications, (2) an anatase form and (3) a rutile form of TiO₂, both sourced from Ke Xiang Company due to their low prices compared to P-25. The rutile form of TiO₂from Ke Xiang Company showed abilities almost comparable to P-25 due to similarities in particle size. As a result an optimum mix design was selected incorporating recycled glass, sand, metakaolin, carbon and cement in the surface layer of the block and recycled aggregate and cement in the base layer of the block. The optimum mix design was used to prepare a batch of paving blocks in a factory for field testing by laying a pavement section. The air quality of the field testing site was continuously monitored to analyze the effect of the blocks towards the surrounding air quality. The observations showed that NO concentrations were reduced by 12% at ground level and 8% at breathing zone. After a period of four months the blocks were removed and taken back to the laboratory to analyze whether its ability to remove NO had decreased. Results showed only a slight decrease of 23 % for these used blocks.