New and effective methods for production of cement-based photocatalytic materials

Abstract

Photocatalytic functional cementitious materials have gained significant traction as people become increasingly aware of their living environments. With the natural energy (solar light) alone, various photocatalytic functions, such as air purification, can be realized. Extensive research has spurred the practical application of photocatalytic construction materials on a global scale. However, several challenges still limit their broader applications. Among them, a normally high cost and an impaired photocatalytic efficiency caused by encapsulation of TiO₂ by the cement hydration products stand out. Commonly, a trade-off has to be made between a high efficiency and a low cost. Therefore, achievement of good photocatalytic efficiencies with low amounts of TiO₂ implies considerable cost savings. In this study, two novel methods have been developed to better incorporate nano-TiO₂ into two types of cementitious materials (namely self-compacting architectural mortar and concrete surface layers of eco-blocks). Recycled glasses derived from post-consumer beverage glass bottles were used as fine aggregates to replace natural river sand. The advantages of samples prepared by these new methods over those prepared by the traditional intermixing method were demonstrated in terms of photocatalytic degradation of various substances under various conditions. For the architectural mortars, the traditional intermixing method was first adopted to combine TiO₂ with recycled glass for the purpose of enhancing the photocatalytic efficiency in terms of NOx removal. The mechanical and durability properties of architectural mortars prepared with varying dosages of TiO₂ and recycled glass were also examined. It was found that the light transmittance property of recycled glass was responsible for the highest photocatalytic NOx removal efficiency observed on the samples prepared with 100% recycled glass as aggregate and a dosage of 5% TiO₂. Adding more glass contents, coupled with increasing TiO₂ dosages, led to a slight reduction in both the compressive strength and flexural strength (due to the formation of agglomerates) and an increase in both the ASR expansions and carbonation depths (but all the ASR expansions were below the permissible limit). To fulfil the goal of increasing photocatalytic efficiencies while lowering the cost, an alternative strategy of applying a TiO₂ containing paint (PC-S7) on the surface of architectural mortars was investigated. The superior photocatalytic efficiency of the PC-S7-coated samples over the 5% TiO₂-intermixed counterparts was reflected by a wide range of photocatalytic activities. For example, the PC-S7-coated architectural mortars had both a significantly higher NOx and xylene degradation performance in comparison with the 5% TiO₂-intermixed counterparts which achieved a much lower NOx removal rate and failed to degrade xylene. In addition, the PC-S7-coated architectural mortars delivered a total inactivation of E. coli after 120 min of UVA irradiation, contrasting with a negligible E. coli killing activity by the 5% TiO₂-intermixed samples. Moreover, the PC-S7-coated samples also garnered a robust weathering-resistant ability, reflected by an almost unchanged ability to photocatalytic degrade RhB after exposure to either a facade weathering process or an accelerated carbonation process. Investigation on the photocatalytic architectural mortars gave rise to an intriguing finding that white cement (WC) was superior to ordinary Portland cement (OPC) in boosting the photocatalytic NOx removal activity. It was found that OPC displayed a much higher light absorption ability relative to WC. As a result, less light would be reflected to the surrounding TiO₂ surface, and thereby fewer electron-hole pairs will be generated. Even if the same amount of light does strike on the TiO₂ surface, a stronger charge transfer resistance of OPC makes the electron-hole pairs difficult to separate, and instead, more prone to recombination. These two adverse effects combine to impart an inferior photocatalytic NOx performance to the OPC/TiO₂ mixtures. Overall, the main culprit seems to be a relative abundance of Fe₂O₃ in OPC. As for the concrete surface layers, a simple and effective method (spray method) was developed to obtain a high photocatalytic efficiency with less amounts of nano-TiO₂. Its advantages over the conventional intermixing and dip-coating methods were well demonstrated. Moreover, the effects of various experimental and environmental factors on the photocatalytic NOx removal efficiency of the concrete surface layers fabricated by the new method were examined and compared with that of the samples fabricated by the intermixing method. It was found that the samples prepared by the spray method outperformed the intermix counterparts under all circumstances, demonstrating that the products produced using the new method hold great promise for practical applications in the construction sector.