Environmentally friendly concrete paving blocks for photodegradation of air pollutants

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Environmentally friendly concrete paving blocks for photodegradation of air pollutants


Author: Chen, Jun
Title: Environmentally friendly concrete paving blocks for photodegradation of air pollutants
Degree: Ph.D.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Air -- Purification -- Photocatalysis
Concrete blocks
Department: Dept. of Civil and Structural Engineering
Pages: xxi, 170 leaves : ill. ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2392644
URI: http://theses.lib.polyu.edu.hk/handle/200/5889
Abstract: Heterogeneous photocatalysis has been intensively studied in recent decades because it only requires photonic energy to activate the chemical conversion contrasting with conventional catalysis which needs heat for thermo-activation. Over the years, the theories for photochemical phenomenon including photo-induced redox reaction and super-hydrophilic conversion of TiO₂ have been established. The progress in academic research significantly promotes its practical applications in the field of photocatalytic construction and building materials. TiO₂ modified building materials are most popular because TiO₂ has a high activity and it is traditionally used as a white pigment. The major applications of TiO₂ based photocatalytic building materials include environmental pollution remediation, self-cleaning and self-disinfecting. The advantage of using solar light and rainwater as driving force for pollution treatment has opened a new domain for environmentally friendly building materials. 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.
To further explore of the mechanisms, the photocatalytic NOₓ conversion by TiO₂ blended cement pastes was used as a standard process to evaluate the internal factors which may influence the depollution performance. The chemical composition and microstructure of the TiO₂ modified cement pastes were characterized and analyzed. The active photocatalytic sites related to the surface area of TiO₂ are the key factor in determining the photocatalytic activity. Ordinary Portland cement pastes showed lower photocatalytic activity than white cement pastes probably due to the influence of minor metallic components. X-ray diffraction and TG analysis demonstrated that TiO₂ was chemically stable in the hydrated cement matrix. The decreasing trend of NOx removal ability with the increase of curing age was confirmed again. This could be attributed to the cement hydration products which filled up capillary pores forming diffusion barriers to both reactants and photons. It was also proved that surface carbonation could reduce the NOₓ removal efficiency after the hydration of cement. On the other hand, as a new functional construction material, the basic properties of the photocatalytic cement-based material must be identified. Therefore, the side effects of adding nano-TiO₂ on inherent properties of hardened cements were also examined. Two types of nano-TiO₂ with different particle sizes were blended into cement pastes and mortars. Their effects on hydration and physical properties of cements were investigated. The admixture of nano-TiO₂ powders significantly accelerated the hydration rate and promoted the degree of hydration at early ages. It was demonstrated that the distribution of TiO₂ in cement pastes was not homogeneous. The total porosity of paste sample decreased and the pore size distribution was also altered. The acceleration of hydration rate and the change of microstructure affected the physical and mechanical properties of the cement-based materials. The initial and final setting time was shortened and more water was required to maintain a standard consistence due to the addition of nano-TiO₂. The compressive strength of the mortar was enhanced, especially at early ages. It is concluded that nano-TiO₂ may act as "a catalyst" in cement hydration reactions. 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.

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