Ordinary construction materials, including concrete, can now be made photocatalytically active by the introduction of nanocrystalline titanium dioxide (TiO2), particularly in the tetragonal anatase form. In the presence of water, oxygen, and ultraviolet or near-ultraviolet light, a series of reactions occurs on the TiO2 surface, imparting self-cleaning, biocidal, and smog-abating qualities to the surfaces of the materials which contain it. In particular, the potential for passive smog-abatement by photocatalysis has attracted much attention. This research will characterize the long-term compatibility between photocatalytically active materials and their cement-based substrates. A comprehensive research plan, which relies upon microchemical analysis and nano/micro-scale and bulk measures of mechanical properties (e.g., nanoindentation, microhardness, compressive strength), will be used to better understand the initial material structure and properties, characterize the influence of photocatalysis on surface and bulk properties, and describe the influence of material variables and environmental parameters on performance.

Verification of, and potential improvements to, the long-term performance of cement-based construction materials with smog-abating capabilities is an intrinsic broader impact of this research. Additional impact will be made through the mentoring and training of a diverse group of graduate and undergraduate researchers in a multi-disciplinary research environment, the publication of research findings, and by the research team?s participation in international research conferences. Finally, the research team will develop high-school appropriate tutorials on the topic of photocatalytic construction materials and will seek to make these publicly available through existing web portals at Georgia Tech.

Project Report

Today, with increasing global awareness and regulation of air pollution, interest in the smog-abating property of photocatalytic materials is increasing. Nanoparticles of titanium dioxide (TiO2) are perhaps the most well-known photocatalytic semiconductor, and their use as passive but potentially effective means to reduce atmospheric nitrogen oxides (NOx=NO+NO2) has been relatively recently introduced in construction materials, commercially sold as photocatalytic cements, photocatalytic pavements, self-cleaning tiles, and self-cleaning glass. Prior research has examined the photocatalytic properties of the TiO2 itself as well as TiO2-containing cement-based materials, and the majority of this effort has been on characterizing and enhancing the photocatalytic efficiency. However, relatively little research was performed to assess the potential impact of the photocatalytic reaction on the "parent" or "host" material. In this research, the focus is on the effect of addition of chemically inert TiO2 nanoparticles and the photocatalysis on the composition, structure, and properties of cementitious materials, which contain titania nanoparticles at early and late ages. With the addition of TiO2 nanoparticles, the rate of early cement hydration and the degree of hydration are increased, resulting in decreased setting time and increased compressive strength at lower water-to-cement ratio, but with decreased microhardness. It was shown from modeling that the high surface area of nanoparticles provides nucleation sites for hydration products to form, thus accelerating the rate of hydration through a boundary nucleation effect. These series of results suggest that the TiO2 nanoparticles could be used to optimize cementitious materials to achieve specific early age behavior as well as hardened properties, setting aside the photocatalytic benefit. Further, the accelerated hydration of the cementitious compound tricalcium silicate (C2S) implies a potential pathway to sustainable development by using C2S-rich cements that can be produced at lower temperatures while emitting less CO2 during manufacture. In addition, the photocatalytic efficiency and the effects of the TiO2 on the long-term durability of cement-based materials are investigated to demonstrate their suitability for long-term use in the field. The photocatalytic efficiency of the TiO2 containing cementitious material under NO and NO2 gases are similar at 3 hours of NOx and ultraviolet light exposure. However, the efficiency decreases with long-term NOx and ultraviolet light exposure and with wet-dry cycling, possibly due to carbonation and overgrowth of hydration products. Also, it was found that the NO2 gas has a greater potential to be bound in hardened cement paste than the NO gas, even in the absence of photocatalysis (e.g., without light exposure). Because the amount of NO2 bound is comparable to the amount decreased by photocatalytic reactions, this new observation suggests that the photocatalytic cement-based materials could be used to alleviate NO2 gas through both photocatalysis and binding within the cementitious matrix. Cycles of NOx and wet/dry exposure result in pits on the sample surfaces, as evidenced by scanning eletron microscopy images, suggesting that extensive NOx and wet/drying has a potential to generate surface damage of a cementitious materials. However, microhardness, surface roughness, and x-ray diffraction are found to be insensitive to these changes. In examining salt crystallization, it was found that calcium nitrate, a potential by-product of photocatalysis, could damage cementitious materials by salt crystallization pressure at low relative humidity. With regard to broader impacts, beyond those derived from the potential contributions of the research on sustainable and durable cementitious materials, this grant has provided full or partial support for two doctoral students (Ms. Bo Yeon Lee and Mr. Amal Jayapalan), two masters student (Ms. Sarah Fredrich and Ms. Eva Land), and two undergraduate research students (Ms. Sarah Fredrich and Mr. Dan Glass) at Georgia Tech; an additional two Georgia Tech undergraduate students (Mr. Chris Wenhao and Mr. Matt Treager) and three visiting scholars from Ecole Normale Superieure de Cachan, France have participated in this effort but were not funded. An additional undergraduate student from University of Texas at Austin, Ms. Melinda Jue, participated in this research effort with funding from NSF's NNIN (National Nanotechnology Infrastructure Network) undergraduate research program. Finally, the research topic has been included in various outreach activities and semi- and non-technical presentations by the PI and students involved in the research effort.

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