Effect of titanium dioxide nanoparticles on early age and long term properties of cementitious materials.

摘要

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 its 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 TiO 2 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 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 the latter part of the study, 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/UV 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 NO 2 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 SEM 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. A separate salt crystallization experiment indicates that calcium nitrate, the possible product of photocatalysis, could damage cementitious materials by salt crystallization pressure at low relative humidity.