Fundamental Understanding of NOx Sequestration Capacity And Pathways in Nano-TiO2 Engineered Cementitious Materials
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The ubiquity of concrete in the urban environment and the upscaling of nanomaterial production have prompted interest in the incorporation of titania (TiO2) nanoparticles into cementitious materials. Air purification by TiO2-based cementitious materials occurs by photocatalysts that capture nitrogen oxide species (NOx) from the atmosphere, then oxidizing them into nitrite and nitrate species. Because nitrite- and nitrate-based corrosion inhibitors are effective in improving corrosion resistance in reinforced concrete, there is potential to develop nano-TiO2 engineered cementitious materials that transform atmospheric NOx into corrosion inhibitors. To provide guidelines for engineers and scientists to design such materials, a fundamental understanding of the NOx sequestration capacity and pathways in cementitious materials is needed. This dissertation first examines the effects of TiO2 nanoparticles on cement hydration. The inclusion of TiO2 nanoparticles accelerates the early age hydration of TiO2-modified OPC pastes due to nucleation and growth effects induced by the addition of TiO2. However, TiO2 retards the early age hydration of CAC samples possibly due to the presence of sulfate ions on the surface of the TiO2 nanoparticles. TGA and XRD results reveals that the early age hydration, which is affected by TiO2 inclusion, does not affect the hydrated cementitious phase composition or proportionality after 28 days of curing. The SEM/EDS analyses show that the TiO2 nanoparticles are uniformly distributed in the cementitious hydrates and exhibit no preference to binding with any particular hydrated phases, ensuring a consistent photocatalytic performance of the TiO2-modified cementitious materials. The photocatalytic performances of both TiO2-doped and TiO2-coated cementitious materials are examined in this research. Ordinary portland cement (OPC) samples exhibit higher NOx and methylene blue photodegradation efficiencies than calcium aluminate cement (CAC) samples. The difference indicates that different NOx sequestration pathways occurred in these cements and are likely due to differences in chemical composition and hydrated cementitious phases. For TiO2-coated cementitious materials, the inclusion of a hydrophobic SiO2 layer improves the bonding between the TiO2 coating and the cementitious substrates but compromises the photocatalytic efficiency. Therefore, it is important to consider the bond strength, the desired interactions between the coated surface and water, and the photocatalytic performance when selecting and designing TiO2-coated cementitious materials. This research also develops a novel experimental approach that combines water-based wet chemical extraction, UV-visible spectrophotometry, and ion chromatography to quantify the NOx sequestration capacity in both plain and TiO2-modified cementitious pastes. Compared to plain cement pastes, TiO2-modified cement pastes exhibit higher NOx uptake (in terms of nitrite and nitrate detected in the material) due to the activation of photocatalytic reactions, a greater surface area, and an increased amount of micropores from the addition of TiO2. The detection of nitrite and nitrate ions in unmodified cement paste demonstrate these cementitious materials’ intrinsic NOx sequestration capacities, which are related to the surface-related catalyzed heterogeneous reactions and the alkaline environment.