Self-cleaning Performance Improvement And Deterioration Mechanisms Of Nano TiO₂ Hydrosol Modified Cementitious Materials

It is difficult to maintain buildings'aesthetic quality requirements in concrete structures exposed to the service environment for a long time.Comfort,mobility,energy conservation,and respect for the environment are the key points of future urban development.Nanometer photocatalysts are added to concrete to reducing the air pollution near buildings and the dirts on buildings'facade.In this work,low dosage of nano TiO₂ hydrosol was used to prepare photocatalytic self-cleaning cement-based materials to achieve the high degradation rate of organic pollutants.This study investigates the dispersion and photocatalytic activity of the nano TiO₂ particles in cement base material.The effects of the formation and degradation of cement hydration products on nano TiO₂particles'electronic structure have been studied.The photocatalytic self-cleaning mechanisms of nano TiO₂ hydrosol modified cement-based materials,and the self-cleaning performance decay in carbonization and pure water wetting and drying cycles are investigated.The main innovative achievements and conclusions are summarized as follows:(1)The presence of Al³⁺in the water environment improved the photocatalytic performance of TiO₂ hydrosol,while Na⁺,K⁺,Ca^2+,and Mg²⁺reduced its photocatalytic performance.The negative effect of bivalent ion on TiO₂ photocatalytic performance is greater than that of a monovalent ion.The effect of Al³⁺on the photodegradation rate of Rhodamine B may be related to the strong hydrolysis of Al³⁺in an aqueous solution.The photocatalytic process of TiO₂ in the presence of all ions followed the pseudo first order reaction kinetic model.The p H value decreased due to different cations,which increased the reaction kinetic constant.It proposed a new view on the relationship between the coagulation and photocatalytic performance of TiO₂ hydrosol in cationic hydrolysate electrolyte,indicating that nano TiO₂hydrosol can be used as an effective photocatalyst in the ionic environment.(2)Nano TiO₂ particles disperse stably in an aqueous solution in the form of hydrosol,but the synthesis temperature will affect the crystal morphology,stability,and hydrodynamic particle size of nano TiO₂ particles.Stable nano-dispersed anatase TiO₂ hydrosol was synthesized,and the influence of synthesis temperature on the physicochemical properties and stability of TiO₂ hydrosol was discussed.The photocatalytic and self-cleaning properties of the synthesized nano TiO₂ hydrosol on the surface of cement paste were characterized by rhodamine B degradated rate and NO_x conversion rate under UV irradiation.Compared with commercial nano TiO₂ powder(Degussa P-25),an extremely low amount of nano TiO₂hydrosol showed better photocatalytic and self-cleaning properties under the same experimental conditions.(3)The self-cleaning performance of photocatalytic cement slurry is related to the dispersion of nano TiO₂in the hardening matrix.The existence of TiO₂ hydrosol delayed the early hydration of hardened cement paste and improved the photocatalytic self-cleaning performance of hardened cement paste.Additional surface defects caused by TiO₂ during the cement hydration process are the main reasons for improving the self-cleaning performance of hardened cement slurry.The morphology and pore size distribution of hydration products also contribute to the surface electron capture effect to enhance the self-cleaning performance.A new mechanism for photocatalytic self-cleaning of hardened cement slurry with different concentrations of TiO₂ hydrosol enhanced by photocatalytic performance and cement hydration was proposed.(4)Carbonation of cement hydration products affects the microstructure and morphology and reduces decontamination capacity of photocatalytic cementitious materials.The carbonization products at different carbonization times not only had a significant impact on the bandgap energy and surface defect state of TiO₂-mortar prepared by nano TiO₂ hydrosol with different dosage,but also had a significant impact on the exposure area of nano TiO₂,and also had a particular impact on the photogenerated electrons and the recombination ability of holes.The photocatalytic degradation of Rhodamine B in TiO₂-mortar surface meets the pseudo-second-order kinetic model.A semi-quantitative evaluation method was established to characterizing the influences of the phase evolution of carbonized products on the self-cleaning performance of photocatalytic cement-based materials.(5)The dry-wet cycle of pure water affects the microstructure of cement mortar and the distribution of hydration products,making the self-cleaning capacity of photocatalytic mortar with low nano TiO₂ hydrosol content fluctuate regularly with the number of dry-wet cycles.However,it does not reduce its self-cleaning capacity below the expected value.The distribution area of nano TiO₂,cement hydration products,and carbonization products on cement mortar surface is correlated with the number of dry and wet cycles.The dry and wet alternations caused periodic changes in the exposed area and particle surface state of nano TiO₂on the mortar surface.In summary,this work analyzed the effect of the cation in the concrete pore solution on the photocatalytic activity of nano TiO₂ hydrosol.The effects of cement hydration and different degradation processes on the photocatalytic self-cleaning properties of nano TiO₂ hydrosol modified cement-based materials,and their mechanism were analyzed.The effects of cement-based materials on the dispersion and particle surface structure of nano TiO₂ hydrosol were revealed.This work can provide a theoretical basis and method guidance for applying nano TiO₂ hydrosol in photocatalytic self-cleaning cement-based materials.