Evaluation of the effect of TiO2 treatments on the reflectance to solar and luminous radiation of coating mortars.

• Autor(a) principal: Dantas, Sérgio Roberto Andrade
• Data de Publicação: 2021
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/3/3146/tde-07102021-111423/
• topic: Argamassa; Dióxido de titânio (Dispersão; Efeito; Tratamento; Avaliação); Fotocatálise; Mortar; Photocatalysis; Radiação ultravioleta (Deterioração); Reologia (Degradação; Processos); Revestimento de fachadas; Rheology (Degradation; Processes); Titanium dioxide (Dispersion; Effect; Treatment; Evaluation); Ultraviolet radiation (Deterioration; Facade coating)

As soon as a mortar is applied to a facade, a continuous degradation process begins and will last during its entire service life. At the worksite, the mechanisms for evaluating the properties of mortars, particularly their applicability, are usually carried out in a sensorial and tactile way. It is common practice for masons to increase the amount of water without any control, only using their experience and preference of mortar consistency. Based on empirical knowledge, such procedures induce many errors that impair the mortar quality and do not enable the measurement of any possible rheological parameters to quantify the material workability. For cement matrixes, the rheological properties of dispersions are governed by the microstructure of the system, and particle dispersion poses a challenge that needs to be addressed for the use of nanomaterials in those mixtures. In mortars with TiO2 addition, particle characteristics such as size and morphology can account for the different photocatalytic activities. Moreover, the surface roughness can affect how TiO2 particles can be activated by solar radiation due to the incidence of shadow caused by the different roughness scales existing on surfaces. The materials used on the facades do not respond only to the building\'s aesthetic condition but have an essential role in their thermal balance, where materials that present high reflectivity values reduce thermal gains and result in an efficient passive technique to reduce demand for electrical energy in the thermal conditioning of environments and improve indoor and outdoor thermal comfort. Furthermore, the increase in solar reflectance and thermal emittance on the facades can be a feasible strategy to help reduce urban heat islands. However, the constant contact of those surfaces with degradation agents can significantly diminish those benefits over time with the exposition to the corrosion caused by acids, accelerated wear, fouling due to soot particles, and the direct incidence of UV radiation that causes discoloration and the appearance of stains. As a result, the coatings become more opaque, affecting their thermal balance and visual aesthetic over time. In this context, using coatings treated with TiO2 helps in the maintenance of these coatings over time. It maintains the solar and lightness reflectance of the building facades, making them more efficient by maintaining their initial reflectance values over long periods. Thus, the study evaluated surfaces experimentally treated with TiO2 - powder and suspension forms - and compared them with white painted surfaces, observing the maintenance of the initial characteristics of reflectance to solar radiation and lightness reflectance of these surfaces, aiming at its low maintenance and better performance, showing itself as a possible alternative to painted mortars. Reflectance to solar radiation measured was according to ASTM E1918-16 and ANSI/ASHRAE-74 procedures. The measurements were carried using two Kipp & Zonen pyranometers sensors, Delft / Holland model CM3, with wavelengths range of 305 - 2800 nm and maximum radiation of 2000 W/m². The signal generated varied from 0 - 50 mV and the sensitivity from 0 - 35 ?V / W / m². A datalogger ALMEMO, model 2890-9 with a 6.5-digit display and RS232 output, was employed for data acquisition. While, to the lightness reflectance, a procedure based on the ASTM D2244-21 was adopted. The measurements were done using a Color Guide Sphere diffuse/8° spin spectrophotometer with geometric measurements of 45º/0º circular, with 400 - 700 nm measurement area, 20 nm spectral resolution, and photometric range 0 - 100%, 0.01 resolution. The illuminant D65 and observer with 10° and reproducibility < 0.02 ?E* were considered. Results indicated that painted specimens maintained their initial reflectance to solar radiation after three years of exposure ending with 73% of their index. The comparison between the painted specimens and specimens treated with TiO2 powder showed a difference between 28.25% regarding their reflectance to solar radiation. On the other hand, the comparison between the samples treated with TiO2 in powder and specimens treated with TiO2 in suspension form shows a difference of 8.5% in reflectance index to solar radiation, not indicating a preference since both showed the same effectiveness in mean after three years of exposure. Concerning the lightness, after thirty-seven months of the natural exposition, the white painted specimens had the best results than specimens with TiO2 suspensions and specimens with TiO2 in powder. Beyond, regarding the absorptivity, the white painted specimens could be considered with low absorptivity. In contrast, the specimens treated with TiO2 suspension and those treated with TiO2 powder could be medium absorptivity. About the aesthetic perception, the study concludes that there are no significant differences in the white painted specimens for the final user. In contrast, for the other specimens with TiO2 (powder and suspension), the final user can notice the color difference very well in three years of exposure. Therefore, this research aims to contribute to filling a gap concerning reflectivity studies of different treatments of TiO2 (powder and suspension) on mortar by making a comparative assessment with painted mortars and exposed to the urban environment in the long term, demonstrating not only the effectiveness of the photocatalytic process by measurements of reflectance to solar radiation and luminous reflectance but also the maintenance of those properties in the mortars over time.