Aging of photoactive building envelope materials containing TIO2 nanopowers

This work is dedicated to the study of photocatalytic building envelope materials, in particular mortars modified with the addition of TiO2 and bituminous membranes coated with paint containing TiO2, and to the evaluation of their performance variations with aging. This is important in order to investigate the behavior of these materials during their service life and to assess the possible recovery of photoactivity if it is lost or decreased by surface deactivation due to aging. Before describing materials and performed tests (ch.3), a general overview about fundamentals and applications of TiO2-based photocatalysis in the field of building materials is presented (ch.1), together with an introduction on the impact of building envelope materials on the energy needs of buildings (ch.2). The materials photocatalytic efficiency was analyzed by rhodamine B photodegradation tests, before and after aging by atmospheric exposure: aged materials lost significantly their photoactivity, in particular during the first year of exposure, still aged samples subjected to artificial irradiation and periodical rinsing presented a significant recovery in photoactivity (ch.4). Further analyses were based on the study of the solar reflectance of materials, concentrating on NIR part of sunlight spectrum, which is related to the energetic performance: the higher the reflectance, the higher the reduction of heat gain during hot days, and consequently the decrease in energy needs related to air conditioning. Freshly prepared TiO2-containing materials presented higher values of reflectance with respect to correspondent non photoactive ones and maintained higher reflectance throughout their lifetime, but an unexpected effect was noticed, namely an increase in the NIR reflectance of aged samples with respect to new ones, despite the soot depositing on the surface in contact with the polluted atmosphere. In order to investigate this behavior, laboratory tests were performed on freshly prepared membranes, trying to reproduce this effect and find the responsible mechanism, which turned out to be related to the disappearance of CaCO3 and to its possible conversion in Ca(NO3)2 (ch.5).