Building stones durability by UVA radiation, moisture and spray accelerated weathering

Abstract

Photodegradation is the alteration of the materials due to exposure to sunlight. This phenomenon is evident in paint, ink, pharmaceuticals and polymers. In polymers, photodegradation is caused by ultraviolet radiation (300–400 nm), which activates the breaking of C-C bonds and the formation of hydroperoxides, thermolabile substances. Although to a lesser extent, photodegradation also affects more resistant materials such as building stones. In these, it causes a change of color and luminosity due to the oxidation of organic matter or to the change in the valence of ions as Fe2+, Mn2+. In outdoor weathering, photodegradation is associated with other environmental factors such as rain, moisture, temperature, and condensation, which, in conjunction with sunlight, significantly increase the decay of the natural stone. The following research illustrates an accelerated degradation test on building stone by the UVA light, moisture, spray accelerated weathering tester by reproducing the ASTMG154 Cycle 7. To simulate outdoor weathering, materials are exposed to alternating cycles of UV light and moisture at controlled temperatures. It simulates the effects of natural sunlight and artificial irradiance using special fluorescent UV lamps in the UVA, UVB, and UVC section of the electromagnetic spectrum. A series of physical/aesthetic parameters (e.g., CIE L*a*b* color, gloss, roughness, water vapor permeability) of a selected group of building stones from Portugal is evaluated. Weathering shows an increase of roughness and a sensitive decrease of gloss.