Protocol for assessing the effectiveness of protective coatings for roofing slate

Abstract

Spain is a world-wide leader in roofing slate production, quarriying more than 600,000 tons of slate of great quality and generating around 300 euros million in sales each year. However, an enormous quantity of slate plates is considered as a low quality product or discarded every year as waste. The application of protective products on roofing slate tiles helps to commercialise slate with higher oxidation rates, reducing wastes and environmental problems. The present protocol serves to evaluate the new protective products that are now used by slate producers. A combination of three technological tests is proposed here, along with a visual questionnaire to grant quality indices. Each test is oriented to clarify critical properties for the future use of the roofing slate, as follows: (i) Thermal cycles were used to determine the oxidation rate of iron sulphides; (ii) Slate behaviour in acid urban atmospheres was interpreted by exposition of slate tiles to SO2 gas; (iii) Effectiveness of the protective layer under saline corrosion and solar radiation was obtained by exposition to saline fog and UV-irradiation. Physico-chemical tests have been performed in the Technological Centre of the Slate (Orense, Spain) whereas the chemical-structural characterizations of natural, impregnated and altered slate plates were carried out by X-ray diffraction and optical and electronic microscopy in the University of Santiago de Compostela (NW Spain). The quantitative analyses of the alteration grades have been determined using a freeware program (IMAGEJ) on the scanned images of roofing slate tiles. The protocol here presented has been experienced with the more important protective slate products nowadays, i.e., siloxanes, organic resins and polyurethanes.