Avaliação da alterabilidade de rochas ornamentais por ataque químico: uma abordagem com análise digital de imagens

Abstract

Ornamental stones have been present throughout human history and, since ancient civilizations, have been widely used in construction, sculptures, and decorative applications. However, their use without proper technical criteria still persists, highlighting the need for greater awareness regarding their properties and appropriate applications. Thus, this study aims to evaluate the chemical resistance of Branco Ceará and Verde Ubatuba granites when exposed to hydrochloric acid (HCl), potassium hydroxide (KOH), sodium hypochlorite (NaClO), and ammonium chloride (NH₄Cl) through chemical attack tests and digital image analysis. Surface alterations were assessed through visual inspection and quantified using the open-source software ImageJ by generating RGB channel histograms. The results indicate that Branco Ceará granite exhibited the highest chemical resistance, showing only minor alterations when exposed to HCl, while the remaining reagents did not significantly affect its surface. In contrast, Verde Ubatuba granite proved to be more susceptible to chemical attack, presenting noticeable surface modifications confirmed by variations in the histogram parameters. Potassium hydroxide, sodium hypochlorite, and ammonium chloride exhibited a low capacity to induce alterations, as they did not produce significant changes in the chromatic parameters, indicating limited interaction with the constituent minerals under the experimental conditions adopted. It was concluded that the mineralogical composition directly influenced the chemical resistance of the studied rocks, with Branco Ceará granite exhibiting minimal variation in RGB histogram parameters, maintaining mean values close to 205 and preserving its main peak at 234 even after HCl exposure. Conversely, Verde Ubatuba granite demonstrated greater susceptibility to chemical attack, evidenced by changes in pixel intensity distribution. Furthermore, digital image analysis using ImageJ proved to be an efficient tool for identifying and quantifying surface modifications, contributing to an objective assessment of the alterability of ornamental stones and providing support for their appropriate application in different environments.