Effects of protective treatments on particle deposition and colour variation in stone surfaces exposed to an urban environment

Because stone surfaces in outdoor environments are exposed to natural and human-induced physico-chemical agents, they are often treated with synthetic coatings to prevent their deterioration. To enable the best possible preservation of stone materials, it is crucial to examine the performances of those protective treatments. Many studies have been conducted to inspect the water-repellence properties of a variety of coatings, while their effects on airborne particle deposition are poorly established. In this work, the effects of the application of different protective treatments to marble surfaces exposed to an urban environment were investigated in relation to particle deposition and colour variation. Five commercial products for the preservation of stone buildings, including siloxanic, fluorinated and TiO2-based coatings, were selected and applied to Carrara marble samples, which were exposed outdoors in the historic centre of Florence (Italy). The morphology and the chemical composition of particles deposited on the samples surfaces after one year were investigated by combining scanning electron microscopy and energy dispersive X-ray microanalysis (SEM-EDX). Cluster analysis and principal component analysis were then applied to SEM-EDX data, to identify the main sources of the deposited particles and to assess the performance of the applied products. Moreover, changes in surface colour due to both the application of the products and the outdoor exposure were evaluated using spectrocolorimetry. Based on the experimental results, all the treated surfaces showed a lower proneness to particle deposition and colour alteration in comparison to an untreated surface. In particular, among the tested products, organosilicon-based and fluorinated acrylic polymers were identified as the best performing protective coatings. Additionally, the experimental and statistical methodology used for this study proved a suitable approach for the investigation of deposition phenomena on stone surfaces.