Abstract
Protective coatings, in recent years also from nanocomposite formulations, are commonly applied onto architectural stone and stone artefacts, mainly to prevent absorption of condensed water and dissolved atmospheric pollutants into the porous stone structure. While standard protocols to assess a coating’s performance are available, understanding the response of the coating-stone system is a complex task, due to the interplay of various factors determining the overall behaviour. Characterization techniques allowing one to correlate the extent and nature of surface modification upon treatment with the most relevant physical properties (i.e., water absorption and surface wettability) are thus of great interest. Electrokinetic analysis based on streaming current measurements, thanks to its sensitivity towards even minor changes in the surface chemical composition, may fulfil such requirement. Indeed, by involving the interaction with a testing aqueous electrolyte solution, this technique allows one to probe not only the outer surface, but also the outermost layer of the pore network, which plays a crucial role in the interaction of the stone with condensed atmospheric water. In this work, a correlation was found between the extent of surface modification, as determined by streaming current measurements, surface wettability and capillary water absorption, for three lithotypes with different mineralogical and microstructural properties treated with two nanocomposite formulations (one water based and one in alcoholic solvent) containing organosilica precursors and titania nanoparticles.
Highlights
The protection of natural stones used in historical architecture is generally carried out through the application of suitable water-repellent coatings, which prevent the capillary absorption of condensedAppl
Streaming current measurements were performed on the three untreated lithotypes at various pH within different ranges, depending on the chemical stability of each given stone, to obtain their respective isoelectric points (IEP, Table 4); for this purpose, titration curves were drawn by fitting the zeta potential values calculated from the streaming current data (Figure 2)
IEPs can be seen to reflect the mineralogical composition of the lithotypes under study quite closely
Summary
The protection of natural stones used in historical architecture is generally carried out through the application of suitable water-repellent coatings, which prevent the capillary absorption of condensedAppl. The protection of natural stones used in historical architecture is generally carried out through the application of suitable water-repellent coatings, which prevent the capillary absorption of condensed. Since the effectiveness of these coatings mostly relies on their ability to modify the interaction of the treated surface with water, understanding the extent of surface modification, both in terms of effective surface coverage and change of chemical properties, is a key issue for the prediction of protective efficacy [2]. The effects of a hydrophobic coating on the surface properties can be studied at the macroscopic level by evaluating the behaviour of the treated stone with respect to established physical parameters, among which capillary water absorption and surface wettability have the highest relevance.
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