In the field of stone protection, the introduction of inorganic nanoparticles, such as TiO₂, ZnO, and Ag in polymeric blends can enhance the protective action of pristine treatments, as well as confer additional properties (photocatalytic, antifouling, and antibacterial). In the framework of the “Nano-Cathedral” European project, nanostructured photocatalytic protective treatments were formulated by using different TiO₂ nanoparticles, solvents, and silane/siloxane systems in the blends. The results about the characterization and application of two promising nano-TiO₂ based products applied on Apuan marble and Ajarte limestone are here reported, aiming at investigating the complex system “treatment/stone-substrate”. The nanocomposites show better performances when compared to a commercial reference siloxane based protective treatment, resulting in different performances once applied on different carbonatic substrates, with very low and high open porosity, confirming the necessity of correlating precisely the characteristics of the stone material to those of the protective formulations. In particular, the TiO₂ photocatalytic behavior is strictly linked to the amount of available nanoparticles and to the active surface area. The alkyl silane oligomers of the water-based formulation have a good penetration into the microstructure of Ajarte limestone, whereas the solvent-based and small size monomeric formulation shows better results for Apuan marble, granting a good coverage of the pores. The encouraging results obtained so far in lab will be confirmed by monitoring tests aiming at assessing the effectiveness of the treatments applied in pilot sites of historical Gothic Cathedrals.

The development of nanocomposites combining photocatalytic, antifouling and protective features has provided interesting and promising results in the last years. However, few data about the behaviour of the nanomaterials applied on stone surfaces are available in the literature. In the framework of the EU-Horizon 2020 project "Nano-Cathedral", nanostructured protective treatments have been designed with different nanoparticles (TiO₂, Ag, ZnO), solvents and silane/siloxane-based polymeric matrices. The innovative formulations have been applied on 6 lithotypes, selected among the stones used in five medieval cathedrals (Vitoria-Gasteiz, Ghent, Cologne, Vienna and Pisa) and a contemporary theatre (Oslo Opera House), which are emblematic of different European geological and environmental areas. The treated stone specimens have been fully characterized to evaluate the surface optical and morphological compatibility, the reduction of water absorption by capillarity, the change in wettability and water vapour permeability properties. The selected treatments fulfil all these requirements and exhibit good photocatalytic and antifouling properties once applied on stone specimens. Different accelerated ageing procedures have also been performed in order to evaluate the stability of the polymeric matrices in the presence of photoactive TiO₂.

The use of hybrid nanocoatings for the protection of natural stones has received increasing attention over the last years. However, the interaction of these materials with stones and, in particular, its modification resulting from the blending of nanoparticles and matrices, are yet little explored. In this work, the interaction of two nanocomposite coatings (based on alkylalkoxysilane matrices and TiO₂ nanoparticles in water and 2-propanol) with two different porous stones is examined in detail by comparing their absorption behaviour and protection performance with those of the respective TiO₂-free matrices. It is shown that the protective effectiveness of both matrices is not negatively affected by the presence of TiO₂, as the desired water barrier effect is retained, while a significant photocatalytic activity is achieved. The addition of titania leads to a partial aggregation of the water-based matrix and accordingly reduces the product penetration into stones. On the positive side, a chemical interaction between titania and this matrix is observed, probably resulting in a greater stability of nanoparticles inside the protective coating. Moreover, although an effect of TiO₂ on the chemical stability of matrices is observed upon UV light exposure, the protective performance of coatings is substantially maintained after ageing, while the interaction between matrices and nanoparticles results in a good retention of the latter upon in-lab simulated rain wash-out.

During the last ten years, photocatalytic nanocomposites combining titania nanoparticles with silicon-based matrices have received increasing attention in the stone conservation research field, because they offer an effective multifunctional approach to the issue of stone protection. However, much work still has to be done in studying the behaviour of these nanocomposites in real environmental conditions and understanding to what extent they are able to retain their effectiveness and compatibility once applied on outdoor surfaces. The latter is a key information that should lie at the basis of any successful conservation and maintenance campaign. The present study provides insight into this relevant topic trough laboratory testing by assessing the artificial ageing of two silane-based photocatalytic nanocomposites, previously selected through an accurate testing on different natural stones. Three accelerated ageing procedures, based on artificial solar irradiation, heating and rain wash-out, allowed simulating about two years of outdoor exposure to some of the weathering factors to which stones are normally subjected. The results provided quite accurate information about the long-term behaviour of the products and on the role that the stone properties play therein. It was shown that, when the products are able to penetrate deeply enough inside the stone pores, they retain much of their hydrophobising and photocatalytic properties and maintain a good compatibility with the stone substrates, even after partial chemical degradation of the alkyl-silica matrices has occurred on the very stone surface.

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.

The characterization of protective coatings applied on natural stones is often a complex task due to the difficulty of identifying and quantifying the various factors contributing to the overall behaviour of the coating-stone system. In particular, linking information about the coating-stone interaction to macroscopic effects in terms of physical behaviour of treated stones can be especially arduous owing to the inherent structural complexity of stone substrates. Electrokinetic analysis based on streaming current measurements, having already proved the ability to sense even minor changes in the chemical composition of different materials surfaces upon treatment, may provide useful insights in view of better understanding the extent of stone surface modification. In particular, involving the interaction of stones with a water-based solution, the streaming current technique could extend the characterization of stone surface to the outermost part of the pore network, which is part of every treatment-induced modification of surface properties. In this work, the effectiveness of streaming current measurements as analytical tool for the characterization of coatings applied on natural stones is assessed by considering different lithotypes and coatings and trying to correlate the results of electrokinetic analysis with the physical behaviour of treated stones, with specific regard to wettability and capillary water absorption.