On the Different Strategies to Reduce the Decay of Stone-Built Cultural Heritage

Deterioration phenomena of ancient and modern stone cultural heritage are natural and unrestrainable decay processes mainly arising from water percolation into stone building materials [1-11]. Thus, the application of hydrophobic/superhydrophobic coatings to stone surfaces is mandatory to protect them from the deleterious effects of water exposition. Here, in order to avoid both natural and artificial stone decay, mainly due to the interaction with atmospheric pollutants (both gases such as NOx and SO2 and particulate matter) i) commercially available Si-based resins have been applied as protective agents [3,8,10,11], ii) new polymeric coatings have been synthesized with satisfactory water repellency and improved durability, thanks to the combined use of fluorinated and long alkyl chain monomers and without the use of any photo-stabilizers agents [2,4,5,6,7] and finally iii) hybrid coatings containing home-made TiO2 and SiO2 nanoparticles mixed with the commercial silane polymers have been tested to obtain complete buoyancy and self-cleaning properties [1,9]. Both natural stone materials (Carrara, Botticino, Candoglia marbles and Angera, Vicenza stones) and mortars (an Air hardening calcic lime mortar, ALM, and a natural hydraulic lime mortar, HLM) have been used as substrate to be treated. The results concerning the physico-chemical characteristics of the bare stones compared with those of the treated samples will be presented and discussed. In order to evaluate the stability of the applied coatings towards degradation induced by solar radiation and interaction with the atmospheric pollution, accelerated ageing tests under UV irradiation (also exposing the samples in a Q-UV tester cabinet) and exposure tests in a typical polluted urban environment have been carried out. For the samples characterization the following analyses have been performed: contact angle measurements, SEM-EDS (Scanning Electron Microscopy with X-ray microanalysis), IC (Ion Chromatography), colorimetric tests by DRS (Diffuse Reflectance Spectroscopy) followed by CIELab elaboration, porosity measurements, water absorption by capillarity and water vapor permeability. The properties of the home-made and commercial hydrophobizing polymers in terms of macromolecular structure, molecular weights, thermal features, and water repellency were determined. Furthermore, the long-term behavior of these polymers was estimated by means of accelerated aging tests exploiting UV radiations. Their behavior over time was checked via Size Exclusion Chromatography (SEC) by evaluating Mn and D data of aged polymeric samples and by Fourier Transform Infrared (FT-IR) spectroscopy: all the synthesized polymers seem to be unaffected by UV aging. Thus, the present stable resins were applied on both natural (marble) and artificial (mortar) stone substrates and their wetting properties together with their absorption by capillarity and water vapour permeability were successfully assessed and compared, showing potential features for cultural heritage protection.