Six-year long monitoring of green bio-consolidants products applied to natural porous stones

Biorestoration, thanks to its affinity to green chemistry, has a large application in stone conservation, but most of the studies performed are focused on the use of the bacteria for bio-cleaning purposes. Few studies were conducted on bio-consolidants and their long-lasting properties. This research had the aim of experimenting with a multi-analytical approach to the effectiveness of green bio-consolidants products compared to conventional ones applied to natural stones. A six-year-long monitoring was applied (first tests were conducted in 2016). Pietra Colombina, which belongs to the same geological formation of the Pietra Gallina di Avesa, near Verona, was chosen and tested. This stone is a fine grained white limestone, sometimes milky, compact and homogeneous, more mechanically resistant than the Pietra Gallina Stone and Avesa Stone, more tender and richer in fossils. Samples were treated with ethyl silicate, Biominéralisant calcite (Biocal + Nutrical) and Nanorestore® consolidant products. Colourimetric analysis, Water Absorption evaluation (WA) by sponge test and Evanescent Field Dielectrometry (EFD) non-invasive techniques were adopted and combined with conventional methods, such as Drilling Test Measurement System (DRMS) in order to evaluate treatment performance. Results showed how the products efficiency change in time: the ethyl silicate treatment continues to keep an hydrophobic effect, while Biominéralisant and Nanorestore® effectiveness return at the initial state. Moreover, the study highlights how the EFD analysis, originally applied to frescoes and concrete building moisture control, is a valid tool for on-site monitoring of treatment performance. The potential of the method and its encouraging results may represent the crossing of a new technological and practical frontier for porous materials.