Hydrophobic and self-cleaning surface treatments for the protection of ”Pietra Leccese” and ”Carparo” limestones

In the present work we present the study of a newly formulated hydrophobic coating based on long-chain siloxane compounds and nano-sized titanium dioxide, for the conservation of two types limestone, the Lecce and the Carparo stone, widespread in the Apulian region, Italy. ”Pietra Leccese” is a marly biocalcarenite aged between Middle and Upper Miocene, consisting of 93-95% of calcium carbonate forming bioclasts and intraclasts embedded in calcitic cement. The use of Pietra Leccese was particularly widespread during the Baroque period. Carparo is a limestone dating back to the late Pleistocene. It is yellowish in color consisting mainly of coarse fossil fragments. It was typically used in Salento in the architectural field since ancient times as a more resistant material than ” Pietra Leccese ”. The main forms of degradation of these stones are due to their high porosity which facilitates to chemical and physical phenomena such as capillary rise, salt crystallization, atmospheric pollution with the formation of black crusts. The stones were first coated with an organic / inorganic hydrophobic treatment based on tetraethyl orthosilicate (TEOS), polydimethylsiloxane (PDMS) and / or polymethyl methacrylate (PMMA) and ZrO2. Subsequently a second coating based on selenium doped titanium dioxide (TiO2/Se) was carried out. The inorganic phases are homogeneously dispersed in the organic matrices, as confirmed by FTIR and Raman spectroscopies. TiO2 nanoparticles are anatase phase with a small contribution of brookite phase as confirmed by XRD and are rod-shaped 10-15 nm in length and 3-5 nm in diameter as observed from TEM measurements