Recent advances on the use of phosphate treatments to provide heritage stones with anti-fouling ability

Formation of calcium phosphates (CaP) was proposed years ago to consolidate and protect stone substrates from dissolution in rain, thanks to the lower water solubility of CaP (especially hydroxyapatite) compared to calcite, and to enhance the mechanical properties of the stone. CaP can be formed in situ, by reacting the calcium-rich substrate with an aqueous solution of a phosphate precursor, typically diammonium hydrogen phosphate (DAP). Given the good protective ability provided by so-formed CaP coatings, the possibility to provide CaP coatings with anti-fouling ability is being investigated within the “SECURE-COATS” project, the first results of which are here presented. Two different strategies have been explored, namely (1) embedding antimicrobial metal nanoparticles (NPs) into CaP coatings, by suspending them in the DAP solution used to form CaP and (2) applying the NPs on already-formed CaP coatings. The results of the present study show that, in the first case, not enough NPs are available on the stone surface to provide effective anti-fouling activity. On the contrary, when the metal NPs were applied onto already-formed CaP coatings, they exerted effective anti-fouling action against two model bacterial strains. After accelerated ageing by dropping a simulated artificial rain onto functionalized stone samples, the anti-fouling ability was retained, thus indicating good durability of the coatings. However, in spite of the initial acceptable colour change right after treatment, the samples exhibited a marked colour change with time, mostly