Oxalate and oxamate derivatives: novel synthetic strategies in the design of materials for the restoration of marble and biomicrite limestone substrates

Air pollution, and in particular the presence of sulphur and nitrogen oxides, results in acidic rain determining the dissolution of calcite in marble and limestones. This degradation process induces evident roughness of the stone surfaces and the partial loss of carved details in artefacts. The formation of an artificial coating of calcium oxalate, obtained by treatment with ammonium oxalate, was proved to be a very promising technique for the protection of stone items. The functionalization of oxalic acid to give monoesters and monoamides (oxamates) allows tailoring the solubility of the relevant ammonium and calcium salts. In this context, theoretical calculations carried out at Density Functional Theory (DFT) level can help predicting the capability of the investigated compounds in interacting with the calcium carbonate lattice. We summarize here the experimental (X-ray diffraction, Mercury Intrusion Porosimetry, FT-MIR and FT-Raman spectroscopy, SEM) and theoretical (DFT PES and NBO analysis) investigations on the effects induced by the variation of the functionalized anions in oxalate and oxamate salts on the efficiency of the treatment of marble from the Cimitero Monumentale di Bonaria in Cagliari (Italy) and limestone samples from Cava Flore (Santa Caterina di Pittinuri, Oristano, Italy).