Evaluation of the application conditions of artificial protection treatments on salt-laden limestones and marble

Soluble salts contaminating limestones and marbles used as building and artistic materials play a relevant role in the deterioration processes of the substrates. Although desalination operations are carried out prior to protection and/or consolidation, a certain amount of salt remains inside the stones. When a surface treatment is chosen, the evaluation of its compatibility with the residual saline content is therefore needed. In the present work, specimens of three lithotypes characterized by a very different porosity – Lecce stone and Ançã stone, both highly porous, and the less porous Gioia marble – were contaminated with salt and then treated with two protective products, the organic polydimethylsiloxane and the inorganic ammonium oxalate (NH4)2(COO)2·H2O. Aim of the research was to select the best application conditions of the two products on salt-laden stone specimens, investigating as well the dependence of the protective action on the procedures adopted to apply the products. The performance of different concentrations and contact times of the products was tested in the laboratory, paying special attention to the possible drawbacks due to the salt. The study was carried out applying different methods: colorimetric measurements before and after the application of the products; water absorption by capillarity to investigate variations in water-interaction features; SEM–EDS analyses to evaluate the distribution of products and salt on the substrates.