Protection coatings for stone monuments and artefacts of cultural heritage made of calcitic materials

Abstract

The degradation of marble monuments and statues is an ever-growing concern due to increased industrialization, extensive urban development, and persisting environmental quality problems. Preservation of the built cultural heritage and artefacts necessitates the development of novel materials and methods in order to increase their resistivity against the detrimental impacts of atmospheric water and pollutants. Over the past few decades, numerous protective coatings have been introduced to ensure the integrity of cultural heritage and prevent their degradation by reducing the rates of building materials deterioration. Protective coatings designed for cultural items are generally expected to adhere to established restoration standards, including transparency, reversibility, compatibility with the surface, long-term durability, straightforward synthesis, cost-efficient maintenance, and non-toxicity. Among coatings most often used for the protection of calcareous stone, poly acrylates and nanoparticles of metal oxides play significant roles in conservation and restoration activities. Graphene derivatives, including graphene oxide (GO), have garnered significant attention as protective coatings. In this study, we have studied graphene oxide-based structures as potential coatings for historical monument protection. Specifically, the resistance to dissolution of Dionysos marble (DM) specimens (1.5 x 1.5 cm x cm) were treated with Polyacrylic acid, MW 2000 (PAA2000), Hydroxy ethylidene, -1-1 phosphonic acid, sodium salt (HEDP) solutions and with GO suspension. DM, consists mainly of calcite (>98% w/w). All compounds tested for the treatment of DM, possessed functional groups capable of interactions with calcitic marble surfaces. The specimens were equilibrated with the solutions and suspension as follows: 2-5x10^-5 mol/L for PAA and HEDP 2x10^-5-5x10^-4 % w/v GO suspensions in water. Equilibration was done by immersing DM test slabs in the solutions and the suspension in 50 mL vials, capped and rotated end over end to ensure homogeneity for 24 hours at 25⁰C. Past equilibration, the specimens were rinsed with triply distilled demineralized water and air dried. The samples were mounted into special holders in special reactors, allowing the flow of calcium carbonate unsaturated solutions (σ=0.89, pH 6.50) with a flow rate of 4.5 mL·h^-1 in contact with both specimen surfaces. From measurements of pH and calcium concentration at the outlet of the reactors, the rates of dissolution of the specimens for each treatment tested were calculated. It was found that the equilibration of the marble specimens with GO suspensions was the most efficient, yielding a dissolution rate of 70% lower in comparison with the respective of the untreated marble. PAA-treated specimens did also retard the rate of dissolution of marble but to a less extent (ca. 30%). HEDP treatment was ineffective in retarding the dissolution rate of the DM specimens, possibly because of the enhancement of the calcitic material solubility in the presence of HEDP or because of structural rearrangement of the adsorbed phosphonate species on DM calcitic grains.