New insights on conservation of marble artworks from computational chemistry

Abstract

Architectural elements made of marble are subject to deterioration when exposed outdoors. A promising route to prevent deterioration is treating marble with an aqueous solution of diammonium hydrogen phosphate (DAP) to form hydroxyapatite (HAP). In this way, dissolution of marble can be prevented (as the solubility of HAP is orders of magnitude lower than calcite’s solubility) and cracks can be sealed (as HAP creates new bridges between calcite grains). To improve the treatment performance, the addition of alcohol to the DAP solution has recently been investigated with encouraging results. Here, we combined computational chemistry and experimental tests to identify alternative organic solvents that can further increase the treatment efficacy. First, we used density functional theory and molecular dynamics to screen alternative solvents (water, ethanol, isopropanol, and acetone), comparing their binding affinity and competitive adsorption behaviour onto calcite. Then, we treated marble specimens with DAP solutions with the additives and characterized the resulting performance. The results of the study indicate that acetone binds to calcite less strongly than the other solvents and does not completely displace water from the surface, which is expected to be beneficial. Nonetheless, the coating formed using acetone showed similar composition, thickness, and protective ability as the coatings formed with the other solvents. In terms of consolidation, all solvents were effective in improving surface and bulk mechanical properties. Overall, the extent of interaction with the calcite surface does not seem to be a critical factor. This can be explained by considering that, in addition to the different binding affinity to the calcite surface, the solvents may weaken the hydration shell of the phosphate ions in the DAP solution, thus facilitating the formation of HAP. Based on our results, this latter aspect seems to be predominant. Additional computational studies are in progress to elucidate this aspect.