Consolidation of porous carbonate stones by an innovative phosphate treatment: mechanical strengthening and physical-microstructural compatibility in comparison with TEOS-based treatments

For preservation of stones used in Cultural Heritage, affected by weathering processes that threaten their cohesion and mechanical properties, the application of consolidants is a common practice. However, available consolidating products generally exhibit some drawbacks that hinder their performance, in terms of either mechanical efficacy, compatibility with the substrate and/or durability. Ethyl silicate is currently the most widely used among stone consolidants; nevertheless, its reduced efficacy on calcitic substrates, together with its temporary hydrophobicity, its tendency to crack and its common formulation with volatile organic solvent, make the research for alternative consolidants for carbonate stones necessary. In this paper, a recently proposed new consolidation treatment based on the formation of hydroxyapatite inside the stone was tested on two different porous carbonate stones (Globigerina Limestone and Giallo Terra di Siena), and compared with TEOS-based treatments, frequently used for the consolidation of these lithotypes. The results show that the hydroxyapatite treatment exhibits a good efficacy in terms of mechanical properties and, compared to TEOS, it causes less pronounced alterations in open porosity and water transport properties. This makes the new treatment a potentially valid alternative to TEOS, especially in those situations where the possible presence of water behind the consolidated layer (e.g. in case of rising damp, condensation or infiltration) might threaten the durability of the consolidation intervention.