Effect of solvents and pH on in situ formation of hydroxyapatite for stone conservation

In situ formation of hydroxyapatite (HAP) inside the cracks and on the surface of calcareous substrates (e.g. marble and limestone) has been proposed for consolidation and protection of cultural heritage. HAP can be formed in situ by reacting the substrate with an aqueous solution of diammonium hydrogen phosphate (DAP). Taking inspiration from literature on bioceramics and from recent computational results, in the present study different strategies were explored to improve the consolidating efficacy of the DAP solution, namely by increasing pH or by adding organic solvents. A 1 M DAP +1 mM CaCl2 solution was taken as a reference and compared to 10-fold more diluted solutions (0.1 M DAP + 0.1 mM CaCl2) at pH 8, 10 or 12 and with addition of 20 wt% ethanol, isopropanol, or acetone. The solutions were applied onto weathered marble specimens and the formation of the new phases was characterized by FEG-SEM and XRD, while the strengthening ability was assessed in terms of bulk mechanical properties and surface properties. The results of this study indicate that apatite phases (mostly likely carbonate HAP with possible stoichiometric HAP) were formed as the result of all treatments, except at pH 12 when undesired ammonium calcium phosphate hydrate was formed. In terms of strengthening efficacy, when in-depth consolidation is needed, the more concentrated DAP solutions are able to provide higher strengthening ability. When consolidation of the most superficial layer is desired, less concentrated solutions are able to provide satisfactory results, allowing for a substantial reduction in the use of DAP with economic and environmental advantages.