In situ Synthesis and Characterizations of Bio-ceramic Based Hydroxyapatite in the Re-mineralization of Calcium-rich Matrices

The research develops hydroxyapatite (HAP) based, inorganic mineral system with improved properties for the consolidation – providing the binding mechanism to reestablish the loss of cohesion – of powdery porous building materials (calcareous stone, marble, cement) and decorated architectural surfaces (rock-art, wall paintings and mosaics) of archaeological, historic and artistic value. The interdisciplinary nature of this research focused at the interface of materials science, biotechnology and conservation science develops tailored sustainable conservation treatments, which consider fundamental aspects of substrate chemistry and structure to prescribe and metricate treatment protocols. The scientific approach exploits biomimetic principles to induce the formation of HAP crystals by triggering reactions between the Ca-rich substrates of wall paintings and ammonium phosphate precursors such as diammonium hydrogen phosphate (DAP, NH4)2HPO4), and ammonium dihydrogen also known as monoammonium dihydrogen phosphate (MAP, NH4H2PO4). Tests were carried out on simulated wall painting panels and on original archaeological substrates in situ. The consolidating effect, influence of the solution and conditions such as concentration and contact time on the extent of hydroxyapatite formation was evaluated through a series of structurally and compositionally sensitive analytics including scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, thermogravimetry, X-ray diffraction, micro-Raman Spectroscopy, mechanical and fluid-transport analyses. Thermodynamic modeling was also performed to evaluate the stable phase equilibria between CaCO3 and (NH4)2HPO4 / NH4H2PO4 solutions and interpret the HAP formation in aqueous solution. Calculations demonstrated hydroxyapatite precipitation in the systems consisting of CaCO3/(NH4)2HPO4. From a purely scientific perspective, the research develops new information to relate: (1) the chemical interactions, thermodynamics and kinetics of reactions between the phosphate reactants and CaCO3, the main component in the stone, plaster or mortar, at various pH levels to the formation of orthophosphates such as HAP and rate/extent of consolidation (2) the impact of HAP formation within the decohesive layers in terms of microstructure-linked, fluid-transport, mechanical, durability and optical properties and (3) the chemical interactions of the reactant precursors with different pigments present in the surface paint layers. Acquired data show the potential of this treatment for the consolidation of powdery calcium carbonate-based surfaces and their protection from weathering and deterioration induced by passage-of-time and environmental action linked effects. These efforts develop quantitative materials science-based structure/property relations to engineer practical, prescriptive consolidation solutions in relation to the reactant chemistry, the substrate properties and the treatment application method relevant to the materials