Study of innovative phosphate-based treatments for the conservation of Cultural Heritage

The use of diammonium hydrogen phosphate (DAP) solutions to promote the in situ formation of hydroxyapatite (HAP) is receiving increasing attention as an innovative approach for the consolidation of carbonate-based materials. This PhD research aimed at extending the performance evaluation of DAP-based consolidants to increasingly complex and representative systems, thus bridging laboratory experimentation with real-world scenarios. Four main objectives were identified. (1) Objective 1: to evaluate the influence of different application methods on the performance of DAP-based treatments. Laboratory tests compared traditional techniques such as brushing and poultice application with an innovative agar-gel delivery system. The study was extended to a real-case application on weathered marble busts from the Château de Fontainebleau (France). (2) Objective 2: to investigate the DAP effects on substrates altered by soluble salt contamination. After identifying a reliable salt quantification method, controlled amounts of salts were introduced into marble samples. These salts were found not to hinder HAP formation nor compromise mechanical performance. Laboratory results were further validated through a case study on a naturally weathered marble slab from the Père-Lachaise Cemetery, Paris. (3) Objective 3: to explore the consolidation effectiveness of DAP treatments on mineralogically complex substrates, such as lime mortars, further subjected to accelerated weathering. (4) Objective 4: to examine the interaction of DAP-treated substrates with microorganisms, both in terms of preventing potential biological susceptibility induced by DAP (by exposing samples outdoors for six months at the Monumental Cemetery in Bologna) and in terms of developing antimicrobial functionalities. To achieve this and actively prevent biological degradation, ionic substitutions into the HAP lattice were explored. Overall, this PhD research showed that the DAP-treatment demonstrated consistent performance and long-term durability even in challenging conditions, while also showing the potential for imparting additional antifouling and antibacterial functionalities beyond simple protection and consolidation.