Conservation of built heritage: preservation of challenging carbonate substrates by phosphate treatments

The present research aimed at: i) optimizing the ammonium phosphate-based (DAP) formulations for protection and consolidation on marble, assess ii) interaction of DAP with salt-contaminated substrates; iii) durability of DAP in comparison with a common consolidant for carbonate substrates, the nanolimes (NL); iv) performance of different DAP formulations on “fresco” mock-ups in comparison with NL. The formulation optimization was investigated by comparing a low concentrated solution with solvents addition and pH increasing to a more concentrated solution. The results indicated that the most concentrated solution led to significantly higher bulk mechanical properties, while the low concentration led to comparable or superior improvements for superficial layer consolidation, with potential economic and environmental advantages. The second aim compared DAP with NL on lime-mortars pre-contaminated with sodium sulfate. The results highlighted that DAP outperformed NL with salt contamination; DAP consolidating effectiveness increased and carbonated hydroxyapatite (HAP) formed. Moreover, lime-mortars durability was assessed by subjecting the samples to freeze-thaw and salt crystallization cycles. The DAP treatment showed significant improvements in comparison with NL, both immediately after consolidation and after aging. The DAP improved mechanical properties with the formation of new calcium phosphates (CaP). Finally, the most performing formulations were used for “fresco” mock-ups consolidation and compared with NL and NL pre-treated (as calcium source) DAP formulation to assess whether CaP might form without color and phase variation. HAP was found for the condition with NL pre-treatment that presented the highest mechanical properties improvements for bulk consolidation, even though the final layer might be optimized. In addition, the low concentrated DAP formulation led to highest superficial mechanical properties improvement, together with the best color preservation for all the pigments. To conclude, the research successfully achieved the proposed aims, providing valuable guidance for conservators and restorers in consciously choosing the most suitable consolidant for calcium-rich substrates.