Marble consolidation by ammonium phosphate (DAP): Do soluble salts in the substrate interfere with in situ apatite formation?

Abstract

The use of aqueous solutions of diammonium hydrogen phosphate (DAP) to form hydroxyapatite (HAP) in situ, on the surface and inside the pores and cracks in heritage building materials (e.g. stones, mortars, frescoes, stuccoes), is receiving increasing attention, because of the advantages that this method offers compared to alternative treatments. However, heritage substrates are often affected by the presence of soluble salts when conservation treatments are applied, which can interfere with the expected chemical reactions. Therefore, the present study aimed at assessing the effects of three types of salt (NaCl, NaNO₃ and Na₂SO₄) present in a reference type of substrate (marble) when treated with DAP solutions. Marble samples were preliminarily contaminated with increasing amounts of the three salts, to reach levels of contamination that can be regarded as low, medium and high according to existing standards. Then, the salt-contaminated samples were consolidated by treatment with a solution containing 1 M DAP+ 1 mM CaCl₂, applied by poultice. The possible interference of foreign ions deriving from salt dissolution on in situ HAP formation was investigated by XRD and SEM-EDS, aimed at assessing whether ionic substitutions in the HAP crystal occurred and/or foreign ions were adsorbed onto the new phases. The results of the study indicate that, independently of the initial salt content, HAP and/or carbonate HAP (containing CO₃^2- ions from the substrate and from the atmosphere) were formed. These new phases proved to be resistant to accelerated ageing, consisting in immersion in water for 24 h. Therefore, the presence of NaCl, NaNO₃ and Na₂SO₄ (even in high amounts, corresponding to high risk) was found not to negatively affect the outcome of the DAP treatment in the conditions investigated in this study.