Characterization of newly-formed calcium phosphates stratigraphies crystallized on calcium carbonate substrates: a multi-methodological approach

The diammonium hydrogen phosphate [DAP, (NH4)2HPO4)] treatment is an effective and promising consolidating agent for carbonatic substrates (Matteini et al., 2011, 2013). The reaction with calcite [CC, CaCO3] at room temperature is non-stoichiometric and induces the formation of hydroxyapatite [HAP, Ca5(PO4)3(OH)] (Ni & Ratner, 2003) and other metastable phases (Possenti et al., 2016). Due to the complexity of the DAP reaction mechanism, several studies have been focused on the interaction between the consolidating product and the carbonatic substrate, in order to define the crystallization sequence and the identification of the formed calcium phosphates. We have recently been carried out a series of experiments on Carrara marble specimens treated by capillarity, paper poultice and immersion in DAP solutions with different molarities. The first experimental findings showed that the treatment involves the formation of a shell-like layer around the calcite grains, composed by a mixture of crystalline and amorphous calcium phosphates. These phases are arranged in layers, with different crystals size and morphology. Since the straightforward characterization of calcium phosphates mixture is challenging (Drouet, 2013; Karampas & Kontoyannis, 2013), we have developed and improved a new multi-analytical approach (based on the combination of X-ray diffraction techniques, vibrational spectroscopies and morphological-microchemical analyses). In addition, the reaction kinetics has been investigated (with a non-destructive protocol) with time-resolved experiments performed at the MCX beamline at Elettra (Trieste, Italy). Our first experimental results suggest a correlation between the specific crystalline phase to the morphology and the position of the newly-formed products of the shell structure. The reaction products and their position into the porous matrix govern the performance of the mineral-inorganic treatment. The obtained data of this study provide useful recommendation concerning the treatment methodologies in the conservation works (DAP molarities and treatment time).