Thermodynamic modeling was carried out using GEM-Selektor v.3.6 (GEMS) and the Cem-data18 database for a temperature of 25 °C and a pressure of 101 325 Pa. In order to simplify the modeling, we assume that only CA reacts within the CAC1 system at a water-to-solid ratio of 100 and 0.3 within the timeframe studied herein. The formation of crystalline katoite was excluded in all modeling. At a water-to-solid ratio of 100, amorphous Al(OH) 3 was selected as the representative aluminum hydrate phase, as it provides a better fit with our experimental results at low CA degree of reaction, although a comparison with microcrystalline Al(OH) 3 is also provided in supplementary information for higher CA degree of reaction (see Figure and S4). In these diluted conditions, no anion-containing AFm phases are expected to form for proton concentration up to 200 µmol, all acid systems could consequently be modeled. At a water-to-solid ratio of 0.3, microcrystalline Al(OH) 3 was selected based on the previously modeled hydrated CAC systems by Manninger et al. (see Figure ). Since, in these conditions, anion-containing AFm phases formation is expected and observed even at low proton concentrations, the HClO 4 system was not modeled, as no thermodynamic attributes are available, to the best of our knowledge, for monoperchlorate (i.e., a ClO - 4 containing AFm phase). All relevant thermodynamic attributes for hydroxylapatite modeling (i.e., molar volume, standard molar Gibbs energy of formation, enthalpy, entropy, and heat capacity) were sourced from Hemingway and Robie .