Mineralogical, microstructural and mechanical characterization of marble consolidated by ammonium phosphate after thermal ageing and salt contamination

Aqueous solutions of diammonium hydrogen phosphate (DAP) have been proposed for consolidation of weathered marble, but few studies have investigated the influence of contaminants in the substrate. The present study was aimed at assessing the effects of NaCl, introduced in controlled amounts into thermally aged marble samples, on the outcome of the DAP-based treatment, tested in two formulations. The effects of the salt presence on the treatment outcome were evaluated in terms of chemical-mineralogical (SEM-EDS, XRD), microstructural (FIB-SEM), mechanical properties (ultrasonic pulse velocity, abrasion resistance and compressive strength), and chromatic compatibility. The stability of the new phases formed after treatment was assessed by immersing the samples in water for 24 h. The results of the study indicate that, independently of the salt presence, carbonate hydroxyapatite is formed as the reaction product between the DAP solution and the marble substrate. So-formed carbonate hydroxyapatite yields a surface coating with a few micrometers thickness, which exhibits good resistance to immersion in water. Mechanical characterization showed that, for all the investigated salt concentrations, no decrease in the mechanical benefit occurs, compared to the salt-free condition. Therefore, the presence of NaCl inside the marble substrate was shown not to negatively affect the outcomes of the DAP-based treatment.