Interaction between sodium chloride and ammonium phosphate on Carrara marble: two laboratory approaches

The present paper aims at investigating the interaction between NaCl and diammonium hydrogen phosphate (DAP) solutions, in view of field consolidation of NaCl- contaminated substrates by DAP. Two different approaches were tested on marble: part of the specimens was treated by a single solution containing both NaCl and DAP (“NaCl+DAP” samples) and part of the specimens was subjected to a two- step treatment, consisting on preliminary contamination by NaCl and subsequent consolidation by DAP (“NaCl then DAP” samples). For each approach, different NaCl and DAP concentrations were tested. The new phases, formed after treatment, were characterized by FT-IR and SEM and their consolidating effect was assessed by ultrasonic measurements, in terms of increase in dynamic elastic modulus (Ed). To investigate the influence of the NaCl contamination on the durability of the consolidating treatments, consolidated specimens were repeatedly immersed in water, the solubilized ions were analyzed by ion chromatography and the possible decrease in consolidating efficacy was assessed by Ed. The results of the study point out that, following either approach, new calcium phosphate (CaP) phases were formed, identified by FT-IR as hydroxyapatite and/or octa-calcium phosphate. These new phases led to significant increases in Ed, which experienced some re- duction when the consolidated specimens were immersed in water. Nonetheless, the residual Ed was sensibly higher than in the untreated condition, indicating that a significant mechanical benefit was maintained. The “NaCl then DAP” samples, which resemble the actual situation in the field, seem to provide more reliable results than those obtained following the alternative approach. In fact, the combined treatment (“NaCl+DAP”) led to lower increases in Ed and less pronounced formation of new CaP, possibly because the presence of sodium and chloride ions in the DAP solution significantly altered the mechanisms of CaP formation.