Ammonium Phosphates as Consolidating Agents for Carbonatic Stone Materials Used in Architecture and Cultural Heritage: Preliminary Research

Consolidation of renders and plasters

“…Recently, more attention has been paid to the use of inorganic consolidating materials. An important advantage is that hydrophilic inorganic products are free of the problems caused by hydrophobic properties of organic-polymeric products [24] and are less susceptible to microbiological attack than organic compounds. Limewater was recommended in some literature of the 20th century [6] for consolidation of mural paintings, particularly for fixation of released surface paint layers.…”

“…The natural conversion of calcium carbonate to calcium oxalate led to experiments in Italy, using a treatment based on ammonium oxalate to protect stone and lime plaster against acid rain [22,23]. Later, ammonium phosphates were proposed for consolidation of calcareous substrates and tested on limestone [24,25,26] and mortars. The most important physicalchemical characteristics of common consolidation products are given in Table 6.1.1.…”

Phosphate-based treatments for conservation of stone

“…Similarly, when ethanol was added to the DAP solution, the calcium phosphate film was found to be mainly composed of OCP, but also in this case a beneficial effect was found, because ethanol addition prevents crack and pore formation [13]. On the contrary, when phases with solubility higher than calcite are formed (e.g., brushite), no positive outcome can be expected from the treatment, because any protective and consolidating effect will disappear after contact with rain [10].…”

“…pH values ranging between 5 and 11 have been explored [9][10][11]. While pH<8 was discarded (because soluble brushite forms in these conditions alongside HAP) [10], no practical benefit in terms of acid protection was obtained by increasing the pH>8 [11], so in the end not adjusted DAP solutions at pH around 8 are usually adopted.  concentration of the phosphate solution.…”

“…Alongside DAP, also ammonium dihydrogen phosphate (ADP) and ammonium phosphate (AP) have been investigated as precursors [9,10]. However, since the main difference between these phosphate salts is their speciation in PO 4 3-, HPO 4 2-and H 2 PO 4 -ions and since the relative amount of these species actually depends on the solution pH, the nature of the phosphate precursor is basically irrelevant, as long as the pH of the phosphate solution is controlled [9].…”