Conservation of calcareous stone monuments: Screening different diammonium phosphate based formulations for countering phototrophic colonization

Stone degradation is a complex process induced by chemical, physical and/or biological factors. This process was accelerated in the last 50 years, by the worsening of the air quality, which caused acidic rains and an increasing of particulate deposition associated to contamination by soluble salts. Consolidation of degraded stone monuments is among the most important and at the same critical conservation goals. It is aimed at assuring higher physical stability, and therefore durability, to the monument. The inorganic consolidants have a good chemical-physical-mineralogical affinity with the stone material, and di-ammonium phosphate (DAP) is one of the last generation water-soluble agent for the consolidation of the carbonate-calcareous stones. Nevertheless, because of its content in phosphor and nitrogen, DAP could favor biological growth in very special humid contexts. In order to counter this potential drawback different formulates based on DAP, used in conjunction with washing procedures with water or biocides, or by adding biocides directly to the DAP solution were tested in this experimental work. Two types of calcareous stones with different porosity were chosen for the experiments (Gioia marble and Gottardo stone). The best results were obtained when a mixture of diammonium phosphate and benzalkonium chloride (BAC) water solutions was applied.