Flos Tectorii degradation of mortars: An example of synergistic action between soluble salts and biodeteriogens

The definition of Flos Tectorii, originally suggested by F.S. Brancato in the 1980s’ (Brancato, 1986), refers to a unique form of deterioration identified on both aerial and hydraulic mortars used in the external walls of historic and contemporary buildings. It is clearly recognizable due to the development of peculiar concentric (occasionally sub-circular) macroscopic forms. At present, there is still a lack of information about the full interpretation of this phenomenon regarding the interaction between the different physicochemical and biological factors that lead to this particular type of degradation. The present study was aimed to examine whether the specific mineralogical and textural characteristics of plasters can promote or hinder the development of Flos Tectorii and, at the same time, to explore if the presence of biodeteriogens is involved in it. The analytical results seem to suggest that the chemical and mineralogical composition of historic mortars does not significantly influence the advance of this phenomenon. At the same time, the isolation of some Actinobacteria suggests that they could act as biodeteriogens in the presence of hygroscopic soluble salts, leading to selective intergranular decohesion of the affected material. Anyhow, the biodeteriogenic role of microorganisms needs to be further investigated.