Oil-fly ash and black crust formation in monuments

Many scientists threw sharp evidence on the important role that carbonaceous oil-fly ash (COFA) plays in the gypsum formation at the surface of calcareous stones, an alteration that is usually named black crust. In the crusts they found, inside the calcite-gypsum mix of crystals, oil-fired carbonaceous particles consistently prevailing over other common particles (e. g., coal-fired ash). They claimed a marked catalytic behaviour in the humidity-assisted oxidation of atmospheric sulphur dioxide to gypsum, caused by the abundant carbon fraction and/or by the metal (such as vanadium, nickel, calcium and so on) oxides that COFA contains. The subsequent investigations, aimed to prove this behaviour, have nevertheless produced results somewhat inconclusive. Recent our investigations definetely demonstrated that: a) COFA is carrier of strong primary acidity (SPA) in the atmosphere, in the form of sulphuric acidity and hydrolyzable heavy metal ions; b) SPA is immediately transmitted to water on COFA impact with it We also found: c) the origin of the SPA formation in fly ash, d) the cause of its set up only in oil-fly ash and e) the serious alterations to which calcareous stone (e.g., marble and calcarenite) surfaces may undergo when contacted with COFA under relative humidity values > 40% and excluding both gaseous sulphur dioxide and liquid water. Thus, COFA has been shown to be responsible of serious damages to stones, whose black crusts are the signal in an advanced stage. Prevention is desirable, but the current law regulations govern particulate emissions in the atmosphere on the exclusive basis of the concentration in the fumes and, recently, of the particle size. As our studies show, there is urgent need to monitor and limit particle emission as a function of acidity and reactivity. Some proposals are presented.