Oil-fly ash and black crust formation in monuments

摘要

Over the period 1981-1986 many articles focussed heavily on the important role that carbonaceous oil-fly ash (COFA) plays in the formation of gypsum on the surface of urban limestone. The resulting mixture of calcite and gypsum crystals, helped along by the contribution of rain or humidity, forms a thin layer which grows on the stone surface as this is steadily eroded. The stone alteration that follows is usually known as black crust. The black colour derives simply from the intimate presence of carbonaceous (>= 60%) oil-fly ash. Other black powders have been found in stone deposition but are not usually associated with surface degradation~(1-5).