Large scale distribution of fly-ash particles inside weathering crusts on calcium carbonate substrates: some examples on French monuments

The presence of calcium sulfate (gypsum) superficial alteration crusts on calcium carbonate (calcite) substrates supposes sulfur participation within the chemical and mineralogical transformations. The most frequently evoked origin of this weathering crust formation lies in the action of atmospheric sulfur under the form of gaseous species, and of acid rains, microorganisms, etc., to be found in very polluted urban atmospheres or in cleaner rural areas. But, in addition, in the last 10 years, the scientific community has turned its attention to the role that industrial fly ash might play in sulfur and catalysts transportation, both necessary for the various sulfation reactions. In order to evaluate this fly ash contribution to the sulfated crusts formation on various substrates, the following were studied: 1) two types of fly ash industrial sources: coal and heavy fuel combustion in electrical power plants; 2) atmospheric aerosols fly ash content in one monument area; and 3) chemical and mineralogical nature of fly ash embedded in sulfated crusts. These studies, which were done at the Fontevraut Abbey, the Le Mans Cathedral, the Saint-Trophime Primatial in Arles, the Porte Noire in Besancon, the Porte des Allemands in Metz, and the statues of the National Assembly in Paris, have confirmed the constant presence of fly ash in these numerous alteration crusts. Their part in sulfur, calcium, and catalysts transportation and then in the sulfation reactions cannot be neglected. -- AATA