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Rôles des apports atmosphériques solides et gazeux, et de la nature du substrat dans les altérations superficielles des monuments. Approche experimentale et modelisation

Objective

The objective is to characterize growth of gypsum on well characterized substrates and to model the mineralogical weathering on the surface of stones using thermodynamic computer codes.

Six varieties of rock were exposed for one year in various meteorological stations, more or less polluted, including Milan, and two types of stones, Berne sandstone and Jaumont limestone were found to have a high capability to accumulate a high content of sulphur when exposed in polluted open atmosphere. The interaction between these stones and atmosphere close to real temperature, relative humidity, sulphur dioxide and nitrogen dioxide levels in Milan is studied in a climatic chamber composed of 9 cells: 3 for the Berne sandstone (naked, covered with fly ash, and covered with microsoots), 3 for the Jaumont limestone, and 3 for an inert substrate (quartz fibers) in the same conditions. In each cell there are enough specimens for analysis after 3, 6, 9 and 12 months of exposure.

Fly ash and microsoots particles deposited on the surface of some specimens have been studied by analytical scanning electron microscopy (ASEM) and transmission electron microscopy (ATEM) to determine their morphology, granulometry, chemical composition etc. Bulk analysis for the major and trace components and micro-X-ray diffraction have been also performed. Water vapour adsorption has been measured on these particles.

These experiments have permited a better estimation of respective roles of the different atmospheric parameters in the superficial weathering of cultural items. An evaluation of potential catalytic power of fly ash and microsoots for the formation of gypsum crusts has been carried out. The gypsum formation occurs under the surface of both types of stones even in the absence of fly ash. However, the presence of fly-ash is required for the development of a gypsis black crust above the stone surface. The effect of microsoots is not so exactly determined.
Data (depth profiles of sulphur concentrations) will be obtained by accurate mineralogical and chemical study of specimens of six varieties of stones previously exposed during one year in various locations. The two specimens of stones choosen for the experimentation on the basis of their "reactivity" will be characterized chemically, mineralogically and physically.

The experimental growth of gypsum on the substrates, in conditions close to real atmospheric temperature, relative humidity, SO2 and NO2 levels. The eventual catalytic role of fly-ash and micro-soots will be evaluated by comparison of results in presence and in absence of these materials. The results of these will be evaluated by means of analytical electron microscopy, X-ray diffraction and bulk chemical analysis.

Thermodynamic modelling will be carried out of the observed or induced modifications, taking into account the chemical, mineralogical and physical caracteristics of the substrates and the dry contributions.

These experiments will permit a better estimation of the respective roles of the different atmospheric parameters in the superficial weathering of cultural items.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Université de Paris XII (Paris-Val de Marne)
Address
61 Avenue Du Général De Gaulle
94010 Créteil
France

Participants (3)

Centre National de la Recherche Scientifique
France
Address
1 Rue Blessig
67084 Strasbourg
Ecole Polytechnique Fédérale de Lausanne
Switzerland
Address
Mx-g Ecublens
1015 Lausanne
UNIVERSITY OF BOLOGNA
Italy
Address
Via Zamboni 67
40127 Bologna