Communication Dans Un Congrès Année : 2024

Weathering of the tuffeau building materials under the microscope. Focus on gypsum

Résumé

Detailed multi-method examination of stone weathering features play a pivotal role in safeguarding historical buildings. Many landmark buildings in France including the famous Loire castles are built with tuffeau, a fine-grained turonian limestone indigenous to the Loire Valley. This stone, comprising calcite, quartz, opal, micas, and clay minerals, is primarily characterised by its considerable porosity, ca. 45%, and modest durability [1]. It frequently shows several alteration features like granular disintegration, flaking, black crust, scaling and especially spalling [2]. Two samples of tuffeau weathered by black crust were taken from a house located on the Notre-Dame-de-Recouvrance street in the town centre of Orléans (France) during restauration work. These samples (length: 5 cm, width: 2 cm, height: 1 cm) were studied by energy-dispersive X-ray spectrometry (EDS), electron backscattered diffraction (EBSD) and micro-XRF mapping. Additionally, isotopic analyses of S and O were performed on the black crust. The unaltered part of the sample consists of crystals of calcite, quartz, plagioclase, spherules of opal with occasional presence of larger (ca. 200 µm) mica flakes and (ca. 50 µm) ilmenite and monazite. When approaching the alteration zone on the surface, the plagioclase disappears while the amount of opal spherules increases. Calcite and opal spherules show curious patterns, one can often observe that calcite either was dissolving at the points of contact with opal or has grown later incorporating the spherules. This kind of mineral relationship is especially prevalent in the altered zone above 1.5 cm from the surface. This zone also appears to be more porous and opal spherules dominate near cracks. This is a zone of active dissolution and re-crystallization of the calcite under the influence of capillary water. An increasing amount of tiny gypsum grains appear when approaching the surface. The black crust is just a 0.5-1 mm surface zone. It consists of tiny (<2 µm) grains of gypsum and opal with single grains of barite and traces of Fe, Mg as well as Pb and P. The S and O isotopes indicate atmospheric pollution as main source of sulphates. The location of gypsum is completely different between the black crust pattern and the spalling pattern in the tuffeau deterioration. Gypsum is more concentrated at the surface in the case of black crust or flaking, and it is more concentrated in 1 cm depth in the case of spalling. The recrystallization processes as well as crystallization of the gypsum in the first 1.5cm from the surface of the tuffeau might constitute an initial trigger of spalling. Indeed, spalling is the main pattern of degradation of tuffeau and the precise mechanisms of tuffeau alteration still need to be elucidated, which is the aim of the ongoing DETRESSE project funded by the Centre-Val de Loire region. References: [1] Beck K. and Al-Mukhtar M. (2010) In: Geological Society, London, Special Publications, 331, 103-111. [2] Janvier-Badosa S. et al. (2015) Environ Earth Sci 74, 2209-2221. [3] Kloppmann W. et al (2011) Sci Total Environ 409:1658–1669

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hal-04621908 , version 1 (24-06-2024)

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  • HAL Id : hal-04621908 , version 1

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Maria Naumenko-Dèzes, Kevin Beck, Guillaume Wille, Wolfram Kloppmann. Weathering of the tuffeau building materials under the microscope. Focus on gypsum. European Mineralogical Conference 2024, Aug 2024, Dublin, Ireland. ⟨hal-04621908⟩

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