Abstract
Dust and powder fall in medieval churches in coastal regions has been observed. Cores from borings in nine churches have been analysed and the influence of sodium chloride on thermal expansion and hygroscopic shrinkage has been tested. NaCl forms the main part of the water-soluble salts in the investigated medieval bricks and is found in the form of very small crystals in pores smaller than 15 μm. Pore size distribution is determined and shows that medieval bricks have pores in the hygroscopic range. It is demonstrated that thermal expansion and hygroscopic shrinkage increase with an increase in NaCl content, and a model to determine the relative thermal expansion is proposed. The dust and powder fall seems to be at a peak at relative humidities around 75%, the crystallization point of NaCl.
Resume
On a observé dans les régions côtières des chutes de poussière et de poudre dans les églises du Moyen-Age. On a analysé des échantillons prélevés dans 9 églises et on a étudié l'influence du chlorure de sodium (NaCl) sur la dilatation thermique et le retrait hygroscopique. Le chlorure de sodium constitute la partie essentille des sels solubles à l'eau dans les briques médiévales examinées, et se rencontre sous la forme de très petits cristaux dont les pores sont inférieurs à 15 μm. On détermine la distribution dimensionnelle des pores. Celle-ci montre que les pores des briques médiévales se situent dans un intervalle hygroscopique. On démontre que la dilatation thermique et le retrait hygroscopique augmentent avec une teneur accrue de NaCl et on propose un modèle de détermination de la dilatation thermique relative. La chute de poussière et de poudre semble à son maximum à des humidités relatives avoisinant 75%, point de cristallisation du NaCl.
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Larsen, E.S., Nielsen, C.B. Decay of bricks due to salt. Materials and Structures 23, 16–25 (1990). https://doi.org/10.1007/BF02472994
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DOI: https://doi.org/10.1007/BF02472994