Salt transport in plaster/substrate layers

We have investigated how transport and accumulation of salt in a plaster depends on the underlying masonry material. To this end, moisture and sodium profiles have been measured non-destructively with a Nuclear Magnetic Resonance (NMR) technique during drying of plaster/substrate systems. The same plaster is applied on two substrates of which the pores are either an order of magnitude larger or smaller than those of the plaster. The moisture and salt transport and the salt accumulation differed significantly for these two systems. In a plaster/Bentheimer sandstone system (the pores of the plaster are smaller than those of the substrate) all salt is removed from the substrate and accumulates in the plaster. In a plaster/calcium-silicate brick system (the substrate has a considerable amount of pores that are smaller than those of the plaster) some salt crystallizes in the plaster layer, but a significant amount of salt remains within the substrate itself. The salt transport from substrate to plaster is quantified in terms of an efficiency number ɛ, which can be estimated from the pore-size distributions measured by mercury intrusion porosimetry.RésuméNous avons étudié de quelle manière le transport et l’accumulation de sel dans du plâtre dépend du matériau de construction sous-jacent. Pour ce faire, les évolutions de l’humidité et de la teneur en sodium ont été mesurées de manière non destructrice à l’aide de la résonance magnétique nucléaire lors du séchage de systèmes plâtre/substrat. Un plâtre identique est appliqué sur deux substrats dont les pores sont un ordre de grandeur plus gros ou plus petits que ceux du plâtre. Le transport de l’humidité et du sel mais aussi l’accumulation du sel diffèrent de manière significative dans chacun de ces deux dispositifs. Dans un système plâtre/grès de Bentheimer (où les pores du plâtre sont plus petits que ceux du substrat), le sel a complètement disparu du substrat et se concentre dans le plâtre. Dans un système plâtre/brique de calcium-silicate (où le substrat possède un nombre significatif de pores qui sont plus petits que ceux du plâtre), une certaine fraction du sel cristallise dans la couche de plâtre mais la majeure partie demeure localisée au sein du substrat. Le transport du sel du substrat vers le plâtre est quantifié en termes d’efficacité ɛ. Celle-ci peut être estimée à partir de la distribution des pores suivant leur taille que l’on mesure grâce à de la porosimétrie par intrusion de mercure.

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