Abstract
Drying of masonry specimens was monitored by means of a two-dimensional (2D) magnetic resonance imaging (MRI) technique. The external surfaces stayed wet for longer if NaCl was present instead of pure water only. This corroborates many practical observations that salts aggravate dampness in masonry. A slower evaporation process and not hygroscopicity was the cause. That suggests that salt-induced dampness may, in general, arise simply from changes in the drying process of masonry materials. That also implies that the height and depth at which crystallization occurs in walls may depend on the relative equilibrium humidity (RHeq) and other properties of salts that influence drying of porous materials. Evaporation rates of free surfaces of pure water and saturated NaCl solution were measured by a gravimetric technique. The results indicate that slow drying of salt-contaminated materials is not due only to the lower RHeq of salt solutions. The effective surface of evaporation is likely to be reduced perhaps due to blocking of pores by salt crystals. Final salt-distribution maps of the specimens show that: (a) salts may affect the inner materials of the masonry, even in evaporation-induced processes that lead crystallization to occur predominantly on the external surface; (b) distinct internal distribution patterns occur if masonry composition varies.
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Acknowledgments
MRI drying experiments were performed at the Centre for Material Research with Magnetic Resonance of Eindhoven University of Technology (TUE). The remaining work was done at the National Laboratory of Civil Engineering (LNEC), in Lisbon. We are thankful to LNEC for sponsoring the two stays of T. Diaz Gonçalves at TUE and the Calouste Gulbenkian Foundation in Lisbon for co-sponsoring the first of these stays. We are indebted to Henk Huinink for his continued cooperation and for the useful discussions we had. Thanks to Kristina Terheiden for helping reviewing this paper. We are also grateful to the referee of this paper for his/her quick answer and useful comments, particularly for drawing our attention to the need of discussing the influence of the boundary conditions in the HMC measurements. We also thank LNEC technicians João Júnior, who made the specimens, and Luís Nunes, who carried out the MIP measurements. We are also thankful to Angela O’Driscoll for the valuable editorial suggestions.
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Gonçalves, T.D., Pel, L. & Rodrigues, J.D. Drying of salt-contaminated masonry: MRI laboratory monitoring. Environ Geol 52, 293–302 (2007). https://doi.org/10.1007/s00254-006-0461-4
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DOI: https://doi.org/10.1007/s00254-006-0461-4