ORIGINAL ARTICLE Drying of salt-contaminated masonry: MRI laboratory monitoring Teresa Diaz Gonc ¸alves Æ Leo Pel Æ Jose ´ Delgado Rodrigues Received: 24 May 2006 / Accepted: 7 August 2006 / Published online: 2 December 2006 Ó Springer-Verlag 2006 Abstract Drying of masonry specimens was monitored by means of a two-dimensional (2D) magnetic reso- nance imaging (MRI) technique. The external surfaces stayed wet for longer if NaCl was present instead of pure water only. This corroborates many practical observa- tions 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 indi- cate 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 pat- terns occur if masonry composition varies. Keywords Magnetic resonance imaging  Drying  Porous materials  Salt decay  Ancient masonry Introduction Moist surfaces are recurrently found in salt-loaded walls. But, as recently stated by an experienced building pathologist concerning the common case of rising damp, ‘‘... the examination of two closely affiliated phenomena, salt damage and rising damp, leaves us with far less in the way of definitive intervention strat- egies than we might wish, primarily because our understanding of the phenomena is limited’’ (Harris 2001). It is therefore, essential to understand the mechanisms of salt-induced dampness. Hygroscopicity of soluble salts can explain some situations of patho- logic dampness. Hygroscopicity refers to the capability of the salts or salt solutions for absorbing moisture from the air. This happens if and when the environmental relative humidity (RH) is higher than the relative equilibrium humidity (RH eq ) of the salt or salt solution. In that case, a negative vapour pressure gradient exists between the salt-contaminated surface and the sur- rounding air. Hence, water vapour diffuses from the environment to the surface. In contrast, when the environmental RH is lower than the RH eq of the salt or salt solution, the vapour pressure gradient is positive. Therefore, water vapour diffuses in the opposite direction, that is to say, from the material to the envi- ronment. Hence, the salt-contaminated material dries. Electronic supplementary material Supplementary material is available in the online version of this article at http://dx.doi.org/ 10.1007/s00254-006-0461-4 and is accessible for authorized users. T. D. Gonc ¸alves (&)  J. D. Rodrigues National Laboratory of Civil Engineering, Lisbon, Portugal e-mail: teresag@lnec.pt L. Pel Department of Applied Physics, Technical University of Eindhoven, Eindhoven, The Netherlands 123 Environ Geol (2007) 52:293–302 DOI 10.1007/s00254-006-0461-4