Seasonal changes of moisture and salt content of historic porous limestone walls; obtained form surface measurements and depth profiles

Moisture has an important and essential role in the transportation of salts in masonry structures, therefore understanding the changes in moisture content provides valuable insights into the diagnostics of historic buildings, namely allowing the prediction of salt weathering. In this study, the thick walls of a fortress were studied. The Citadella is a landmark historic building in Budapest, that was built from Miocene porous limestone in the middle of the 19th century. Besides external cladding with porous limestone, the cores of the walls also contain volcanic tuffs. The walls are high (12-16 m) and their thickness is more than 1.5 m, which makes it difficult to record the moisture distribution. The sources of water are partly linked to direct precipitation on the wall surface or from capillary rise from pavement surfaces and from the soil. To assess the moisture distribution and salt content both on-site and laboratory analyses were performed during the dry summer period and wet autumn period. Not only the wall surfaces but also the subsurface zones and wall interiors were studied. Dry drillings were made to assess the in-depth moisture profile and salt content. Using an on-site moisture test it was possible to identify the moist and water-saturated zones of the ashlars and renders. Salt content and composition were measured on drilled dust samples and small samples obtained from the wall surface. Optical microscopy, XRF and XRD and Thermogravimetric analyses allowed the identification of salts. According to laboratory tests, the major salts responsible for the damage of external walls are gypsum, halite and hygroscopic nitric salts. The salt distribution within the depth shows seasonal variations. It was possible to detect the moisture distribution and salt content changes in depth and along vertical profiles. The results of the current research can be used to understand the moisture and salt distribution in thick natural stone walls and help in the diagnostics of historic structures and evaluate the salt weathering processes under various climate conditions.