Salt Decay of Sandstone
This project investigates the main factors involved in salt-induced weathering of building stone within an environmental context. These three factors have been identified as: sandstone type, cleaning regime and geographical location. These factors were first investigated by looking at the variation of related weathering indicators such as: porosity and pore size distribution, chemical analysis and superficial changes in laboratory and artificial weathering studies. Then, the results were correlated with those from field studies. In all cases, it can be proposed an index of importance with respect to conservation of the building heritage which suggests that geographical location and sandstone type dictate the main differences in weathering. This means that if the aim is to reduce salt-induced weathering of existing sandstone buildings then chemical cleaning can be safely applied at certain locations and for certain sandstone types.
Keywords: sandstone, salt weathering, stonecleaning, environment, porosity, chemical analysis, colour changes and building conservation
The richness of architectural heritage of Scotland is recognised throughout the world, yet that heritage is constantly under threat and in need of protection. As an initial step to provide an understanding of the "state of the art" in the field of heritage conservation a review has been produced which presents a summary of the work in salt weathering on masonry to date, and addresses the importance of certain environments (marine and urban) and masonry treatments (chemical cleaning) in the acceleration of decay mechanisms. Two conclusions can be drawn from this review; firstly, there is a need to investigate the effects of marine environments on sandstone buildings and the interactions of marine salts (chlorides and sulphates) with stone and secondly, the important effects of chemical cleaning of buildings on the formation of soluble salts (CaSO4.2H2O, Na2SO4, Na2CO3.H2O, NaF, KF).
Sandstone buildings and monuments are exposed to both external and internal weathering processes, the interaction of these processes will depend on the nature of the environment and the degree to which this is affected by both the features of the building and the location of the stone within the building. Indeed, through the normal processes of wetting/drying, heating/cooling and freezing/thawing, a natural breakdown of stone occurs. This is intensified by salt crystallisation, clay expansion and the internal processes involved in mineralogical conversion. The visual evidence of breakdown can be readily found in surface powdering, delamination and contour scaling (Webster, 1992; Namorado Rosa et al., 1993). Recent research by Maxwell (1992) has shown that these natural processes are actually exacerbated on cleaned structures. Many monuments and historic buildings are exposed to marine environments. In addition, many have undergone cleaning processes over the last several years, often by chemical treatments (Urquhart, 1994). There is a need now to monitor this heritage and to predict, through research, the consequences of chemical cleaning in marine environments. This review forms part of an investigation being conducted by the Masonry Conservation Research Group with sponsorship from Historic Scotland.
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