A comparative study of hygroscopic moisture content,electrical conductivity and ion chromatography for salt assessment in plasters of historical buildings

It has long been known that highly soluble salts or hydrate-forming salts can damage porous building material. There is a real challenge to provide salt assessment in compliance with good monuments diagnosis and restoration practices. The salts present in the masonry can be analysed qualitatively and quantitatively by means of several techniques. Nowadays, the standard analytical equipment as ion chromatography (IC) found in several laboratories is a reliable method for analysing a large variety of anions and cations in solutions (or building materials water extracts), in order to meet international standards and regulations. However, preliminary study and diagnosis of soluble salt present in monuments do not necessarily require such a precise (SD < 5%) and sensitive (ppm or ppb range) analyses. Some simple-to-use techniques, such as hygroscopic moisture content (HMC) and electrical conductivity (EC) of extracted salt solutions, are widely in use to provide indications on the presence of salts.This paper presents a comparison of salt assessment obtained by IC, HMC and EC methods, and provides the domain of their application to specific questions linked to the damaged building diagnosis and preservation.The comparison of the results obtained by IC, EC and HMC analyses highlights the limits of the EC method in providing quantitative information on soluble salt content. The EC technique seems to require some cautions when used for materials containing other high soluble phases such as portlandite. On the other hand, the HMC technique seems to be very useful in the field of historical masonry investigation since it can provide reliable semi-quantitative distribution of hygroscopic salts.