Soluble Salts In Stone Artifacts: Reliable Quantification And Influence On Consolidation

The presence of soluble salts in historical architectural stones is a significant challenge for their conservation, as salt crystallization induces mechanical stress and cracking. The accurate determination of the amount of salts present in a stone is crucial for effective conservation strategies. While salt extraction and quantification are relatively straightforward in highly porous materials, they are more challenging in less porous ones, such as marble. Therefore, the goal of the present study was twofold: (1) to identify a reliable method for salt quantification in marble and (2) to evaluate whether a critical salt content exists, above which desalination is essential to ensure that consolidation of salt-contaminated marble by diammonium hydrogen phosphate (DAP) is successful. First, two different methods for salt extraction were compared, namely the European standards EN 16455:2014 for Cultural Heritage and EN 772-5:2016 for masonry elements. Carrara marble and Lecce stone were used as representative substrates with low and high porosity, respectively. Samples of the two substrates were artificially contaminated by immersion in aqueous solutions of NaCl with increasing concentration. The soluble salts were then extracted from the contaminated samples by the two methods and the salt amount was quantified by ion chromatography. Unlike the case of porous limestone, in the case of Carrara marble significant differences were found between the two extraction methods. Afterwards, the effects of consolidating salt-laden marble by diammonium hydrogen phosphate (DAP), aimed at forming hydroxyapatite (HAP), were investigated. The presence of salts when HAP is formed in situ is particularly relevant, as formation may be altered or even inhibited by the presence of foreign ions. XRD and SEM-EDS analyses showed that, even in the presence of high salt concentrations, the new phase formed after the consolidation is carbonated HAP, independently of the salt content. The presence of carbonate HAP was confirmed even after sample immersion in water, to test the stability of the new consolidating phases.