Drying of porous building materials possibly contaminated with soluble salts: summary and findings of the DRYMASS research project

Abstract

Moisture causes severe and recurring anomalies in the built heritage, but removing it from the solid and thick masonry walls of old constructions is not easy, especially when soluble salts are present. These salts crystallize during drying, may harm the materials and their aesthetics. Moreover, the salts themselves and the alterations they cause affect the underlying drying process through mechanisms that require clarification. The DRYMASS project aimed at improving the current understanding of drying and of how soluble salts can influence it. It also had the objective of verifying if drying can be accelerated by means of surface layers such as coatings, which would be of high technical interest for conservation. The work of the project involved evaporative drying tests on porous building materials contaminated with different salts. It included also a novel use of optical profilometry for monitoring salt decay processes, and NMR measurements to determine the crystallizing Na2SO4 phases. Main conclusions were that salts hinder drying because they reduce sorptivity and also, when compact efflorescence or a salt crust occurs, because these types of deposit obstruct vapour transport. It was also concluded that lime coatings can accelerate drying of several types of porous building material initially saturated with pure water. However, this effect may not manifest when salts are present because efflorescence morphology incorporates a component of chaotic uncertainty.