Evaluation of compatibility and durability of a hydraulic lime-based plaster applied on brick wall masonry of historical buildings affected by rising damp phenomena

Abstract

In the framework of the CNR Cultural Property Project for the Safeguard of the Arsenale, it was decided to test some new macroporous plaster designated to resist salt crystallization in order to prevent crystallization phenomena that are damaging the brick wall masonry of Venetian historical buildings.To understand the influence of rising damp and salt crystallisation on the durability of a plaster, in-field experiments were carried out: cores at different heights and different depths were drilled before and after plaster application in order to obtain a vertical and horizontal distribution of moisture and soluble salts.The determination of moisture content was carried out by gravimetric method while the salt content was obtained by ion chromatography measurements of chlorides, nitrates and sulphates.From the first results artificial hydraulic lime-based plaster seems to have a better performance in reducing moisture and chloride content. If this trend is confirmed in the future samplings, as the composition of plaster is the same in both areas, the methodology of application will be the discriminating factor to obtain better performances.Notwithstanding the absence of any damp proof course the state of conservation of the plasters seems to be satisfying. This is probably due to the different position of the evaporation front in the thickness of a macroporous plaster in comparison with a traditional plaster. In fact, in the first case, the evaporation front, and, consequently, the area in which soluble salts are accumulated, is confined to the inner part of the plaster, close to the brick wall, while in a traditional plaster salts are generally scattered throughout the whole thickness, but tend to concentrate mainly near the external surface. Due to this behaviour the traditional hydraulic lime-based plaster shows a superficial exfoliation caused by the crystallisation of soluble salts in the sub-surface; on the contrary, in our case, the artificial hydraulic lime-based plaster does not show either presence of efflorescence or of crypto-florescence.