Soluble and non water soluble sulphates in damage layers on hydraulic mortars

The present work attempts to fill the gap in knowledge on the behaviour and durability towards atmospheric multipollutants of hydraulic mortars used as a construction material in buildings and monuments. Different types of ancient and recent hydraulic mortars were collected from the Arsenal in Venice. A summary of the characterization of the samples is presented, along with the identification of the formation products found on the surfaces due to the reaction between the mortars and atmospheric multipollutants. A series of analyses was performed by means of optical microscopy (OM), X-ray diffractometry (XRD), scanning electron microscopy (SEM-EDX), thermal analysis (DTA-TGA) and ion chromatography (IC). The results prove that sulphation processes occur on hydraulic mortars, leading to the formation of a gypsum layer on the external surface of the samples. In addition, non water soluble sulphates were quantified by calculating the difference between the water soluble sulphate measured by IC and the total sulphates measured, using a methodology specifically set up for this purpose. The resulting data reveal that the sulphation of hydraulic mortars is not limited to gypsum formation alone, but that secondary damage processes also occur.