Study of nitrate contaminated samples from a historic building with the hygroscopic moisture content method: Contribution of laboratory data to interpret results practical significance

Abstract

Abstract The assessment of moisture and soluble salt sources affecting the masonry materials of a historic building located in the Old Town of Prague triggered the present experimental research focused on the use of the hygroscopic moisture content (HMC) method for the evaluation of nitrate salts in building materials. The historic building under study had been recently subjected to a rehabilitation intervention that encompassed the injection of a damp-proofing chemical at the ground level and the application of renovation mortar coats. The applied renovation plasters and renders showed striking moist stains within 6 months after their application. To unveil the reason behind the moist-related problems observed, several samples of repair plasters and renders, as well as of the underlying historic masonry materials, were collected from representative affected areas of the building. The HMC method combined with ion chromatography (IC) was used to investigate the nature and amount of soluble salts in the specimens. Nitrates were generally found in a high content and located at high levels (ca. 3 m height) in the walls. The IC results and the generalised surface wetting observed on-site led us to infer that nitrocalcite was one of the main salts present. The results obtained instigated a laboratory study with the HMC method to assess the hygroscopic behaviour of single nitrate salts and salt mixtures. The selection of single salts and the design of the salt mixtures was based on the IC data of the selected historical materials’ samples aiming at replicating their hygroscopic behaviour. The results of the case study indicated that the rehabilitation strategy implemented reduced the rising damp phenomena, but that the choice of the type of mortar coats may have contributed to drawing existing moisture (and salts) to higher levels in the walls. The evaluation of the hygroscopic behaviour of samples with the HMC method at a range of RH provided a better understanding of the on-site behaviour of the salt-contaminated materials in the building. The results of the laboratory study showed that the salt mixtures prepared simulated well the hygroscopic behaviour of the building samples.