NHL-based mortars in restoration: Frost-thaw and salt resistance testing methods towards a field related application

Renders, plasters and masonry mortars based on natural hydraulic lime (NHL) are very common materials in the alpine area of Tyrol as shown by numerous application fields in cultural heritage preservation building sites. A sufficient resistance to frost damage and the crystallization of building salts are the most important requirements in regard to a long-term durability in critical environments. Up to now, the literature provides different types of frost-thaw and salt resistance testing methods, but not perspicuously tailored for NHL- or lime-based mortars. Hence, the present study aimed to assess cyclic frost-thaw and salt exposing test methods concerning the long-term durability of NHL-based mortar materials. Furthermore, we intended to relate the testing procedures to application fields occurring on building sites under consideration of the exposure conditions occurring in the alpine region. In order to cover a broad spectrum of possible applications, we operated with several testing methods mentioned in the literature, working with varying test setups. Moreover, we applied interstitial test procedures, which have been partly adapted and aligned in some details, to close the gap between the different methods. Regarding the investigated materials, we focused on different mortars actually applied on historic objects in Tyrol. Thus, the comparison with the effective properties occurring in the specific application field of a building becomes facilitated. Within the frame of the current study, it was figured out that it is reasonable to distinguish between dynamic and static testing setups. In addition, it was observed that it is required to confine the use of the materials under question in consideration of a static or dynamic application area at the building object. If a general assessment is required, we recommend a combined test consisting of both methodical approaches, operating with the equal frost-thaw cycle. Moreover, the results reveal that parts of the operated testing series (e.g. a combination of methods) allow useful estimations about the durability of the materials in relation to their application range. Key parameters ascertainable from porosity and water transport measurements additionally help to understand the experimental data and to evaluate the reliability of the results.