Assessing the influence of the mixing method on porosity and durability of NHL-based renders based on key parameters

Abstract

Natural hydraulic lime (NHL) is a common binder material in historic mortars and becomes increasingly used in recent restoration works for the preservation of cultural heritage. These plasters, renders and masonry mortars are suitable for applications, where elevated water permeable characteristics are required. At the building site different workmanship methods may be used, which can be simulated by e.g. different mixing times. Hence, within the present study two NHL-based recipes using hydrated lime of different origin (dry powder and lime putty suspension) were evaluated with regard to the effect of different mixing methods and water content on fresh and hardened mortar characteristics based on a comprehensive experimental program. Additionally, the porosity was evaluated by direct and indirect methods for assessing the water transport behaviour. Although durability and the underlying water transport properties are strongly related to porosity, this study focused primarily on this essential interrelation described by key parameters found in literature and evolved within this study. The results revealed that the mixing method and its associated water content as well as the type of hydrated lime affect strongly the fresh and hardened mortar properties. The pore structure characteristics that resulted from different fresh mortar treatment are crucial for the developing of durability. The observed interrelation between the aforementioned characteristics is essential for a deeper understanding of NHL-based systems. Generally, a longer treatment time leads to a higher durability against e.g. like frost-thaw and salt exposure. Considering the evaluated characteristics, it is possible to apply a prediction model based on the introduced key parameters to obtain information about the estimated resistance of NHL-based renders against damaging processes. Thus, the gained insights are beneficial for a comprehensive scheduling of restoration approach with specified framework requirements and arranged general material selection.