Correlation between porosity and resistance to freeze-thaw cycles of natural hydraulic lime plasters

The need to develop materials with a low environmental footprint has led several heritage material companies to formulate plaster mixtures that they label as green. Many of them use Natural Hydraulic Lime (NHL) as a binder, either alone or in combination with other binders such as air lime or cement. These materials are now widely used in the maintenance of heritage buildings, but their compatibility with historical materials and their durability still needs further investigation. Research into the compatibility and durability of conservation materials has highlighted the importance of studying porosity, which plays a key role in the development of decay patterns. The average diameter, shape, connectivity and distribution of pores greatly influence the ability of a plaster to resist to the main damage mechanisms to which it is exposed during its life. The aim of the present study is to examine the behaviour of different plasters when subjected to freeze-thaw cycles and to investigate how porosity influences their resistance to temperature variations. Five NHL based commercial plasters, labelled green were selected. Of these plasters, numerous 40x40x160mm samples were made and subsequently subjected to porosity tests and freeze-thaw cycles to assess their durability, to further investigate the influence of the microstructure on the plasters' ability to withstand temperature cycles.