Patterns of halite (NaCl) crystallisation in building stone conditioned by laboratory heating regimes

The crystallisation of soluble salts within the pores of the stone is widely recognised as a major mechanism causing the deterioration of the stone-built architectural heritage. Temperature, in turn, is one of the main controls on this process, including salt precipitation, the pressure of crystallisation and the thermal expansion of salts. Most laboratory experiments on decay generated by salts are just carried out with convective heating regimes, while in natural environments building stones can undergo radiative and convective heating regimes. The thermal response of stone to these different heating regimes is noticeably different and might influence the crystallisation patterns of a salt within a stone. The aim of this work is to raise awareness on the different patterns of crystallisation of NaCl within a porous stone tested with different heating regimes (convection and radiation) and the implications that this could have on the design of experimental modelling of natural weathering conditions in laboratory simulations. Results show that heating regime affects the sodium chloride distribution within a stone with high percentage of microporosity. In this case, radiation heating facilitates the generation of subefflorescences, while convection heating promotes efflorescences. This has a clear implication both on the stone decay in natural environments and on the methodologies for testing salt decay, as subefflorescences are more destructive than efflorescences. In this sense, the use of convective heating in laboratory experimentation might underestimate the potential damage that sodium chloride may generate. This counsels the use of radiation heating test methods in addition to convection for the laboratory study of salt crystallisation.