Salt weathering is one of the crucial causes of building stone decay. For assessing the durability of stones against salt weathering, a standardised test procedure (EN 12370:2020 Determination of resistance to salt crystallisation) is used: the mass loss of the stone is measured during sodium sulphate crystallisation cycles. Porous, Miocene limestone test specimens collected from the Sóskút quarry (Hungary) were subjected to salt crystallisation cycles in laboratory conditions. The limestone with open porosities of up to 32 V% is composed of ooids, various types of bioclasts and other carbonate grains. Besides the classical measurements of mass changes, the ultrasonic pulse velocity was also recorded after each crystallisation cycle. The ultrasonic pulse velocity values of quarry stones and salt-laden porous limestone specimens were compared. The results indicate that when salt crystal clogs the pores, an increase in ultrasonic pulse velocity is observed. Additional salt crystallisation cycles reduce the pulse velocity. This negative shift in pulse velocities is linked to the opening of micro-cracks, indicating the damage of the studied limestone. Our tests have proved that ultrasonic pulse velocity testing device can detect salt crystallisation damage in a non-destructive way. Its application is recommended in the condition assessment of salt damaged stone heritage structures.
The crystallisation of salts is one of the critical processes of weathering on stone-built monuments. Salt crystallisation can occur in a wide range of different environments like urban areas [1], deserts [2], forests, coastal areas, etc. This phenomenon has been observed under a vast range of relative humidity and temperature [3]. A clear understanding of the salt weathering process and critical parameters involving this phenomenon is demanded [4]. In recent years many studies regarding salt crystallisation and the main parameters affecting this phenomenon have been concluded [5-7]. According to Martinez et al. [5], salt mixture crystallisation is susceptible to temperature and relative humidity changes. Sato and Hattanji [6] investigated salt weathering of three types of rock (tuff, porous and dense sandstone) with sodium chloride, sodium sulphate, and magnesium sulphate under changing humidity conditions. The results showed that the most harmful salt is NaCl, but MgSO4 also damaged all tested lithotypes. On the other hand, Na2SO4 had no effect on salt weathering in high humidity conditions.