Mirabilite and heptahydrate characterization from infrared microscopy and thermal data
Résumé
Sodium sulfate is widely regarded as the most damaging salt for porous building materials. To understand this complex and incompletely understood phenomena, it is important to predict which phases will crystallize under a range of environmental conditions and which phase transitions will cause damage. In this study we present data on the crystallization of sodium sulfate hydrates from infrared spectroscopy and temperature data.
This work documents the crystalline phases and their transitions under controlled environmental conditions (temperature and relative humidity, RH). The phase changes in a salt crystal coming from a droplet of sodium sulfate solution (28 and 30 wt. % of Na2SO4) subjected to evaporation are observed using FTIR-ATR spectroscopy. As Na2SO4 hydrates contain more OH content than anhydrous phase, the OH absorption band enables the distinction between these two types of phases. The temperature signal are acquired using a type K thermocouple in a solution of sodium sulfate (28 and 30 wt.% of Na2SO4) that is cooled down from +40 °C to +5 °C. As the temperature decreases, crystallization events are recorded though the temperature signal. According to temperature monitoring experiments, heptahydrate crystallizes more easily than mirabilite.
The importance of the environmental conditions and the protocol establishment are discussed. The approach of these methods can help us to understand the crystallization sequence of sodium sulfates and later better apprehend the damage of building stone.