Sodium sulfates are well known to be the most damaging salts in building materials and rocks. Unfortunately, the crystallization processes of sodium sulfates are not completely understood. In addition, the metastable heptahydrate has long been neglected in scientific works on salt damage until recently. In this study, we use temperature monitoring and differential scanning calorimetry to detect and identify the crystallization of sodium sulfate hydrates (i.e., mirabilite and heptahydrate) upon cooling/ heating a bulk solution. The presence of impurities seems to play a major role in the crystallization sequence and can explain the crystallization of mirabilite and ice close to -10 °C. The crystallization of heptahydrate does not seem to be sensitive to the presence of impurities and does not always occur prior to the crystallization of mirabilite as commonly observed. The heptahydrate and mirabilite show different and characteristic thermal signatures that enable to distinguish each other. The shape, the intensity and the duration of the peak of temperature due to the crystallization depict these differences. Therefore, the thermal signatures can be used in further experimental studies to estimate the role of the different sodium sulfate hydrates involved in the salt weathering of rocks.