In the literature, several intermediates have been identified during the dehydration of MgSO4.7H2O. However, the published results are at some points conflicting on which intermediates are formed [2-6]. The dominant natural occurring magnesium sulfates on earth are Epsomite (MgSO4.7H2O), Hexahydrate (MgSO4.6H2O) and Kieserite (MgSO4.H2O). Since it is not clear which other intermediates are formed, it was decided to measure the dehydration of MgSO4.7H2O anew. The dehydration of MgSO4.7H2O was studied by means of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA involves the measurement of the mass change as function of time and DSC involves the measurement of heat as function of time, in which both are subjected to a predefined temperature program. The experimental results indicated that dehydration of MgSO4.7H2O proceeds in three discrete steps as illustrated in Figure 1:

The above Figure shows that the first dehydration reaction occurs at low temperature (<50°C) and involves the loss of one water molecule. The largest number of water molecules is dehydrated during the second reaction, which also allows storing the largest amount of energy of the three dehydration reactions: almost 10 times more energy compared to water (~2.3 GJ/m3 compared to 0.25 GJ/m3 for water in the temperature range 25-85°C). Additionally, the reaction occurs between approximately 60°C and 200°C: a temperature range which can be almost completely covered using a vacuum tube solar collector (Tmax=150°C). For these two reasons the second dehydration reaction is most interesting

for seasonal solar heat storage. Figure 1 also shows that the last water molecules are dehydrated at a high temperature (~275°C). In contrast to the first two dehydration reactions, the third dehydration reaction involves an exothermic process (indicated by the positive DSC signal). Ruiz et al [3] suggests that the final transition to MgSO4 includes an exothermic reaction due to recrystallization of an amorphous precursor. This suggestion was further investigated by performing X-ray diffraction (XRD) experiments as shown in Figure 2:

When the temperature of the sample is increased from 25°C to 55°C, the XRD measurement shows that MgSO4.7H2O is converted to MgSO4.6H2O. This result is in agreement with the results obtained by the TGA-DSC experiments (see also Figure 1). No peaks are observed in Figure 2 between 80°C and 276°C, indicating that an amorphous state is formed when MgSO4.6H2O is dehydrated further. This observation is in agreement with the findings of Ruiz et al [3].

The XRD measurements shows that the material only recrystallizes at T>276°C. It should be noted that recrystallization only occurs when a small amount of water is present (0.2 water molecules per MgSO4 molecule, see Figure 1) after which an exothermic reaction is observed. This could be explained by the hypothesis that below the critical water content of 0.2 water molecules per MgSO4 molecule, a spontaneous exothermic crystallization occurs that expels the last water that is present in the material. The XRD measurements confirm the observations from TGA-DSC experiments that the material is completely dehydrated at 300°C.