Hydration–dehydration behavior and thermodynamics of MX-80 montmorillonite studied using thermal analysis

• Adsorption–desorption of water on MX-80 is determined from TGA and DTA analysis.
• From DTA analysis, three types of water behavior exist at various hydration states.
• Surface area of DTA allows to determine the dehydration enthalpy per mole of water.
• A maximum enthalpy is 12 kJ/mol for the adsorption and desorption processes.
• Enthalpy of formation of water for adsorption and desorption processes are provided.

The thermal dehydration of natural bentonite clay MX-80 chosen as a possible future backfill material, was investigated using thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The aim of this work is to provide a better understanding of the thermodynamics of the hydration–dehydration process of MX-80. The data obtained from thermogravimetry derivative curves at different relative humidities were used to determine the adsorption–desorption isotherm of MX-80. The total amount of water varies from 0.35 to 5.62 and from 0.78 to 6.12 mol adsorbed H2O/mol of clay upon adsorption and desorption, respectively, for a RH between 11 and 91%. Furthermore, the heats released upon adsorption and desorption are not completely similar. Moreover, the analysis of DTA signals obtained at various hydration states provides insights about three types of water behavior in MX-80. Therefore, the surface area of DTA curves was taken into account to determine the dehydration enthalpy per mole of water; the values do not correlate with the amount of adsorbed water in MX-80, and the maximum enthalpy was approximately 12 kJ/mol for the adsorption and desorption studies. The values obtained were combined with the standard enthalpies of the formation of liquid water to obtain the corresponding enthalpy of the formation of water relative to dehydrated MX-80.