Investigation of the long-term stability of quartzite and basalt for a potential use as filler materials for a molten-salt based thermocline storage concept

This work addresses changes of the molten salt properties and in the microstructure of the natural stones depending on the exposure time in molten salt at temperatures of approximately 560 °C. In the first step of the material investigation the natural stones were isothermally stored in Solar Salt at a maximum temperature of 560 °C for up to 10.000 h. After the thermal treatment the microstructure of the stones was investigated by QEMSCAN (Quantitative Evaluation of Minerals by Scanning electron microscopy). By means of this analysis method the changes in the microstructure of quartzite and basalt was detected and arising stone components are identified. The melting temperature und enthalpy of Solar Salt was measured and compared with the salt properties before the thermal treatment. Additionally, the specific heat capacities of basalt and quartzite depending on the temperature were determined. The results are essential to verify the suitability of quartzite and basalt as potential filler materials in modern thermocline storage concepts.