In the current study pre-formed geopolymer half shells were fabricated from a combination of fly ash and black slag. Five (5) geopolymer shells were filled with a chloride-based phase change material (PCM) with a melting point of 540 degrees C for use in a high temperature thermal energy storage system (TESS). Additionally, three (3) of these shells contained an inner coating of alumina (various manufacturers) while another geopolymer shell was pre-fired at 610 degrees C. The half shells were cycled around the melting point of the PCM (500-580 degrees C) for five (5) cycles, with the mass change of the samples recorded. Of the five (5) samples only the Aremco-coated sample suffered less PCM loss than the control. Furthermore, the melting point and latent heat of PCM samples from the geopolymer half shells were measured by differential scanning calorimetry (DSC) to find if degradation occurs. In all samples the PCM melting point was not affected while the latent heat was significantly affected. Additionally, the Aremco-coated sample resulted in PCM segregation.