The work reported in paper concerns the use of diatomite to form-stabilise sodium nitrate, a phase change material (PCM) for medium temperature thermal energy storage applications. The composite was found to be able to retain up to 70% of the nitrate salt. X-ray diffraction (XRD) analyses suggested an excellent chemical compatibility between diatomite and the salt. Scanning electron microscope (SEM) analyses demonstrated an even distribution of the salt within the diatomite structure. Differential scanning calorimetry (DSC) measurements showed that melting temperature of the material was approximately 307.8 degrees C with a latent heat of 115.79 kJ/kg. Mechanical characterization of the composite material showed a compressive strength of the composite materials as high as 22.17 MPa. The composite materials were found to have a fairly low thermal conductivity of similar to 0.5 W/m K and an addition of graphite could give a substantial thermal conductivity enhancement (similar to 6-fold with an additional of 10 wt% graphite). (C) 2017 Elsevier Ltd. All rights reserved.