This work explores the use of sodium sulfate and diatomite to formulate composite materials for high temperature thermal energy storage applications. Sodium sulfate in the composite functions as a phase change material (PCM) and diatomite as a structural skeleton for shape stabilization. It is found that sodium sulfate and diatomite have an excellent chemical compatibility with the PCM melting temperature at around 880 degrees C It is shown that the composite containing 45% diatomite gives an optimal formulation in terms of energy density, salt leakage and mechanical strength. The results also suggest that the composite with the optimal formulation has an application window of 890-980 degrees C Failures occur to the composite materials at temperatures above 1000 degrees C. (C) 2014 Elsevier B.V. All rights reserved.