Ca2+-doped MgCO3 forms a porous matrix that can encapsulate polyethylene glycol 6000 (PEG) using a simple impregnation method to form a shape-stabilized functional phase change composite for use in thermal energy storage. A facile hydrothermal route was developed to prepare unique rhombohedral anhydrous MgCO3. The rhombohedral building blocks disintegrated during a hydrothermal reaction in the presence of 15 mol % Ca2+ (15CaMgCO(3)) to produce very small particles with a porous structure. The hydrothermally synthesized 15CaMgCO(3) powders were used to encapsulate PEG and form a phase change material that did not leak liquid PEG during phase transitions. This PEG/15CaMgCO(3) shape-stabilized composite phase change material displayed reproducible behavior over a large number of thermal cycles. Differential scanning calorimetric (DSC) results indicate that the melting temperature of the PEG/15CaMgCO(3) material is 61.59 degrees C, and the latent heat is 193 J/g. The composites have significantly higher thermal enthalpies, high thermal conductivities, and less supercooling than those of long-established shape-stabilized phase change materials (PCMs). The presence of OH- on the surface of the PCM probably mitigates the supercooling effect. These results were confirmed with DSC and thermogravimetric analysis, and the characteristics of the PCMs were investigated by using scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and pore size distribution data.