Abstract Thermal energy storage composites were developed by incorporating granular phase change materials (PX25 from Rubitherm®) into cement paste. Without prevention, however, a significant amount of phase change material (paraffin) leaked from the PX25 during the fabrication process. To prevent paraffin leakage during fabrication, three types of nano-SiO 2 powder (hydrophilic fumed silica (fs1), hydrophobic fumed silica (fs2), and precipitated silica (ps2)) were chosen to modify PX25. The influence of dosage of these modifiers was investigated with reference to PCM leakage during fabrication. Paraffin leakage was eliminated when hydrophobic fs2 or ps2 was used in a 2.3% and 9.0% mass fraction (relative to PX25), respectively. However, hydrophilic fs1 did not prevent leakage of paraffin. Microstructural and mechanical analyses of the thermal energy storage composites were used to analyze the prevention mechanism of hydrophobic nano-powder against paraffin leakage.