Performance of Cement Mortar with Inorganic Na2SO4·10H2O-Na2HPO4·12H2O Shape-Stabilization Phase Change Materials

Phase change material (PCM) is a strategic choice for storing energy and regulating the building temperature. It has been successfully integrated into cement-based materials, significantly affecting mechanical properties. The main feature of PCM is thermal mass control, given the sensitivity of the hydration process to temperature. In this study, multiple methods were applied to investigate the changes in cementitious mortar caused by the addition of PCM. Eutectic hydrated salt and expanded perlite (EP) were used to fabricate an inorganic shape-stabilized PCM composite known as EPC. The influence on hydration is directly reflected in internal temperature change, volume shrinkage, and mechanical properties. In addition, microcharacterization was used to reveal the underlying reasons. The EP-based mortar did not exhibit weaker mechanical properties but showed a higher volume shrinkage. The alkalinity of the PCM was not sufficient to trigger the activity of amorphous silica (SiO2) in the EP, and this difference was attributed to its physical properties. The EPC demonstrated remarkable temperature regulation during hydration, replacing the temperature peak with a plateau. The residual sodium sulphate decahydrate (Na2SO4·10H2O) on the surface of the EP promoted ettringite generation near the aggregate, contributing to a slight volume expansion in the early stage and a decrease in the cement–aggregate bond quality. This phenomenon significantly accounts for the strength loss in the PCM mortar. The heat flow during hydration was altered, resulting in lower peak temperatures and a lower final hydration level. In conclusion, not only is PCM incorporation critical, but the choice of PCM type is also crucial for concrete properties. Therefore, the selection of the PCM should consider its potential effect on hydration, placing a higher demand on leakage prevention.