AbstractUsing the high-pressure differential thermal analysis (HP-DTA) system in a cubic multianvil high-pressure apparatus, we measured the melting points of portlandite, Ca(OH)2, up to 6GPa and 1000C. We detected endothermic behavior at the temperature and pressure conditions of 800C and 2.5GPa, 769C and 3.5GPa, 752C and 4.0GPa, 686C and 5.0GPa, and 596C and 6.0GPa, respectively, due to melting of portlandite. By insitu X-ray studies under pressure, the melting of portlandite was observed at 730C and 4.32GPa and at 640C and 5.81GPa, respectively. Results of both HP-DTA and X-ray studies were consistent within experimental error. The melting is congruent and has a negative Clapeyron slope, indicating that liquid Ca(OH)2 has higher densities than crystalline portlandite in this pressure range.