The hydration of a tricalcium silicate paste at ambient temperature and at 200°C under high pressure (up to 1000 bar) has been studied. Two high pressure cells have been used, one allows in-situ electrical conductivity measurements during hydration under high pressure. The hydration products were characterized by thermal analysis, X-ray diffraction and 29 Si NMR measurements. The pressure has a large kinetic effect on the hydration of a C 3 S paste at room temperature. The pressure was seen to affect drastically the hydration of a C 3 S paste at 200°C and this study evidences the competition between the different high temperature phases during the hydration.