Understanding the chemical reactions underlying cement hydration under increased temperature and pressure conditions is important to ensure proper zonal isolation and integrity of oil and gas wells.In this study, the influence of high temperature and high pressure on American Petroleum Institute (API) Class G oil well cement was investigated.For this purpose, cement samples were cured at elevated pressures (1 bar to 550 bar) and temperatures (27 ℃-120 ℃) for 24 and 72 hours, respectively in steel cells.The hardened cement samples were then removed and cement hydration was stopped by immersion in acetone.The cement samples were characterized by Magic-Angle-Spinning Nuclear Magnetic Resonance spectroscopy (29Si MAS NMR), X-ray diffraction (XRD) and Environmental Scanning Electron Microscopy (ESEM).Furthermore, the development of compressive strength of the oil well cement samples during the hydration period was monitored using the ultrasonic method according to API RP 10B.