Abstract: By adding silica flour,it could be possible to prevent the loss of compressive strength of cement in high temperature conditions,and the retrogression of cement.However,the strength retrogression may still occur in some cases even after silica flour is added,which could result in the S-shaped growth of ultrasonic strength development curve under high temperature.In order to prevent cement strength retrogression from influencing cementing quality,it is necessary to eliminate the growth of the S-shaped strength curve.To investigate the cause of this strength retrogression phenomenon,the phases of the cement hydrated at 12,24,48 and 90 h were analyzed by XRD respectively in a study.Results showed that the amount of silica flour in the cement slurry was constant within 24 h,and the strength retrogression that occurred in the cement with 5.0% silica powder could not prevent the conversion of C-S-H phase to α-C2SH phase.Having silica flour for 24 hours prevented strength retrogression because it gradually participated in the hydration process.By increasing the specific surface area of silica flour to improve its reactivity,the phenomenon of strength retrogression disappeared.The research suggest that,reactivity cleficiency of silica flour is the primary cause of S-shaped strength development curve in cement slurry.Therefore,the addition of silica flour with higher specific surface area is one of the solutions to prevent the S-shaped strength retrogression of cement slurry.