S形水泥浆强度发展曲线成因分析及消除方法

水泥石在高温时会出现强度衰退现象，添加硅粉可以防止高温下水泥石强度的衰退，但在研究和实际应用中发现添加硅粉后水泥石在高温下还会出现强度衰退现象，导致水泥浆高温下的超声波强度发展曲线呈现S形。为了防止水泥石发生高温强度衰退影响固井质量，消除S形强度发展曲线，利用X射线粉末衍射仪（XRD）分别对抗高温高密度水泥浆在强度发展过程中12，24，48和90 h等时间点处的水泥石进行物相分析，根据物相分析结果探究S形强度发展曲线的成因。结果表明:水化反应前24 h，硅粉未参与反应，且5.0%的微硅粉不足以阻止C-S-H凝胶向低强度α-C2SH转变，因而水泥石出现高温强度衰退现象；水化反应24 h后，硅粉开始参与水化反应，阻止水泥石发生强度衰退。研究表明，硅粉活性不足是导致水泥浆强度发展曲线呈现S形的原因，增大硅粉比表面积可以提高其活性，防止水泥石出现短暂高温衰退现象，消除S形水泥浆强度发展曲线。 

Origin Analysis and Elimination of the S-Shaped Strength Development Curve of Cement Slurry

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.