Synergistic Effects of Tricalcium Aluminate and Gypsum on Performance of Oil Well Cement Slurries

To better control the quality of oil well cement, class G cement clinkers with different contents of tricalcium aluminate (C₃A) were mixed with different kinds of gypsums, and study was conducted on the synergistic effects of C₃A and gypsum on the gelling time, thickening property, compressive strength, permeability and hydration products of the cement at 80℃. It was found that at this temperature, the higher the C₃A content, the shorter the gelling time and thickening time of the cement. High C₃A content is beneficial to the enhancement of the early strength of the set cement, the disadvantage of this is that the strength of the set cement in later stage will have a negative growth. To avoid this deficiency, the C₃A content in a cement should be controlled to less than 3%. Gypsum in the cement helps extend the gelling time of the cement, in this way the dihydrate gypsum is better than bassanite, and bassanite is in turn better than anhydrite. Compared with bassanite and anhydrite, dihydrate gypsum can extend the thickening time of the H42L (a retarder) treated cement slurry much longer, and the use of dihydrate gypsum is much more beneficial to the development of the compressive strength of set cement. At 80℃, the synergistic action between C₃A and dihydrate gypsum promoted the hydration of calcium silicate’s early stage mineral phase, the hydration product of C₃A was mainly C₃AH₆, the amount of which produced was closely related to the content of C₃A in the cement and the types of gypsum. The results of this study are of theoretical and referential importance to the control of the quality stability of oil well cement and to the quality improvement of well cementing operation.