This study assessed the rheological and mechanical properties of Nigerian Ewu-Obi Calcium Bentonite (ECaB) as cement partial replacement for shallow depth cementing operations using experimental design and response surface methodology. Rheological properties, Thickening Time (TT), and Compressive Strength (CS) were measured using a 12-speed rotational viscometer, High-Pressure High Temperature (HPHT) Consistometer, and Ultrasonic Cement Analyzer, respectively. The effect of the increase in the concentration of bentonite clay as well as its interaction with other cement additives (accelerator and antifoam) on plastic viscosity (PV), yield point (Yp), fluid loss (FL), TT, and CS were investigated. The result shows that the optimum replaceable percentage of ECaB in class G cement, accelerator, and antifoam is 23.5, 7.5, and 0.95 wt %, respectively. At optimum conditions, the PV (17.5±1.35 cp), FL (20 ml/30 min/100 psi), TT (228±16.4 min), and CS24 (614±0.57 psi) obtained agreed with the API standard and compared favorably with literature.