Green Environment and sustainable building become important issues due to their essential effects on the global system. This study aimed to investigate the potential use of high calcium oxide clay as a substituting portion for ordinary Portland cement (OPC). The clay was locally sourced and thermally treated at 530 ℃, 610 ℃, 670 ℃, and 750 ℃ with a fixed duration of 2.5 h and utilized in all mortar mixes as a 10% replacement of cement. The raw and calcined clay labeled (NZ) were characterized by Thermogravimetric Analysis (TGA), X-ray fluorescence (XRF), and X-Ray diffraction (XRD). Mortar’s flowability, hardened density, and compressive strength were obtained to detect the change in workability and compressive strength properties. The interoperated data of TGA, XRD, and XRF showed high contents of calcium carbonate (calcite) before and after calcination. Depending on the clay composition and the calcining condition, the results of fresh mortar revealed that the increase in calcination temperature of the clay attributed to a reduction in mortar flowability as reflected in more superplasticizer demand. At the same time, the impact of calcined clay at 670 ℃ showed significant improvement in the hardened density at day 7 and 28 with no noticeable increment in compressive strength in all samples compared to the control. The replacement of cement by clay possesses high calcium oxide and calcined at precise temperatures producing more densified mortar.