Construction materials sustainability is a significant contemporary issue worldwide. This work aimed at evaluating the role of thermally treated high-grade metakaolin (MK) in the performance of ordinary Portland cement (OPC) mortar. MK was developed by the thermal treatment of locally sourced clay, namely ADM, collected from Adam, Oman. The reactivity of ADM clay was determined through XRD analyses and compression tests of mortar containing 10 wt.% ADM-MK. In the first phase, the calcination temperature (460, 530, 590, and 640 ℃) was determined at constant burning duration (3 h). In the second phase, different calcination durations (1.0, 1.5, 2.0, and 3.0 h) were examined to determine the optimum one. The results proved that, at a constant calcination duration of 3 h, the cement mortar containing ADM-MK calcined at 590 ℃ showed the highest compressive strengths at 7 days of curing, with an increase of 12%. However, a decrease or increase in the calcination temperature and duration resulted in a lower effect. Additionally, optimizing calcination duration at 1.5 h for ADM-590 resulted in higher strength development at both early and later ages. The X-ray diffraction (XRD)-analysis proved that the temperature of 590 ℃ is adequate for transforming most of the crystalline aluminosilicate minerals into an amorphous structure with high pozzolanic reactivity. Further increasing in the calcination temperature to 640 ℃ was found to have no effect. The production of MK from local clay at low temperatures and holding time strongly contributed to enhancing the mass production and commercial viability of MK-blended high-performance concrete in Oman.