Infrastructure construction is continuously growing due to urbanization and population growth. This sector produces huge amounts of cement and concrete annually, contributing to greenhouse gases, energy consumption, and resource depletion. A large amount of carbon dioxide (CO2) is emitted in ordinary Portland cement (OPC) production by a calcination process that requires intensive energy consumption at around 1500 °C. Hence, introducing the sustainability concept in this industry is crucial in reducing the carbon footprint. Different researchers around the world are investigating various sustainable materials that can be utilized as partial replacements for traditional construction materials, which can preserve the environment. One of these materials is supplementary cementitious materials (SCMs), which increase the mechanical properties of concrete/mortar, enhance the durability performance, lower carbon emissions, and are economical. In this study, two raw clays labeled as HMR-C and HMR-1 were collected from Al Hamra city in Oman. These materials were utilized as SCMs, and pozzolanic material in a partial replacement for OPC. A preparation process of screening, grinding, sieving, and thermal treatment was conducted for both clays. The various physical, chemical, and microstructural properties of clays were obtained by material characterization. The X-ray fluorescence (XRF) analysis showed that the combined total of (silica, alumina, and iron) oxides in calcined HMR-C clay is 91%, and in HMR-1 is 81%. Hence, both clays were classified as pozzolanic. Additionally, the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) findings revealed that the suitable calcination temperature of HMR-C is (650 °C and 700 °C), and in HMR-1 is (1000 °C and 1050 °C). The calcined clay was utilized as a replacement for OPC at different replacement ratios by weight with a constant w/c ratio of 0.30 in different mortar mixes. The potential of utilizing the HMR-C and HMR-1 calcined clays as pozzolana in mortar was examined in two phases. In the first phase, binary mixes were designed and produced with replacement percentages of (7%, 12%, 20%, and 30%). Depending in the 28-day compressive strength of this phase, an optimum replacement level of 12% and optimum calcination temperature of 650 °C for HMR-C and 1050 °C for HMR-1 was selected for the second phase. Different composite mixtures of calcined clay and various conventional SCMs were designed and produced in this phase. All the binary and composite mortar samples were experimentally tested to obtain their various properties. Immediately after mixing, a flowability test, temperature, and fresh density were performed. Also, the compressive strength and hardened density were tested at 1, 7, 28, and 91 days. Additionally, the flexural strength of calcined clay mortar samples was recorded at 28 days. Moreover, the ultrasonic pulse velocity (UPV), water absorption, and porosity were tested at 91 days. Furthermore, the effect of submerging mortar samples in a combined solution of (5% sodium sulfate and 5% sodium chloride) and (5% magnesium sulfate and 5% sodium chloride) for 9 weeks was tested. Similarly, acid resistance was conducted on mortar samples at 3 months after 28 days of wet curing; these acids are sulfuric acid, citric acid, nitric acid, hydrochloric acid, acetic acid, and phosphoric acid. The major outcomes of this study indicated that as the percentage of HMR-C and HMR-1 clay replacement rises in the blended paste, the normal consistency also increases, and the setting time is higher than OPC. It was also observed that the flowability of HMR-C decreases as the percentage of replacement of Portland cement with calcined clay increases. In contrast, HMR-1 calcined clay increases the flowability compared with the OPC. In binary mixes, a reduction in the early strength at 1 day is recorded up to around -45%, and an enhancement in strength is observed at 7, 28, and 91 days (42.6%, 21.5%, and 8.6%), respectively, while the composite mixes resulted in lower strength compared with the plain OPC. An increase in flexural strength is recorded in some samples of HMR-C and HMR-1 binary and composite mixes. The porosity and water absorption are lower in HMR-C binary mixes compared to the HMR-1 mixes, and no significant difference is recorded in UPV. The compressive strength test after exposure to high temperature (800 °C) showed an increased range of (3.98 to 17.7) % in HMR-C that was calcined at 700 °C. Moreover, the compressive strength after exposure to (5%Na2SO4 +5%NaCl) and wet/dry cycles resulted in an increase in strength in both clays. A loss in strength is recorded in all mixes of both clays after exposure to (5%MgSO4 +5%NaCl) and wet/dry cycles. Weight loss, degradation, and color change are observed after submerging all the HMR-C and HMR-1 mortar samples in various acids for 9 weeks. Strength increase is detected in most of the HMR-1 samples after exposure to H3PO4 and C2H4O2 compared with the plain OPC. An increase is also observed after the HCl, HNO3, and H2SO4 acid attack for both clays.