he present study examines the effect of fly ash (FA) and micro-silica (MS) as a potential cementitious material to fabricate the composite cements (CCP; OPC-FA-MS). The physical, chemical, mechanical, and electrochemical properties of CCP were studied using different techniques. The results indicate a significant increase in the compression strength (CS) and the gel/space ratio (X) upto 90-days for the composite cement OPC-FA-10% MS. While the mix containing 70% OPC-10% FA-20% MS (C0MS20) shows a significant decrease in X value and CS from 1 to 7 days, after that, the X value and compressive strength reveals a remarkable increase at 28–90 days. Moreover, scanning electron micrographs (SEM) shows that the utilization of FA with MS resulted in a much denser microstructure, leading to an increase in the compressive strength. Potentiodynamic polarization of steel rebars in the composite cements filtrate, OPC-10% FA-MS, at 5, 10, and 20 mass% MS, (BS0, CS0 and C0MS20, respectively), was performed at the room temperature in the absence and presence of 5% NaCl as a corrosive environment. The lowest values of the corrosion rates were in the case of C0MS20, due to the formation of a protective film on steel rebars significantly higher corrosion resistance for both free and chloride attack media. Scanning electron microscopy (SEM) of the C-steel immersed in the C0MS20 filtrate confirms its smoothness and better morphology. The protection efficiency of C-steel in the C0MS20 filtrate has been enhanced additionally in the presence of different concentrations of the eco-friendly inhibitor; spinacia oleracea extract. The synergistic inhibition impacts of spinacia oleracea extracts and 20% MS on the steel rebars corrosion were studied.