In this paper, the corrosion of C-steel in OPC-fly ash-composite cement pastes (OPC-FA-CCP) was examined with different fly ash (FA) contents (0, 10, 25 and 50 mass%) and immersion times in the absence and
presence of spinach extract as a green-corrosion inhibitor. The effect of replacing FA in cement composition on its physical, chemical, mechanical and microstructure properties using a variety of techniques
has been also studied. The results indicated a significant increase in compression strength (CS) and gel/
space ratio (X) upto 90 days in the case of OPC-FA-CCP containing 10 mass% FA. At higher FA replacement
levels (25–50 mass%), the CS and X values decreased. SEM confirmed the presence of a surface layer of
amorphous-ill-crystalline C-S-H as a gel product on the FA grains, with prolonged time showing dense
crystalline hydrated fibril-C-S-H products. The C-steel immersed in the pure OPC cement (FA0S) has
the highest corrosion potential (Ecorr = 461 mV) vs. SCE. With the increase of the FA content the Ecorr
shifts to a more negative value with an indication of the decrease in the corrosion resistance of the rebar.
During the first 6 h of immersion, the increased thickness of the passive film leads to a remarkable
decrease in the corrosion rate. As the dipping time increases, the current corrosion density in all tested
OPC-FA-CCP mixtures is unexpectedly reduced. A significant improvement in corrosion resistance of
rebar was achieved in the presence of spinach extract. All the corrosion parameters were also calculated.