The strong demand for traditional Portland cement (OPC) is a significant contributor to the construction-related CO2 emissions whereas the conventional cement systems are still susceptible to chloride intrusion, sulfate attack and the loss of longevity in hostile environments. In this study, eco-friendly cementitious materials were designed by partially substituting OPC by fly ash and blast-furnace slag ground into fine particles which were replaced by 20-50% and measured their mechanical, durability and microstructural profiles against a 100% OPC control blend. The experimental program consisted of compressive, splitting tensile, and flexural strength tests at 7-90 days, water absorption, rapid permeability to chloride, sulfate exposure and rapid carbonation tests. The evolution of microstructure was studied with the help of scanning electron microscopy and X-ray diffraction. The findings indicated that at early ages, strength was reduced with the replacement level, although the 30% replacement mixture gave the highest overall strength with a compressive strength of 56.4 MPa in 90 days as compared to 52.3 MPa in the control. It was the same mix that decreased chloride permeability down to 1985 coulombs, water absorption dropped to 4.21% and sulfate expansion dropped to 0.076%. With the control, the depth of carbonation was 4.2 mm, whereas with the optimum eco-friendly mix, it was 5.6 mm. Microstructural analyses (SEM and XRD) confirmed a denser matrix, low portlandite, and narrow pore structure in SCM blended systems. The optimum mix will optimize the saving of the cement reduced by 30% and save about 102 kg CO₂/m³ of concrete. These results indicate a quantifiable route to the generation of low-carbon, structural binders with long durability to be used in a sustainable structure.
