Heat Resistance of Concrete Incorporating Magnesium Oxide and Fly Ash in Portland Cement Binder System

A cement binder containing around 30% reactive magnesium oxide (MgO) has the potential to reduce its carbon footprint because reactive MgO is produced at 600°C–700°C, whereas clinker requires a much higher temperature of 1,450°C. In this study, investigations have been carried out to evaluate the heat resistance of concrete prepared with a ternary binder designated as OPC-MgO-Fly Ash (OMF) composed of 50% ordinary portland cement (OPC), 30% MgO, and 20% fly ash (FA). Replacing OPC by 50% is a viable alternative to curb CO2 emissions from the cement industry. The concrete samples prepared with the OMF were subjected to elevated temperatures of 200°C, 400°C, 600°C, 800°C, and 1,000°C. Concrete incorporating OMF demonstrated improved thermal performance, with greater compressive strength retention, reduced water absorption, and lower mass loss compared with OPC. At 1,000°C, the OMF-based mix exhibited a 43.40% reduction in compressive strength, significantly less than the 60.20% loss observed in OPC. Water absorption was also lower (5.44% versus 8.33%). X-ray diffraction (XRD) analysis revealed that the hydration products of OPC decomposed below 600°C, whereas the corresponding peaks in the OMF mix remained stable. Fourier-transform infrared spectroscopy (FTIR) analysis showed a reduction in the O–H and Si─ O─ Si bands at 400°C for OPC, whereas these bands persisted in the OMF system up to 600°C. Scanning electron microscopy (SEM) analysis further confirmed the stability of magnesium silicate hydrate (M─ S─ H) phases above 600°C. Thermogravimetric analysis (TGA) results indicated a mass loss in OMF between 600°C and700°C due to the decomposition of M─ S─ H, whereas OPC exhibited significant mass loss at 400°C–500°C associated with the breakdown of hydration products. These findings collectively confirm the superior thermal stability of the ternary binder concrete compared with conventional OPC-based concrete.