Mechanical performance and microstructure evolution of MgO-doped high volume GGBFS-based engineered cementitious composites at room and elevated temperatures

This paper presents a comprehensive study on the effect of MgO on the mechanical performance of ternary or quaternary blended engineered cementitious composites (ECC) primarily composed of cement, silica fume, and a high volume of ground granulated blast furnace slag (GGBFS) at both room and elevated temperatures. The study evaluates the mechanical performance with varying dosages of MgO and GGBFS, focusing on compressive strength, tensile strength, and residual compressive strength, along with strength variation and microstructural evolution analysis at different temperatures. Microstructural characterization was performed using scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry techniques, with further analysis of the interfacial transition zone (ITZ) conducted using synchrotron Fourier-transform infrared microspectroscopy. Test results indicated that 10-20 % MgO can be utilized in combination with GGBFS to develop high-strength and high-ductility ECC even at total cement replacement level as high as 80 %. Furthermore, elevated temperature analysis revealed the beneficial effect of MgO addition, particularly due to its hydration-retarding properties which led to accelerated strength development at later stages and resulted in higher residual strength. A compressive strength retention of over 45 % was observed for mixes containing 60 % GGBFS and 10 % MgO. While silica fume addition was found beneficial in terms of enhanced hydration until 200 degrees C, it led to spalling at high temperatures despite higher dosage of GGBFS. Analysis of the ITZ in the MgO-based matrix further showed the formation of a densely packed calcium silicate hydrate (C-S-H) and calcite around brucite, indicating a strong interface. Magnesium silicate hydrate (M-S-H) was also observed through X-ray diffraction analysis and was found in the matrix till 400 degrees C, possibly contributing to strength development and higher retention.