Research on mechanical properties and hydration characteristics of ultra-high performance concrete with high-volume fly ash microsphere

This study examines the potential of using fly ash microspheres (FAM) in producing ultra-high performance concrete (UHPC). The effects of FAM on the mechanical properties and hydration characteristics of UHPC were investigated. Isothermal calorimetry and X-ray diffraction (XRD) with quantitative analysis were employed to assess the hydration kinetics of cementitious materials. Additionally, scanning electron microscopy (SEM) was utilized to evaluate the microstructure of UHPC. The results demonstrate that incorporating 10% FAM in UHPC led to a significant enhancement in compressive, flexural, and tensile strength. However, as the FAM incorporation increased, the mechanical properties of the specimens exhibited a decreasing trend. Nonetheless, specimens with extensive incorporation of 50% FAM and 15% silica fume still achieved a compressive strength exceeding 100 MPa. Moreover, it was observed that higher amounts of FAM slowed down the hydration of cementitious materials but increased the hydration degree of cement. SEM analysis revealed that an optimal FAM content positively influenced the interfacial transition zone of UHPC, leading to improved microstructure. This study offers insights into the potential for large-scale and high-value utilization of FAM and the development of cost-effective and environmentally-friendly UHPC to reduce the carbon footprint of UHPC production.