Utilization of CaO to accelerate the pozzolanic reaction of excessive silica fume in ultra-high-performance fiber-reinforced concrete: Implications for microstructural and mechanical properties

This study investigates the effect of partially replacing ordinary Portland cement (OPC) with calcium oxide (CaO) in ultra-high-performance fiber-reinforced concrete (UHPFRC). Owing to its higher specific surface area compared with that of OPC, CaO rapidly reacts with free water during the curing stage, forming calcium hydroxide, Ca(OH)₂, and facilitating pozzolanic reactions with residual silica fume. As the CaO replacement ratio increases, the concentration of Ca²⁺ ions in the matrix rises, accelerating the hydration process. At a 3 wt% replacement level, UHPFRC achieves a maximum compressive strength of 195.29 MPa, a 12.7 % improvement over the control sample without CaO. The incorporation of CaO also enhances fiber–matrix interactions, increasing bond strength and fiber pullout energy by 52.7 % and 56.9 %, respectively. While the 3 wt% CaO mixture exhibits the highest tensile strength (17.24 MPa, a 23.2 % increase), the 2 wt% CaO mixture demonstrates superior strain capacity (0.91 %) and fracture energy (121.54 kJ/m³). However, excessive CaO content (>3 wt%) results in microstructural irregularities and reduced flowability, ultimately compromising durability. These findings highlight the potential of optimized CaO replacement to improve both the mechanical performance and durability of UHPFRC, providing valuable insights for the development of high-performance cementitious composites.