Performance optimization of ultra-high strength concrete through the synergistic integration of silica fiber and micro-silica for superior strength and structural resilience

The growing demand for resilient and sustainable infrastructure requires concrete systems capable of delivering superior strength, durability, and thermal performance under aggressive service conditions. This study investigates the combined effects of micro silica (0–20%), silica fibers (0–1.5%), and 5% phase change material (PCM) on the fresh, mechanical, durability, and thermal properties of ultra-high-strength concrete (UHSC) containing 25% fly ash. Twenty UHSC mixtures were prepared and evaluated through workability, strength, impact resistance, durability, and thermal performance tests. The results identified the mix containing 15% micro silica and 1.0% silica fiber (M15–S1.0) as the optimum composition, achieving compressive strengths of 115.3 MPa and 124.1 MPa at 28 and 90 days, respectively, along with split tensile and flexural strengths of 8.2 MPa and 12.4 MPa. Compared with the control mix, impact resistance, toughness index, and normalized failure energy increased by approximately 114%, 22%, and 102%, respectively. Durability performance was significantly enhanced, with sorptivity reduced by about 62%, chloride permeability decreasing from 2680 C to below 400 C, and residual strength retention reaching 92.8% under sulfate exposure and 85.5% under acid exposure after 90 days. Furthermore, thermal conductivity decreased by 35–40%, while specific heat capacity increased by 5–8%, demonstrating the beneficial contribution of PCM to thermal regulation. The findings confirm that the synergistic incorporation of micro silica, silica fibers, and PCM can produce a multifunctional UHSC with superior mechanical performance, durability, and thermal resilience suitable for advanced infrastructure applications.