Mechanical, Durability, and Environmental Performance of Limestone Powder-Modified Ultra-High-Performance Concrete

Ultra-high-performance concrete (UHPC) delivers outstanding durability and strength but typically relies on high Portland cement content. This study evaluates a 20% cement replacement with limestone powder (LP) in UHPC and benchmarks performance under two curing regimes: moist curing (MC) and warm bath curing at 90 °C (WB). Metrics include workability, compressive and flexural behavior, shrinkage, freeze–thaw resistance, chloride transport (surface resistivity, RCPT), material cost, and embodied CO2. LP improved fresh behavior: flow increased by 14.3% in plain UHPC and 33% in fiber-reinforced UHPC (FR-UHPC). Compressive strengths remained in the UHPC range at 28–56 days (approximately 142–152 MPa with LP), with modest penalties versus 0%-LP controls (about 2–5% depending on age and curing). Under WB at 56 days, controls reached 154 MPa (plain) and 161 MPa (FR-UHPC), while LP mixes achieved 145.2 MPa (plain) and 152.0 MPa (FR-UHPC). Flexural performance was reduced with LP: for FR-UHPC, 28-day MOR under MC was reduced from 15.5 MPa to 12.7 MPa and under WB from 14.3 MPa to 10.3 MPa; toughness under MC was reduced from 74.4 J to 51.1 J. Durability indicators were maintained or improved despite these moderate strength reductions. After 300 rapid freeze–thaw cycles, all mixtures retained relative dynamic modulus near 100–103%, with negligible MOR losses in LP mixes (plain UHPC: −1.1% with LP versus −4.7% without; FR-UHPC: −3.7% versus −8.1%). Chloride transport resistance improved: at 56 days under MC, surface resistivity increased from 558 to 707 kΩ·cm in plain UHPC and from 252 to 444 kΩ·cm in FR-UHPC; RCPT for LP mixes was 139 C (MC) and 408 C (WB), about 14–23% lower than respective controls. Drying shrinkage was reduced by roughly 23% (plain) and 28% (FR-UHPC). Sustainability and cost outcomes were favorable: embodied CO2 was reduced by 18.8% (plain) and 15.5% (FR-UHPC), and material cost was reduced by about 4.5% and 2.0%, respectively. The main shortcomings are moderate reductions in compressive and flexural strength and toughness, particularly under WB curing, which should guide application-specific limits and design factors.