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<p>Ultra-high-performance concrete (UHPC) is recognized for its excellent mechanical properties and durability, but its high carbon footprint and cost limit large-scale use. Alkali-activated UHPC (A-UHPC) has emerged as a greener alternative, yet its performance, especially under elevated temperatures, remains underexplored. This study provides a multi-scale comparison of cement-based UHPC (C-UHPC) and A-UHPC, focusing on mechanical properties, shrinkage, thermal resistance, microstructural changes, and simplified CO<sub>2</sub> and cost assessments. At ambient conditions, fiber-free prismatic (40 × 40 × 160&#xa0;mm) and cubic specimens (70.7&#xa0;mm) were tested for compressive and flexural strength, ultrasonic pulse velocity, porosity, and capillary absorption. At elevated temperatures, hybrid fiber-reinforced specimens were used to assess thermal resistance. Microstructural analyses of PP-fiber-reinforced paste samples were conducted using SEM, MIP, TGA, FTIR, and XRD. Results indicate that C-UHPC exhibits higher strength, lower shrinkage, and a denser microstructure at room temperature due to C-(A)-S-H gel formation, but experiences rapid deterioration above 600℃ from matrix cracking and hydration loss. In contrast, A-UHPC, though initially weaker, shows more gradual degradation due to the formation of stable crystalline phases, such as nepheline and augite. Microstructural analysis confirms distinct transition pathways and damage patterns between the two systems. Environmentally, A-UHPC reduces CO<sub>2</sub> emissions by over 40%, though it incurs a moderate cost increase due to alkali activators. This integrated performance-environment-cost evaluation, seldom addressed in UHPC research, highlights the trade-offs between strength and sustainability. These findings provide a foundation for designing greener, thermally resilient UHPC systems for low-carbon construction.</p>

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A multi-scale investigation of cement-based and alkali-activated ultra-high-performance concrete: from ambient and high-temperature performance to microstructural and environmental analysis

  • 期刊论文

  • 发表日期：

    2026年2月22日

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  • Yunfeng Qian,
  • Dingyi Yang,
  • Zhenxiang Jie,
  • Kailu Yang,
  • Guosheng Ren

摘要

Ultra-high-performance concrete (UHPC) is recognized for its excellent mechanical properties and durability, but its high carbon footprint and cost limit large-scale use. Alkali-activated UHPC (A-UHPC) has emerged as a greener alternative, yet its performance, especially under elevated temperatures, remains underexplored. This study provides a multi-scale comparison of cement-based UHPC (C-UHPC) and A-UHPC, focusing on mechanical properties, shrinkage, thermal resistance, microstructural changes, and simplified CO2 and cost assessments. At ambient conditions, fiber-free prismatic (40 × 40 × 160 mm) and cubic specimens (70.7 mm) were tested for compressive and flexural strength, ultrasonic pulse velocity, porosity, and capillary absorption. At elevated temperatures, hybrid fiber-reinforced specimens were used to assess thermal resistance. Microstructural analyses of PP-fiber-reinforced paste samples were conducted using SEM, MIP, TGA, FTIR, and XRD. Results indicate that C-UHPC exhibits higher strength, lower shrinkage, and a denser microstructure at room temperature due to C-(A)-S-H gel formation, but experiences rapid deterioration above 600℃ from matrix cracking and hydration loss. In contrast, A-UHPC, though initially weaker, shows more gradual degradation due to the formation of stable crystalline phases, such as nepheline and augite. Microstructural analysis confirms distinct transition pathways and damage patterns between the two systems. Environmentally, A-UHPC reduces CO2 emissions by over 40%, though it incurs a moderate cost increase due to alkali activators. This integrated performance-environment-cost evaluation, seldom addressed in UHPC research, highlights the trade-offs between strength and sustainability. These findings provide a foundation for designing greener, thermally resilient UHPC systems for low-carbon construction.