Experimental investigation of hygro-thermo-fracture couplings in cement paste with mineral fillers via micro-scale splitting tests

The fracture mechanics of heterogeneous cement paste with a low water-to-cement (w/c) ratio under varying relative humidity (RH) and temperature (T) is critical for enhancing the durability of modern cement-based materials such as ultra-high performance concrete (UHPC) in real environmental conditions. Leveraging the rapid and stable equilibrium attainment of microscopic tests in terms of RH and T, this study investigates, for the first time, the micro-scale splitting tensile strength of cement paste incorporating fine limestone filler (LF) at different RH (10, 30, and 80 %) and T (20, 40, and 60°C). First, micrometer-sized cubes (150 × 150 × 150 µm 3 ) were fabricated using cement paste with a constant w/c ratio of 0.40, while substituting cement with LF at 15 % and 30 %, resulting in water-to-fine ratios of 0.40, 0.34 and 0.28. Then, micro-cubes were tested with a nano-indenter equipped with an environmental chamber able to apply varying RH and T conditions. Additionally, X-ray computed microtomography (µ-CT) and mercury intrusion porosimetry (MIP) were used to characterize porosity and pore-network connectivity. The results show that LF reduces porosity and refines pore structure, improving tensile strength but only partially mitigating strength loss at higher RH and T levels. A quadratic model describes the non-linear correlation between strength, RH, and T. Coupled hygro-thermal effects indicate that higher RH slightly offsets thermal weakening in cement paste but has a reduced influence in systems with LF. Finally, the micro-scale splitting test effectively assesses hygro-thermal effects on cement paste fracture performance, providing valuable insights into UHPC durability. • Micro-scale splitting test assessed the fracture of cement paste with limestone. • At low w/f ratios, limestone enhances micro-scale splitting tensile strength. • Limestone reduces porosity and connectivity, while refining the pore structure. • Higher RH and T reduce splitting strength, while limestone slightly mitigates it. • Pore-network connectivity governs the moisture-induced weakening on cement paste.