Micro-scale uniaxial compression assessment of hygro-thermo-mechanical interactions in limestone-filler cement paste at low water-to-fine ratio

The partial cement substitution with limestone filler (LF) enables the development of sustainable ultra-high-performance concrete (UHPC). While LF reduces the carbon footprint of cement production, its influence on the micromechanical behavior of cement paste under varying environmental conditions has not been fully understood. This study presents a twofold originality: first, by employing uniaxial compression on micrometer-sized specimens to characterize the mechanical properties of cement paste containing fine LF; and second, by assessing the effects of varying relative humidity (RH) and temperature (T) conditions, as an effective method of testing under rapidly archived hygro-thermal equilibrium. Moreover, a dual-method characterization approach, including water adsorption isotherms and X-ray computed microtomography (μ-CT) was employed to verify hygro-thermal equilibrium in the specimens and analyze the microstructure, respectively. Micro-prisms (150 × 150 × 300 μm 3 ) were fabricated using a high-precision dicing saw from cement pastes with LF additions. Micro-scale uniaxial compression tests were then conducted under controlled conditions at varying RH (10, 30, and 80%) and T (20, 40, and 60°C), considering their coupled effects. The increased RH and T levels generally decrease both compressive strength and elastic modulus showing a fairly non-linear dependence. Notably, LF not only enhances compressive strength, elastic modulus, and fracture energy, but also mitigates RH and T effects, due to the refined pore structure confirmed by μ-CT analysis. This study advances the understanding of the micromechanical properties of cement paste containing LF under varying RH and T conditions at low water-to-fine ratios, providing valuable insights for the development of sustainable UHPC. • The microstructure of 3D prismatic RVEs extracted from scanned cement paste samples containing limestone filler was analyzed. • Replacing cement with limestone filler results in reduced porosity, refined pore structure, and decreased size and volume of clinkers. • Micrometer-sized prisms (150 × 150 × 300 μm 3 ) were tested under different levels of relative humidity and temperature using an advanced uniaxial compression test at micro-scale. • Limestone filler increases the compressive strength, elastic modulus, and compressive fracture energy of cement paste at micro-scale. • Relative humidity and temperature exhibit a non-linear correlation in their overall weakening effect on the compressive strength and elastic modulus of cement paste.