Effect of limestone filler on micro-scale creep of stoichiometrically saturated cement paste systems under coupled relative humidity and temperature conditions

Reducing time-dependent deformation is critical for the long-term performance of ultra-high-performance concrete (UHPC), particularly when targeting low-clinker, low water-to-fine (w/f) ratio formulations. This study investigates the micro-scale creep behavior of cement paste systems incorporating fine limestone filler (LF) as partial cement substitution under coupled relative humidity (RH) and temperature (T) conditions. All mixtures were designed with a stoichiometrically saturated water-to-cement ratio (w/c = 0.40) to promote a high degree of hydration, whereas the water-to-fine (w/f) ratio was adjusted to form dense, UHPC-like matrices. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were used to analyze the porosity and microstructure of the cement paste systems. Using an advanced uniaxial creep recovery test on micrometer-sized prismatic specimens (150 × 150 × 300 μm 3 ), the viscoelastic response was quantified under seven RH-T combinations. Substituting 30% of cement with LF increased the uniaxial creep modulus by 67% and reduced porosity by 25%. Across all systems, creep deformation increases with higher RH and T levels, and these effects are coupled, each amplifying the influence of the other. Within the investigated range, creep is more sensitive to RH variations than to T, although the inclusion of LF mitigates RH-T-induced creep amplification. Despite the environmental loading, all systems exhibited long-term logarithmic creep kinetics and a consistent recovery index (~ 56%). The study offers original micro-scale evidence that mineral filler addition not only improves the microstructure but also enhances long-term environmental robustness, supporting its role in the next generation of low-carbon, high-performance cement-based materials. • Cement paste micro-prisms were fabricated for uniaxial creep recovery testing. • Limestone filler reduces porosity, enhances hydration, and decreases creep. • Micro-prisms exhibit long-term logarithmic creep and a recovery index of 56%. • RH and T increase creep, while limestone filler mitigates their effects.