Rheological, Thermal, and Microstructural Behavior of Cementitious Systems with Silica Fume and Fly Ash for Self-Compacting Concrete

This study presents a multiscale investigation of cement-based systems modified with silica fume (SF) and fly ash (FA) for self-compacting concrete (SCC). Cement pastes and mortars with replacement levels of 0%, 5%, 10%, and 15% were evaluated for rheological behavior, hydration kinetics, and microstructural evolution using rotational rheometry, semi-adiabatic calorimetry, scanning electron microscopy, and X-ray diffraction. The results show that SF increases plastic viscosity and promotes structural build-up due to its high specific surface area and pozzolanic reactivity, while its influence on yield stress depends on dispersion conditions and superplasticizer efficiency. In contrast, FA reduces both yield stress and viscosity, improving flowability due to its spherical particle shape. Thermal analysis indicates that SF modifies hydration by reducing and slightly delaying the main exothermic peak at higher dosages, whereas FA mainly lowers the peak temperature with limited effect on its timing. Microstructural analysis reveals that SF contributes to a denser, more homogeneous matrix through pore refinement and increased C–S–H formation, whereas FA systems exhibit a more heterogeneous structure with slower early-age development. The results demonstrate a clear relationship between rheology, hydration, and microstructure. The combined use of SF and FA has been shown to be an effective approach to improving the performance and sustainability of SCC.