Joint effects of fly ash fineness and morphology on properties of cement-based materials

This study investigates the joint effects of fly ash fineness and particle morphology on cement-based material properties. Raw fly ash was subjected to gradient ball milling, producing three ground fly ash (GFA) samples with increasing fineness, which were systematically compared to a commercial grade I separated fly ash (SFA). The results reveal a strong coupling effect during grinding: while intensive milling reduced median particle diameter by 67.6%, it severely degraded spherical morphology, lowering spherical particle content to 19.35%. Rheologically, finer GFA improved mortar fluidity by 11.0% and significantly reduced cement paste yield stress and plastic viscosity by 73.3% and 39.1%, respectively. However, SFA demonstrated superior rheological performance at similar fineness due to higher sphericity, underscoring the critical role of particle morphology. Krieger–Dougherty modelling further confirmed that severe morphological degradation in GFA hinders its theoretically predicted viscosity reduction. Conversely, ball-milling enhanced hydration activity, reflected in an 8.3-percentage-point increase in seven-day strength activity index and a 4.6-percentage-point reduction in 28-day porosity compared to SFA. In conclusion, while fineness and morphology impart distinct functional contributions, they jointly govern the overall performance of cement-based materials, offering a scientific basis for selecting appropriately processed fly ash in engineering applications.