Role of fly ash as fine aggregate replacement in the properties of concrete: A comprehensive study of rheological, mechanical, and microstructural properties

Replacing natural river sand with fly ash (FA) in the manufacturing of green concrete could address various environmental problems. Eleven types of concrete were prepared with FA as fine aggregate (CFA) instead of river sand (0 %, 10 %, 20 %, 30 %, 40 %, 50 %, 60 %, 70 %, 80 %, 90 %, 100 %) for relevant measurements. The measured properties include the rheology, water absorption, dry unit weight, compressive, splitting tensile, and flexural strengths, modulus of elasticity, and axial compression. Scanning Electron Microscopy (SEM), Mercury Intrusion Porosimetry (MIP), and air-void characteristics were analysed to elucidate the reinforcement mechanism of CFA. The findings indicate that substituting fine aggregates with FA significantly increases the yield stress and plastic viscosity of the concrete. The green concrete produced demonstrated mechanical properties that satisfy performance requirements, with an optimal FA dosage of 40 %. Additionally, a constitutive model was established for CFA under compression. From a microscopic perspective, the filling and volcanic ash effect of FA promote the formation of Aft, enhancing matrix density and improving the strength of CFA. However, at higher FA substitution rates, an accumulation of unreacted FA particles leads to reduced porosity. The air content, spacing factor, and average chord length of the CFA show a trend of first increasing and then decreasing with increasing FA dosage. An appropriate FA dosage resulted in smaller, more stable air bubbles with a uniform distribution in the CFA system.