Biomass fly ash is a sustainable, eco-friendly cement substitute with economic and performance benefits, being renewable compared to coal fly ash. This study examines using biomass fly ash (BFA) as a sustainable cement substitute, comparing it with Class F fly ash (CFA). With a water–binder ratio of 0.5 and replacement rates of 10%, 15%, 20%, 25%, and 30% (by mass), the research highlights BFA’s promising applications. BFA and CFA were mixed into cement paste/mortar to analyze their reactivity and properties, with hydration products CH and C-S-H evaluated at 7, 28, and 91 days. Compressive strength, micro-pore structure, and drying shrinkage (assessed from 7 to 182 days) were tested. Results showed BFA had similar pozzolanic reactions to CFA at later stages. While compressive strength decreased with higher BFA replacement rates, early-stage performance matched CFA; growth was CFA-10 (18 MPa) and BFA-10 (17.6 MPa). BFA mortars exhibited slightly better deformation properties. BFA-30 cement had superior performance, with a lower drying shrinkage rate of 65.7% from 14 to 56 days compared to CFA-10’s 73.4% and a more stable shrinkage growth rate decrease to 8.4% versus CFA-10’s 6.4% after 56 days. This study concluded that BFA, usable without preprocessing, performed best at a 10–15% replacement rate.

This study investigates the impact of particle size in fly ash derived from different coal sources on the performance of fly ash–cement systems. Utilizing a newly developed flotation classification method, physical properties of fly ash were examined to reveal variations among different particle sizes and coal sources. Thermal analysis was employed to analyze the calcium hydroxide content’s effect on the cement system, while selective dissolution methods were used to assess reaction rates. XRD analysis confirmed particle size effects. Results indicate that flotation classification optimizes the properties of fly ash, enhancing activity and flow values, where some of the ash fractions exhibit overall superior properties. The use of high-volume fly ash (50% fly ash replacement) promotes continued pozzolanic reactions, especially with smaller particle sizes. Reaction rates decrease with larger particle sizes, emphasizing the importance of classification. XRD analysis further supports these findings, revealing that smaller particle sizes favor cement hydration and pozzolanic reactions. Overall, this study provides insights into optimizing fly ash properties for enhanced concrete performance.