One-part binders from woody biomass fly ash and blast furnace slag: reaction mechanisms and microstructural evolution

Woody biomass fly ash (WBFA) is the main by-product of woody biomass energy production. However, its use in cementitious materials remains limited due to its low intrinsic reactivity, largely associated with the scarcity of aluminosilicate phases. At the same time, the high alkalinity and sulphur content in WBFA make it a promising component for formulating cement-free binders without additional chemical activators, when combined with highly reactive precursors. This study investigates the reaction mechanisms and microstructural evolution of binders based on WBFA and ground granulated blast furnace slag (BFS), with the aim of elucidating their synergistic interactions and optimizing performance. Binary pastes with varying WBFA/BFS ratios mixed with water were prepared and characterized by isothermal calorimetry, pore solution analysis, XRD, FTIR, TGA, SEM-EDS, and MIP. The results show that, although increasing WBFA content initially delayed hydration by limiting the dissolution of reactive species, it markedly enhances long-term reactivity and strength through sustained release of alkali and sulphate. The main hydration products are C-(A)-S-H gels, ettringite, Friedel's salt, and hydrotalcite, with their amount and assemblage strongly governed by the WBFA/BFS ratio. Reaction kinetics analysis and thermodynamic modelling confirm the dual role of WBFA as both a reactive precursor and internal alkali/sulphate activator. Among the formulations studied, the mixture with a WBFA/BFS ratio of 50:50 exhibited the best overall performance, achieving the highest compressive strength and lowest porosity. These findings clarify the reaction mechanisms in WBFA-BFS binary pastes, providing practical guidance for designing WBFA-based, cement-free binders for sustainable construction applications.