Alkali-Dissolution-Derived Reactive Elementals Ratios for Relating Precursors Reactivity, Gel Polymerization, and Early-Age Mechanical Performance in Alkali-Activated Materials

Abstract At present, the heterogeneous chemical environments and variable contents of silicoaluminate constituents in diverse industrial wastes present a major challenge for the precise characterization of alkali-activation reaction kinetics and the rational control of alkali-activated material (AAM) properties. To address this limitation, this study proposes the use of reactive Si/Al and Ca/Si molar ratios, which are quantitatively determined through alkaline leaching tests, as key parameters for AAM design and performance regulation. A fly ash-ground granulated blast furnace slag composite system was employed to systematically investigate the effects of reactive Si/Al and Ca/Si molar ratios on early-age reaction kinetics, water state evolution, gel structure development, and mechanical performance. The results show that increasing the reactive Si/Al molar ratio accelerates polymerization, increases the availability of polymerizable silico-aluminate species, and enhances the binding capacity of the gel network for internal water. When the reactive Si/Al molar ratio ≤ 2.61 (Ca/Si ≤ 1), higher reactive Si/Al promotes the formation of Q3–Q4 structures and increases the proportion of bridging oxygen, resulting in highly cross-linked N-A-S-H/C-A-S-H composite gels and superior early-age mechanical performance. In contrast, an increase in the reactive Ca/Si molar ratio favors the formation of chain-like C-A-S-H structures dominated by Q1–Q2 units, which reduces network polymerization. As the reaction proceeds to later stages, the gels further evolve through local structural rearrangement, accompanied by partial ordering and secondary crystalline phase formation. This study provides a more reliable theoretical framework for the rational design and performance control of AAMs derived from multi-source industrial solid wastes.