One-part hybrid alkali-activated binder using gasification forest fly ash as alternative activator at room temperature

The limited adoption of conventional alkali-activated materials (AAMs) in the construction industry is primarily attributed to their dependence on high dosages of chemical activators, the handling and safety challenges associated with highly alkaline solutions, and the requirement for elevated curing temperatures. This study introduces a one-part hybrid alkali-activated binder incorporating gasification forest fly ash (GFF) and ground granulated blast furnace slag (GGBS), produced without external alkaline activators and cured at room temperature, achieving physical and mechanical properties within a practical paste-scale (binder) performance range under the tested conditions. Five AAM formulations were prepared: two control mixes containing only GGBS (one with an alkaline solution and the other without) and three hybrid binders in which GGBS was partially replaced with GFF at 20 %, 40 %, and 60 %, using water as the sole liquid component. The specimens were characterised through solubility, setting time, physical, microstructural, mechanical, and leaching analyses. Water solubility tests showed that GFF released K⁺, Na⁺, and Ca²⁺ ions, generating sufficient alkalinity (high pH) to activate GGBS and accelerate reaction kinetics, thereby reducing setting time. The GGBS60 (40 % of GFF) hybrid binder achieved compressive and flexural strengths of 32.93 MPa and 5.62 MPa, respectively, after 28 days of curing, showing consistent strength development and higher 28-day strength than the NaOH reference under ambient curing conditions. SEM–EDS analyses confirmed the formation of C–S–H and C–A–S–H gels, which contribute to the mechanical strength and microstructural densification of the hybrid matrix. Furthermore, leaching results demonstrated compliance with European heavy metal leaching regulations. Overall, the results demonstrate paste-scale feasibility of using GFF as both an alternative solid alkali source and a secondary precursor in a one-part GFF–GGBS binder, without using additional alkaline activator and meeting the applicable European leaching limits under laboratory conditions, and additional validation in mortar and concrete, including fresh behaviour and durability, is recommended to support future engineering assessment.