Optimisation of activator solutions for geopolymer synthesis: Thermochemical stability, sequencing and standardisation

Geopolymers present a sustainable alternative to conventional binders: however, their commercial viability is limited by a lack of standardised methods for preparing stable activator solutions – alkaline feedstocks critical to geopolymer synthesis. This study combines quantitative 29 Si NMR, thermodynamic stability analyses and solubility modelling to identify optimal preparation conditions that minimise irreversible precipitation risks, optimise stabilised mixing periods and improve reproducibility. Three key findings emerge. First, activator solutions can be prepared and used within 1 – 1.5 minutes (∼ 1 minutes for hydroxide stabilisation and seconds for silica speciation equilibration), in contrast to the ≥ 24-hour equilibration periods often cited in literature. Secondly, solution stability is strongly temperature-dependent with higher solution temperatures promoting thermochemical stability and precursor reactivity, while cooling increases solution viscosity and drives solutions into instability (which may necessitate discarding), as defined by its unique “time-stability window” . Thirdly, the order in which feedstocks are combined critically affects whether a solution becomes unstable, with the optimal sequence of water → alkali-hydroxide → soluble silicate found to ensure greater process reliability. A predictive model and accompanying visual tools enable practitioners to assess solution viability and define stability windows by quantifying initial and final/unstable periods and temperatures based on feedstock composition and solution temperature. These results challenge common preparation practices, contribute to improved reproducibility and quality assurance / quality control (QAQC) in geopolymer research and represents a step toward developing standard operating procedures for geopolymer activator synthesis. • Experimentally validated model quantifies thermochemical stability in activator solutions • Solubility model predicts time-temperature windows for optimised solution stability • Higher temperatures enhance thermochemical stability, cooling increases precipitation risk • Feedstock sequencing affects stability: H 2 O → NaOH → silicate is preferred • Model enables reproducible synthesis and supports SOPs for geopolymer mixing