Influence of activator molarity and waste-glass-to-volcanic-ash ratios on the microstructure of potassium-based alkali-activated pastes

Abstract In this study, binary alkali-activated pastes based on volcanic ash from Mount Etna (Italy) and borosilicate waste glass were synthesized for the first time using potassium hydroxide (KOH) at different molarities (i.e., 7 M and 9 M) and moderate temperature (60 °C). This work aims to define how the reactants involved in the mix design, specifically the solution concentration and solid proportions of the waste precursors, influence the final microstructure and subsequently their physical and mechanical properties. For this purpose, a multidisciplinary approach, including mineralogical, molecular, chemical, and morphological investigations, was applied to elucidate these properties. The physical-mechanical parameters, including density, uniaxial compressive strengths, porosity, pH, and leaching resistance, determined by boiling tests, were quantified. Increasing KOH molarity from 7 M to 9 M contributes to the formation of a more stable Si-O-Si/Al network, enhancing the compressive strength resistance (~21 to 23 MPa) and reducing both weight loss (~7 to 9%) and the open porosity (~20%). The combined effect of higher molarity and waste glass proportion positively influenced the mechanical response, as a result of the formation of a denser and more compact microstructure. Results confirmed that sustainable materials can be produced using potassium-based binders made from volcanic ash and waste glass.