Life cycle assessment of geopolymer concrete using fly ash–slag binder and waste glass-derived alkali activator

The environmental and economic drawbacks of conventional alkali activators in geopolymer concrete (GPC) have motivated the development of more sustainable alternatives. This study evaluated the environmental sustainability of using waste glass-derived sodium silicate (WGSS) as a substitute for conventional sodium silicate (CSS) in fly ash–slag-based GPC, applying life cycle assessment to compute sustainability and economic indices. ISO 14040 procedures were followed, covering goal and scope definition, life cycle inventory, life cycle impact assessment and interpretation. Input data included material and energy consumption, while outputs covered emissions and transport impacts. Four mixes were considered, including one CSS control and three WGSS mixes, comprising one fly ash–only formulation and two mixes with increasing slag content. The analyzed impact categories included compressive strength, embodied energy (EE), global warming potential (GWP), transportation impact (Ti), sustainability index (SI), economic index (EI) and CO 2 abatement cost (CAC). Substituting CSS with WGSS reduced EE (up to 6%), GWP (up to 31%) and Ti (up to 9%) due to waste valorization and local sourcing. Despite substantially lower compressive strength than the CSS reference, WGSS mixes maintained favorable environmental and economic performance. When benchmarked against literature-derived CSS mixes of comparable strength classes, WGSS mixes demonstrated lower GWP and competitive SI and EI values, with negative CAC observed for the most balanced mixes. Sensitivity analysis further confirmed the robustness of these results under variations in transport distance and emission factors. These findings, therefore, highlight WGSS’s potential as a low-carbon activator, thereby offering guidance for sustainable GPC mix design.