Structural performance of reinforced fly ash and slag-based geopolymer concrete beams using waste glass-derived sodium silicate as an alternative activator

Abstract

The pursuit of sustainable construction materials has advanced geopolymer concrete (GPC) as an alternative to Portland cement systems. However, most GPC systems rely on commercial sodium silicate (CSS), whose high cost, energy demand and environmental impacts limit its large-scale sustainable application. This study therefore investigated the structural performance and sustainability of reinforced fly ash–slag (FA–GGBFS) GPC beams using sodium silicate synthesized from waste glass (WGSS) as an alternative activator to CSS. Mix designs derived from oxide composition analysis of the constituent materials were proportioned to produce medium-strength structural GPC and enable comparison between CSS and WGSS activator systems. Mechanical characterization, reinforced beam testing under three-point loading and life cycle assessment (LCA) following ISO 14040 were conducted, with structural performance validated against Eurocode 2. The WGSS concrete mixes with FA:GGBFS ratios of 1:0, 7:1 and 3:1 achieved 28-day compressive strengths of 26.64, 40.17 and 45.74 MPa, respectively, while the control CSS concrete (FA:GGBFS = 1:0) reached 94.75 MPa. Among the WGSS formulations, the 7:1 mix demonstrated the most balanced structural, environmental and economic performance. Beams produced with this mix achieved a flexural moment capacity of 11.56 kNm, comparable to the 11.46 kNm recorded for the CSS control beam, while exhibiting greater mid-span deflection (7.2 mm vs 5.3 mm), indicating improved ductility despite the lower compressive strength. LCA further showed reductions in carbon emissions (~31%) and production cost (~10%) relative to the CSS system. These results demonstrate that WGSS-based GPC can achieve adequate structural capacity relative to design predictions while providing notable environmental and economic advantages for sustainable infrastructure applications.