Strength, microstructure, efflorescence behavior and environmental impacts of waste glass geopolymers cured at ambient temperature

In this study, waste glass powder (d50 = 15.4 μm) obtained by crushing soda lime glass bottles was investigated as a raw material for geopolymer cement. The waste glass (WG) powder and class C fly ash (FC) were mixed at varying ratios (100:0, 75:25, 50:50, 25:75, 0:100) and activated by sodium hydroxide solutions of different concentrations (0 M, 2.5 M, 5 M, 7.5 M, 10 M). The geopolymer pastes were cured at ambient temperature and the compressive strengths at increasing curing ages were tested. The mechanical properties were analyzed with respect to the kinetics and Si/Al ratio of the reaction system. The experimental results suggested that the WG mostly acted as an inert filler at early ages due to the slow reaction rate. However, the strength performance improved significantly after the curing age of 14 days, which was attributed to the reaction between WG and FC. The geopolymer made by mixing WG and FC at a ratio of 1:3 (Si/Al = 3.038) and 5 Mol NaOH solution achieved a compressive strength of 34.5 MPa. The microstructure and minerology of the geopolymers analyzed by the SEM-EDX and XRD suggested the coexistence of sodium aluminate silicate hydrate (N-A-S-H) gel and calcium aluminate silicate hydrate (C-A-S-H) gel in geopolymer matrix. The results of leaching tests indicated the potential environmental impact caused by the leaching of excessive alkali, which should be carefully managed during the mix design. The sustainability analysis including the embodied energy and carbon footprint confirmed the environmental friendliness of WG geopolymer. This study exhibited the feasibility of producing WG geopolymer concrete at ambient temperature and the potential synergy between the waste glass recycling and geopolymer industry.