Sustainable geopolymer synthesis catalyzed by hexafluorosilicic acid: A low-energy approach using phosphate industrial waste

Abstract

This study investigates the utilization of hexafluorosilicic acid (AFS), a by-product of the phosphate industry with negative environmental impacts, as a catalyst in the synthesis of acid-based geopolymers at room temperature. Specifically, the research focuses on the acceleration of the acid geopolymerization reaction to produce phosphoric acid-based geopolymers and examines the influence of varying AFS concentrations on the geopolymerization process, microstructural properties, and mechanical strength. The experimental approach includes quasi-isothermal DSC analysis, temperature monitoring of geopolymer paste over time, vicat automatic tests, compressive strength, FTIR, DRX, SEM, and EDX. Results indicate that geopolymers prepared without AFS remained unconsolidated even after three days at room temperature. In contrast, adding AFS reduced the setting time to as little as 18 min with 7 % AFS by weight of the paste, demonstrating a significant reduction in setting time from several days to few minutes. Isothermal DSC and internal temperature monitoring of the geopolymer paste during setting revealed that minimal AFS additions (1%–5%) effectively accelerate the geopolymerization kinetics by catalyzing the highly exothermic second step, thus enhancing the subsequent steps of geopolymerization. However, precise control of AFS concentration is crucial, as insufficient amounts do not fully catalyze the reaction, while excessive AFS causes a rapid temperature rise (up to 108 °C in less than 10 min), hindering the initial dissolution step and leading to incomplete aluminosilicate source dissolution. Compressive strength tests showed that adding 5 % AFS at room temperature increased strength by 87 % compared to samples without AFS, which required 60 °C for 14 MPa. However, strength decreased with AFS concentrations above 5 %. After 28 days, a 25 % increase in strength was observed compared to 7-day samples, highlighting that most strength development occurs within the first 7 days, while microstructural analyses confirmed that AFS serves as a catalyst without altering the crystal phase or the geopolymer network. This study underscores the potential of AFS to significantly enhance the performance of acid-based geopolymers, providing a sustainable approach to utilizing an industrial by-product while improving material properties.