Improvement of mineral admixtures on the properties of fluoroaluminic acid shotcrete and fluoride ion leaching behavior

Abstract

Fluoroaluminate accelerators have been widely adopted in engineering applications due to their excellent setting acceleration performance, cost-effectiveness, and satisfactory stability. However, their adverse effects on the mechanical properties of shotcrete and the potential risks associated with fluoride ion leaching remain unresolved, posing threats to structural durability and environmental safety. In this study, three mineral admixtures (fly ash, slag powder, and silica fume) were incorporated at different replacement levels (0 %, 5 %, 10 %, and 20 %) with fluoroaluminate accelerators to fabricate shotcrete. The investigation focused on evaluating the effects of mineral admixtures on enhancing the mechanical properties of shotcrete and reducing fluoride ion leaching concentration. Isothermal calorimetry was employed to analyze the influence of mineral admixtures on the hydration kinetics of shotcrete. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP) were utilized to characterize the hydration product composition and internal pore structure of the shotcrete. The results demonstrated that mineral admixtures synergistically interact with fluoroaluminate accelerators to further shorten the setting time and improve the 28 d compressive strength retention rate. Specifically, the incorporation of 20 % silica fume increased the 1 d compressive strength from 8.6 MPa to 9.3 MPa, while the addition of 10 % slag powder enhanced the 3 d compressive strength by 18.4 %. Mineral admixtures were found to effectively reduce the fluoride ion leaching concentration of shotcrete, with slag powder and silica fume demonstrating the most significant improvements in the environmental safety of shotcrete leachate. Furthermore, this study elucidated the mechanisms by which mineral admixtures enhance the mechanical performance and environmental safety of shotcrete through the integration of microscopic analysis results.