Effect of Fluoride in Alkali-Free Liquid Accelerator on Corrosion of Reinforced Shotcrete by Chloride Ion.

Shotcrete is a crucial component of tunnel engineering. To investigate the impact of fluoride-containing alkali-free liquid accelerators on the structural safety of shotcrete, this study prepared shotcrete using three different fluoride-based alkali-free liquid accelerators: magnesium fluosilicate, fluosilicic acid, and hydrofluoric acid. The corrosion of steel rebar within the shotcrete was examined and compared with that of shotcrete prepared using a fluoride-free alkali-free liquid accelerator. Analysis of the hydration products and pore structure of the shotcrete revealed that fluoride-containing accelerators inhibited the hydration of CS, increased the content of harmful pores, reduced the pH value, and decreased the chloride ion binding capacity of the shotcrete. As a result, steel reinforcement within the fluoride-containing shotcrete was not effectively protected during the early stages. Detection of corrosion products indicated that shotcrete containing fluosilicic acid and hydrofluoric acid accelerators led to the rapid formation of loose corrosion products, primarily FeO, thereby accelerating the corrosion rate of the steel reinforcement. Among the accelerators studied, hydrofluoric acid posed the most severe threat to the reinforcement, with the open circuit potential reaching -520.2 mV after 24 days of accelerated corrosion testing. Additionally, the mechanisms by which fluorides influence the corrosion of steel reinforcement in shotcrete were explored through the analysis of concrete hydration products and corrosion products on the steel rebar.