Leaching behavior and environmental safety evaluation of fluorine ions from shotcrete with high-fluorine alkali-free liquid accelerator.

High-fluorine alkali-free liquid accelerator (AF, F concentration was about 2.31g/L) was still used in engineering because of its low cost, excellent stability, and coagulation-promoting effect. The main purpose of this study was to explore the leaching behavior of fluorine ions in shotcrete for tunnel lining with high-fluorine alkali-free liquid accelerator and whether there was fluoride pollution. The setting time and mechanical properties of cement paste and mortar with AF were tested. Under different environmental conditions, F leaching concentration from sprayed concrete was studied comparatively. Moreover, XRD and SEM were used to analyze the crystal composition and micro morphology of hydration products. The experimental results showed that with the increase of AF dosage, the setting time of cement paste was greatly shortened, and later strength of mortar and shotcrete could meet the construction requirements. In addition, when the leaching solution type (including NaCO, NaPO, NaSO, and NaNO) and testing conditions (particle size, soaking temperature, leaching solutions) were different, F leaching concentration changed regularly, and the minimum value was more than 20 mg/L, which might cause fluorine pollution to groundwater and soil. After shotcrete samples were soaked, the CaF peaks' intensity was relatively weaker and Ca(OH) decreased obviously. Meanwhile, cement hydration products became looser and abundant of flaky C-S-H gel transformed into fibrous and chained structure.