Performance Influence and Mechanism Analysis of Liquid Accelerator Prepared with Different Waste Fluorined Acids

This study was conducted to valorize fluoride resources from phosphate ore in cement-based materials. Low-alkali and alkali-free liquid accelerators were prepared from two typical fluorine-containing waste acids: fluorosilicic acid and hydrofluoric acid. Their fundamental properties, including setting time and hydration promotion mechanisms, were examined. The findings demonstrated that all acid-derived accelerators effectively shortened the cement setting time, particularly in the alkali-free formulations. Regarding mechanical performance, the low-alkali accelerator prepared with hydrofluoric acid improved the early age compressive strength of the mortar more effectively than the alkali-free version prepared with the same acid. The accelerator synthesized from fluorosilicic acid was found to yield the highest compressive strength. The underlying mechanism was explored by measuring the early hydration heat release via isothermal calorimetry. Furthermore, XRD, TG, and SEM were utilized to analyze the composition and microstructure of the hydration products. A hydration model was proposed to investigate the effect of the fluorine-containing acid type on the accelerator performance.