The stability of formed CaF2 and its influence on the thermal behavior of C–S–H in CaO–silica gel waste-H2O system

Abstract

In this work, the stability of formed CaF2 and its influence on the thermal behavior of C–S–H in CaO–silica gel waste-H2O system were investigated. The hydrothermal synthesis of calcium silicate hydrates was performed at 200 °C, when the duration of synthesis and the molar ratios of the primary CaO/SiO2 mixtures were 0.55 (16 h), 0.66 (48 h), 0.83 (48 h), and 1.0 (48 h). Later on, synthesized products were calcined in a 600–1000 °C temperature range for 30 min. It was determined that fluoride ions, from silica gel waste, participated not only for the formation of CaF2, but also intercalated into the structure of products during hydrothermal synthesis and, also, exchanged the formation sequence of new compounds. It was found that, during calcination, the chemical composition of synthesized C–S–H affects the stability of formed CaF2, because it remained stable in the system with C/S molar ratio of 0.55 and 0.66. Meanwhile, in the system with higher C/S molar ratio of 0.83 and 1.0, CaF2 fully recrystallized into cuspidine. Moreover, it was estimated that the recrystallization processes and the formation of new compounds influenced the morphology of calcined products. In addition, EDX analysis showed that 9.05 mass% of fluorine are present in the crystal structure of calcination products. The obtained results were confirmed by the XRD, STA, FT-IR, and SEM/EDX analysis data.