Crystal growth of hydrated calcium silicate synthesized from fly ash and lime milk at 100 °C

Abstract

To maximize the high-value application of fly ash, this study investigates the incorporation of amorphous silica derived from fly ash as a silicon source and lime milk as a calcium source into microporous calcium silicate powders through dynamic hydrothermal synthesis at 100 °C for a duration of 2 h or less. The products were collected at various synthesis intervals and analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), and thermogravimetric differential scanning calorimetry (TG-DSC). Results indicate that microporous calcium silicate forms through a dynamic reaction that disrupts Si-O bonds, enhancing the mobility of silica-oxygen tetrahedra and generating H₂SiO₄ ²⁻groups. Ca²⁺ ions also interact with these bonds, resulting in Q1 and Q2 forms of silica-oxygen tetrahedra. Phase transformation of calcium silicate at various intervals was noted, beginning with 3CaO·2SiO₂·3H₂O, shifting to 2CaO·3SiO₂·2.5H₂O, and finally to CaO·2SiO₂·2H₂O at 90 min. Thus, microporous calcium silicate evolves from an amorphous C-S-H gel into a crystalline form, featuring diverse calcium silicate minerals with calcium-silicon ratios and particle sizes between 10 and 20 μm, with sizes increasing until the 80-min.