Influence of the metakaolin mass fraction in sodium hydroxide solution on the synthesis of silica gel from metakaolin

Abstract This study investigates the influence of metakaolin mass fraction in sodium hydroxide solution on the structure of silica gels produced by the sol-gel method. The effect of this ratio was analyzed using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and differential scanning calorimetry coupled with thermogravimetry (TG-DSC). The results revealed that after calcination at 650 °C, there is a complete conversion of kaolinite to amorphous metakaolinite. XRD analyses of the gels show a characteristic halo of amorphous silica with some crystalline reflections mainly associated to halite. The evaluation of this influence indicates that better dispersion of metakaolin promotes interactions that lead to the hydrolysis of sodium silicate and grain growth. At constant soda solution volume, the increase of metakaolin mass fraction reduced the formation of halite as by-product while a starting crystallization toward hydroxysodalite like structure is observed. This was associated to the formation of short chains oligomers within the gel network as the metakaolin mass increase. These short chains exhibit more hydroxyl groups, which are beneficial for optimizing the retention of polar compounds such as water.