Amorphous silica has many applications due to the material's high surface area and pozzolanic properties. In this work, amorphous silica was synthesised from cogon grass (Imperata cylindrica) by combustion at 650 °C. Kinetic parameters of the combustion were determined by fitting thermogravimetric analysis (TGA) data to the Flynn–Wall–Ozawa model, where the corresponding activation energy and frequency factor are 202.5 ± 5.1 kJ mol^− 1 and (7.9 ± 6.7) × 10¹⁵ s^− 1. Energy-dispersive X-ray (EDX) analysis revealed that cogon grass has a relatively low silica content (2.9 wt.%). In addition, X-ray diffraction (XRD) data showed that the purity and amorphicity of silica obtained are significantly affected by the potassium content in the weed. However, the process of acid leaching removed the impurities and increased the purity of silica from 31.4 wt.% to 83.4 wt.%. The primary particle size (a) and the mass fractal dimension (D) of silica particles determined by small-angle X-ray scattering (SAXS) were 19.8 nm and 2.4, respectively.