Synergistic alkali fusion-acid leaching strategy for silica aerogel synthesis from fly ash and performance optimization

Abstract

This study synthesized lightweight hydrophobic silica aerogel blocks from fly ash using a sol-gel method combined with surface-modified ambient pressure drying. The effects of alkali fusion temperature and hydrochloric acid (HCl) concentration on aerogel properties were systematically investigated. After drying, sieving, and compositional analysis of the fly ash, sodium hydroxide (NaOH) fusion and calcination were conducted under varying conditions, followed by acid leaching with HCl at different concentrations. Experimental results demonstrated that NaOH quantity and calcination temperature directly influenced product yield, while HCl concentration critically affected aerogel structure and performance. Optimal conditions were achieved at a fly ash-to-NaOH mass ratio of 1:1.2 and an HCl concentration of 4 mol/L. Under these parameters, the aerogel exhibited a homogeneous and dense chain-like structure, with a specific surface area of 384.61 m2/g, pore volume of 0.36 cm³/g, and average pore diameter of 3.28 nm. Characterization via SEM, TEM, BET, FTIR, and XPS confirmed a uniform three-dimensional network structure, excellent hydrophobicity, and thermal stability. The proposed inexpensive approach produces silica aerogel with superior properties. The preparation conditions of alkali melting and acid leaching of fly ash also provide guiding assistance for industrial production methods.