High yield of large scale SiO₂extraction from fly ash - Structure and physical properties

Nanosilica (n-SiO2) constitutes one of nanofillers that acts as nucleation centers for self-healing concrete contributing to the acceleration of hydration process as well as improving the compactness of concrete. Source of n-SiO2 varies, for example, from rice husk, fly ash, and silica sand. In this work, we present the efficient extraction route of n-SiO2 from the fly ash which contains 49% of SiO2. Further, some characterizations of n-SiO2 were carried out including surface morphology by Scanning Electron Microscope and functionality by FTIR spectroscopy, crystal structure by XRD, particle size distribution by dynamic light scattering, and the water absorption capacity. The developed extraction route in brief can be described as follows: (1) acid leaching using 0.5M citric acid solution (1:3 %wt) under 600 rpm stirring for 1h, (2) alkali treatment using 4 M sodium hydroxide solution (2:9 %wt) under 1000 rpm stirring at 90°C for 2 h, and (3) titration using 10 M citric acid forming silica hydrosol. The yield of n-SiO2 is found 81% at the highest. The resultant n-SiO2 is amorphous (92.14%) where the crystalline impurities root from trisodium citrate and sodium chloride. The particle size is 4.93 nm in which the crystal structure indicates lattice strain of 0.034 with dislocation density of 4.12·1012 cm-2. IR spectroscopy reveals that high absorption of hydroxyl (-OH) and aliphatic carboxylic groups may facilitate rapid hydration and formation of calcium silicate hydrate (C-S-H). The n-SiO2 also shows large water absorption capacity of up to 49.56 ± 13.81 g/g.