Structural and Surface Changes of SiO2 Flint Aggregates under Thermal Treatment for Potential Valorization

Abstract

In order to extend their use, controlled SiOH SiO2 surfaces were fabricated and investigated. A study of the effect of heat treatment on the structural and surface changes of a natural flint SiO2 aggregate subjected to chemical treatment was carried out. The obtained samples were subjected to thermal treatment at three different temperatures: 500, 700, and 1000 °C. The samples were investigated using different techniques. X-ray diffractions (XRD) were used to follow the structure’s evolution with the heat treatment. The decrease in the FWHM of the SiO2-(101) peak showed that the crystalline quality improved upon heating. This result was confirmed by Fourier transform infrared spectroscopy (FTIR). The morphology of the SiO2 samples was characterized using a Variable Pressure scanning electron microscope (VP-SEM), revealing the presence of disordered needles of nanometric sizes (∼500 nm) on the surface of the grains, which could be eliminated by heating at higher temperatures. Furthermore, FTIR spectroscopy also confirmed that heating caused a reduction in OH groups on the surface. Thermogravimetry (TG) was used as the reference method to determine the hydroxyl group content. The OH groups found on the surface of the sample without and with heat treatment at 500, 700, and 1000 °C were 0.83, 0.44, 0.28, and 0.2 mmol/g, respectively. This study allowed us to obtain a controlled SiO2 surface and provides new insights into the use of SiO2 flint surfaces for new applications as a functional filler in polymers/asphalts composites.