Phosphoric acid based geopolymerization: Effect of the mechanochemical and the thermal activation of the kaolin

Abstract The purpose of this work was to study the effect of the mechanical activation of kaolin on the mechanical properties, the microstructure and the structure of the phosphate-based geopolymers and to compare these properties to those of the same materials but obtained from the thermally treated kaolin. Several experimental techniques, such as powder X-ray diffraction (XRD), 27Al, 29Si and 31P Magic angle spinning nuclear magnetic resonance (MAS-NMR), infrared spectroscopy (FTIR), scanning electron microscopy (SEM), as well as the determination of the specific surface and the particle size distribution were adopted to characterize the raw materials and then the obtained geopolymeric materials. The Brazilian tests were used to measure the materials mechanical resistance. The main findings of this work were that the two types of treatment allowed to obtain reactive kaolin for the synthesis of phosphate-based geopolymers. However, some fundamental structural differences were observed between the mechanically activated and the thermally activated kaolins. Indeed, the mechanically activated kaolin (MA) seemed to be more reactive than the thermally activated Kaolin (TA). Hence, the kinetic of the geopolymerization reaction would be more accelerated in the mechanically activated kaolin (MA) and phosphoric acid mixture. Consequently, the obtained phosphate-based geopolymer (GMA) would be denser and of better mechanical properties than those based on thermally activated kaolin (GTA) regardless of the curing time.