Investigation of Phosphate based geopolymers formation mechanism

In the present research, phosphate-based geopolymers were prepared from metakaolin and commercial phosphoric acid H₃PO₄. The paper aims to observe and identify the different chemical early steps of the material formation mechanism, as well as the kinetics of each step. To achieve this goal, an investigation upon the reaction with respect to time has been fulfilled, using different techniques such as atomic absorption measurements, in situ quasi-isothermal Differential Scanning Calorimetry (DSC), pH-metry measurements, Scanning Electron Microscope (SEM) with Energy-dispersive X-ray (EDX) analysis, X-ray diffraction, magic angle spinning Nuclear Magnetic Resonance (MAS-NMR) and Fourier Transform Infrared spectroscopy (FTIR). As a result, a composite structure composed of amorphous aluminum phosphate phases and amorphous silica phases dispersed in a geopolymeric amorphous structure of (-Si-O-Al-O-P-) units with the presence of some terminal-Si-O-P units, was observed. Furthermore, the techniques combination suggested a geopolymeric mechanism composed principally of three great processes: the first one stands for the dealumination process. It was proved that this process is divided into two elementary overlapping steps. The second one corresponds to the condensation process. It was proved that this process is divided into four types of condensation, according to the chemical nature of the reactant units as well as that of the obtained ones. Eventually, the last one represents the polycondensation step, resulting in the finally polymerized 3D structure. In addition, this work monitored the duration and the kinetics of each geopolymeric step.