Effect of phosphoric acid content on the microstructure and compressive strength of phosphoric acid-based metakaolin geopolymers

The phosphoric acid-based metakaolin geopolymers were prepared by regulating H₃PO₄/Al₂O₃ ratios. X-ray diffraction (XRD), thermogravimetry and differential scanning calorimeter (TG-DSC), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to determine the reaction process and phase formation. The results showed that the metakaolin calcined from Kaolinite mainly consisted of quartz crystalline phase and amorphous phase. The diffraction peak for quartz obviously became lower with the increasing of H₃PO₄/Al₂O₃ ratios. The excessive quartz from metakaolin did not totally take part in the chemical reaction. The polymeric structure of -P-O-Si-O-Al-O constitutes the main building block of phosphoric acid-based metakaolin geopolymeric structure. The optimized compressive strength was 29 ± 2 MPa with H₃PO₄/Al₂O₃ molar ratio = 1.3:1. The simulation of the total deformation under 29 MPa load and the total heat flux at 1400 °C of the phosphoric acid-based metakaolin geopolymers with H₃PO₄/Al₂O₃ molar ratio of 1.3:1 based on finite element method verified the failure mechanism and the excellent thermal stability at high temperature.