Upcycling of natural volcanic resources for geopolymer: Comparative study on synthesis, reaction mechanism and rheological behavior

The present study prepared geopolymers based on two kinds of volcanic ash, which were mined from Shanxi Province (central China) and Jinlin Province (northeast-China) and denoted as S1 and S2, respectively. The two different volcanic ash were characterized by quantitative X-Ray Diffraction (XRD), X-ray fluorescence (XRF), and particle size analysis. The geopolymer pastes were obtained with the two volcanic ash and 6, 8, and 10 mol/L of sodium hydroxide solution. The mechanical and rheological properties were tested, and the results were correlated with microstructural analysis performed by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), Fourier Transform Infrared Spectrometer (FTIR Spectrometer), and ²⁷Al magic angle spinning-nuclear magnetic resonance (²⁷Al MAS-NMR). The results showed that the two kinds of volcanic ash have similar chemical composition while S2 has a higher content of the amorphous phase and smaller median particle size compared with that of S1. S2 has higher geopolymerization activity than S1. The maximum 72-hour compressive strength of the S1 and S2 geopolymer 7.8 MPa and 44.8 MPa, respectively, was acquired when 10 mol/L and 8 mol/L NaOH solution were used. It can be concluded from the results of microscopic experiments that the less content of the amorphous phase, coarser particles, and side reaction between unreacted NaOH and CO₂ in the air are the main causes of the lower mechanical strength of S1-based geopolymer. The rheological experiment showed the flow curves of both types of volcanic ash conform to the Bingham model. The yield stress and plastic viscosity coefficient of S2 are higher due to the finer particle size.