MICRO FT-IR STUDY OF THE HYDRATION-LAYER DURING DISSOLUTION OF SILICAGLASS

Hydrothermal hydration of silica glass was experimentally studied with pure water at temperatures of 400-500 degrees C and pressures of 40-9 6 MPa. The dissolution of silica glass occurred at nearly constant rat es based on the weight loss and the hydrated surface layers have optic al anisotropy. The thickness of the hydration layer increased graduall y during the initial stage of the reaction and reached a steady state thickness (0.1 mm) after tens of hours. Concentration-depth profiles o f both OH and H2O species in the hydration layer were obtained using m icroscopic Fourier transform infrared spectroscopy (micro FT-IR). The concentration of the OH group was 0.4-1.3 wt% and that of H2O was abou t one third of the OH group near the surface. These concentrations dec reased exponentially from the surface of the silica glass inward, indi cating diffusion-related processes. Overall reaction rates are interpr eted to be determined by diffusion of water in the hydration layer. Th e diffusion coefficients are estimated to be 10(-15)-10(-14) m(2)/s an d exhibit positive correlations with pressure and temperature. The act ivation energy was approximately 60 kJ/mol. Copyright (C) 1997 Elsevie r Science Ltd.