Kinetics and mechanism of ultrasonic-assisted magnesium oxide hydration.

The kinetics of ultrasonic-assisted magnesium oxide (MgO) hydration was investigated in the present paper. The degree of hydration at different temperature (298-338K) and reaction time (0.25-3h) was determined by thermal gravity analysis (TGA). And the products of the hydration were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectrometer (FT-IR) and Particle Size Distribution analysis (PSD). A coupling model based on characteristic equations for chemical reaction control and inward diffusion control was employed to describe the hydration process. The experimental data indicated that the degree of hydration at 2h under ultrasound reaction condition was 18-25% higher than that of mechanical stirring reaction condition. However, the value was nearly the same at the first hour. Combined with the analysis results of degree of hydration, characterization of products and model fitting, the hydration process was supposed to be a mixed model, which controlled by both chemical reaction and inward diffusion. A reaction mechanism emphasized on the physical effect of ultrasound was proposed, assuming that the hydration product layer was broken and regenerated during the process. The calculated activation energy of 24.2kJmol-1 corroborated the mechanism proposed in this study.