Ultrasonic sound speed measurement as method for the determining the hydration degree of gypsum

This article addresses the sound velocity through slurries as well as non-porous and porous materials. The focus is on using the sound velocity for the microstructure prediction of porous materials, especially gypsum plasterboards, during and after hydration. For a slurry, the model of Robeyst et al. [1] showed a good agreement with experimental data when taking into account an air content of 10 ml per kg of hemi-hydrate. This model takes into account the bulk moduli of the continuous (fluid) and discontinuous (solid) phase as well as the size and shape of the solid particles. The bulk modulus of the fluid is corrected for the presence of entrapped air. For gypsum materials, the best agreement was found between the experimental and theoretical values using a series arrangement according to Ye [2] with a solid sound velocity (cs) of 6800 m/s. Finally, the sound velocity during the hydration of gypsum is studied. The use of linear relation between the amount of hydration-product (gypsum) formed and sound velocity gives a reasonable result. Furthermore a relation between initial volume fraction hemihydrate and hydration time is shown.