This article focuses on the hydration, and associated microstructure development, of β-hemihydrate to dihydrate (gypsum). The sound velocity is used to quantify the composition of the fresh slurry as well as the hardening and hardened-porous-material. Furthermore, an overview of available hydration kinetic and volumetric models for gypsum is addressed. The presented models predict the sound velocity through slurries and hardened products. These states correspond to the starting and ending times of the hydration process. The present research shows that a linear relation between the amount of hydrationproduct (gypsum) formed and sound velocity (Smith et al., J Eur Ceram Soc 22(12):1947, 2002) can be used to describe this process. To this end, the amount of hydrationproduct formed is determined using the equations of Schiller (J Appl Chem Biotechnol 24(7):379, 1974) for the hydration process and of Brouwers (A hydration model of Portland cement using the work of Powers and Brownyard, 2011) for the volume fractions of binder, water and hydration products during the hydration process.
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