Ultrasonic sound speed of hydrating calcium sulphate hemihydrate; part 2, the correlation of sound velocity to hydration degree
In this article the sound velocity through a mix is correlated to the hydration degree of 
the mix. Models are presented predicting the sound velocity through fresh slurries and 
hardened products. These two states correspond to the starting and finishing point of the 
hydration process. The present research shows that a linear relation between the amount 
of hydration-product (gypsum) formed (Smith et al., 2002) and sound velocity can be 
used to describe this process. To this end, the amount of hydration-product formed is 
determined by the using the equations of Schiller (1974) for the hydration process and 
of Brouwers (2010) for the volume fractions of binder, water and hydration products 
during the hydration process. The presented model shows that the induction time and 
gypsum growth rate are linear related to the water/gypsum-ratio.
Hydration kinetics of calcium sulphate hemihydrate: A comparison of models
The kinetic equations derived by Schiller, Ridge, Taplin and, Combe, Smith and Braden, for the hydration of calcium sulphate hemihydrate, based on crystallisation theory, are compared in order to establish which model describes the reaction best. Schiller's, Ridge's and Combe's models show considerable departures from the data, whereas Taplin's model fits the data extremely well. A simple Product model also fits well if the rate is assumed to be temperature dependent.
Hydration of Calcium Sulphate Hemihydrate
IN Nature of October 3, 1964, M. J. Ridge discusses the hydration of calcium sulphate hemihydrate on which he modifies his previous views. The mechanism proposed is somewhat similar to that put forward by me at the fourth international symposium on the “Reactivity of Solids”1 and published in greater detail in subsequent communications2–4. However, Mr. Ridge's theory remains open to several objections.