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.
Kinetics of hydration of calcium sulphate hemihydrate
The kinetics of hydration of α-CaSO4·0.5H2O has been studied by calorimetry, analysis of the liquid phase, measurement of the combined water, and electron microscopic observations. Hemihydrate particles dissolve immediately after mixing with water and subsequently form hydrated layers on their surfaces by intrusion of water molecules, which grow to a thickness of ca. 0.09 µm according to a parabolic law and then undergo destruction, when the dihydrate nuclei are generated for the first time. At this point in time, the induction period terminates, and subsequent hydration of hemihydrate proceeds in accordance with the previously proposed rate equation based on the dissolution–crystallization mechanism.
On the cause of the anomalous setting behaviour with respect to temperature of calcium aluminate cements
Abstract As the temperature of hydration of monocalcium aluminate is increased, the setting time becomes longer until it reaches a maximum value at 28°C. Above this temperature, the setting time decreases rapidly with increasing temperature. Various explanations for this phenomenon are critically reviewed and most are shown to be unable to explain all the observations that have been reported. It is proposed that the anomalous setting behaviour is because none of the hydrated calcium aluminates can form readily in the range 25–30°C. The nucleation of CAH 10 is no longer favoured on thermodynamic grounds, whilst the rate of formation of C 2 AH 8 is very sluggish, and C 3 AH 6 can only be formed via conversion of these metastable hydrates in this temperature range. Hence precipitation of the hydrate crystals to produce a set cement takes longer than at other temperatures.