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.
Mechanism of re-crystallisation in calcium sulphate hemihydrate plasters
The course of a hydration process is uniquely determined by simultaneous consideration of (a) the balance between the matter dissolving from the original phase and growing on the new phase, (b) the mechanism of mass transport (diffusion), (c) the correlation between the mass deposited on the new particles and its rate of growth and (d) the conservation of mass. An estimate shows that, in the case of neat calcium sulphate plasters, diffusion is quantitatively unimportant as a rate-determining factor and the corresponding simplifications are introduced. As a result, the hydration can be described in terms of one single parameter which depends on the slurry density, the rate of growth constant for the new phase, the difference in solubilities of the two phases, the shape of the new phase, and the effective number of nuclei for the new phase. 
 
 
 
Supporting experimental evidence is supplied and discussed.
MAS-NMR spectroscopy studies in the setting reaction of glass ionomer cements.
OBJECTIVES
The main objective is the characterisation of the setting reaction in glass ionomer cements based on experimental ionomer glasses with different fluorine content and a commercial glass ionomer cement liquid by using 13C CP/MAS-NMR, 29Si, 27Al and 31P MAS-NMR spectroscopy in order to receive information specifically about the cross-linking process.


METHODS
Different fluorine containing glass compositions based on 4.5SiO2-3Al2O3-1.5P2O5-(5-z)CaO-zCaF(2) where z=0-3, were mixed with a commercially available polymer liquid to form glass ionomer cements. The cements were subjected to 27Al, 13C CP/MAS, 29Si, and 31P MAS-NMR analysis.


RESULTS
The 27Al spectra showed clearly the formation of six-fold coordinate Al(VI), that may crosslink the carboxyl groups in the poly-acid molecules. A shift towards to more positive values of the carboxyl peak in the 13C CP/MAS-NMR spectra showed clearly the proton dissociation of the carboxyl groups. A shift towards more negative values was observed in the 29Si MAS-NMR spectra, suggesting formation of hydrated silica gel and consequently formation of additional Si-O-Si bonds. 31P MAS-NMR spectra also reflected changes in the coordination state around a PO4(3-) tetrahedron. Increasing the fluorine content of the glasses resulted generally in increased reactivity during setting, due to promoting cross-linking and repolymerisation of the silicate phase, followed by clear changes in the MAS-NMR spectra.


CONCLUSIONS
The cross-linking process during the setting reaction of glass ionomer cements can be followed by MAS-NMR spectroscopy observing the conversion of Al(IV) to Al(VI). The acid base setting reaction is completed in 1 day and no further significant changes in the MAS-NMR spectra can be observed. Further study is required in order to understand the role of phosphorus.