Hydration characteristics and mechanism analysis of β-calcium sulfate hemihydrate based on low-field NMR

Free water plays a crucial role in the hydration process of beta-calcium sulfate hemihydrate (beta-HH) based plaster. To further understand the hydration mechanism of beta-HH, this study monitored water consumption and pore structure evolution of plaster prepared by different water/beta-HH (W/H) ratios using low-field NMR. The hydration characteristics were jointly analyzed using X-ray diffraction (XRD), thermogravimetric (TG), and isothermal calorimetry. In addition, the pore structure of the plaster was characterized by BET and MIP. Results indicated that during early hydration (0-10 min), free water migrated from macropores to micropores in beta-HH samples. The specific surface area and free water content of beta-HH based plaster decreased during rapid hydration period. It is also found that increasing the W/H ratio accelerated free water consumption but resulted in larger pore diameter. Furthermore, signal intensity and transverse relaxation time were found to be exponentially related (y = Ae((-x/B)) + y(0)) to the CaSO4 center dot 2H(2)O crystal content, hydration degree, and heat release. These correlations suggest that low-field NMR correlated well with other measurements and could be viable method to characterize the plaster hydration process.