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

Abstract

Free water plays a crucial role in the hydration process of β-calcium sulfate hemihydrate (β-HH) based plaster. To further understand the hydration mechanism of β-HH, this study monitored water consumption and pore structure evolution of plaster prepared by different water/β-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 β-HH samples. The specific surface area and free water content of β-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) + y0) to the CaSO4∙2H2O 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.