Real-time high-resolution X-ray imaging and nuclear magnetic resonance study of the hydration of pure and Na-doped C_3A in the presence of sulfates

摘要

This study details the differences in real-time hydration between pure tricalcium aluminate (cubic C_3A or _3CaO·Al_2O _3) and Na-doped tricalcium aluminate (orthorhombic C_3A or Na_2Ca_8Al_6O_(18)), in aqueous solutions containing sulfate ions. Pure phases were synthesized in the laboratory to develop an independent benchmark for the reactions, meaning that their reactions during hydration in a simulated early age cement pore solution (saturated with respect to gypsum and lime) were able to be isolated. Because the rate of this reaction is extremely rapid, most microscopy methods are not adequate to study the early phases of the reactions in the early stages. Here, a high-resolution full-field soft X-ray imaging technique operating in the X-ray water window, combined with solution analysis by 27Al nuclear magnetic resonance (NMR) spectroscopy, was used to capture information regarding the mechanism of C_3A hydration during the early stages. There are differences in the hydration mechanism between the two types of C_3A, which are also dependent on the concentration of sulfate ions in the solution. The reactions with cubic C_3A (pure) seem to be more influenced by higher concentrations of sulfate ions, forming smaller ettringite needles at a slower pace than the orthorhombic C_3A (Na-doped) sample. The rate of release of aluminate species into the solution phase is also accelerated by Na doping.