Real-time high-resolution X-ray imaging and nuclear magnetic resonance study of the hydration of pure and Na-doped C3A in the presence of sulfates.

This study details the differences in real-time hydration between pure tricalcium aluminate (cubic C(3)A or 3CaO·Al(2)O(3)) and Na-doped tricalcium aluminate (orthorhombic C(3)A or Na(2)Ca(8)Al(6)O(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 (27)Al nuclear magnetic resonance (NMR) spectroscopy, was used to capture information regarding the mechanism of C(3)A hydration during the early stages. There are differences in the hydration mechanism between the two types of C(3)A, which are also dependent on the concentration of sulfate ions in the solution. The reactions with cubic C(3)A (pure) seem to be more influenced by higher concentrations of sulfate ions, forming smaller ettringite needles at a slower pace than the orthorhombic C(3)A (Na-doped) sample. The rate of release of aluminate species into the solution phase is also accelerated by Na doping.