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.

Analysis of cubic and orthorhombic C3A hydration in presence of gypsum and lime.

Field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) have been used to study the microstructural changes and phase development that take place during the hydration of cubic (pure) and orthorhombic (Na-doped) tricalcium aluminate (C3A) and gypsum in the absence and presence of lime. The results demonstrate that important differences occur in the hydration of each C3A polymorph and gypsum when no lime is added; orthorhombic C3A reacts faster with gypsum than the cubic phase, forming longer ettringite needles; however, the presence of lime slows down the formation of ettringite in the orthorhombic sample. Additional rheometric tests showed the possible effects on the setting time in these cementitious mixes.