The hydration of tricalcium aluminate, in the presence of gypsum, is investigated using in situ "real-time" H-1 NMR spin-spin relaxation, X-ray diffraction, and scanning electron microscope experiments. Aside from rapid ettringite formation within the first 45 min, it is shown that a re-distribution of water within the first 2 h contributes significantly to the retardation of the rate of hydration. From the H-1 NMR component with T-2 of approximately 20 mu s, the continual disappearance of ettringite and the production of the layered monosulfate structure is monitored. In addition, the technique makes possible the monitoring of the change in the quantity of interlayer water in monosulfate, as well as the time scale associated with the decrease in the effective of SO42- interlayer spacing, resulting from the ionic substitution for OH-. The ionic substitution within monosulfate starts after approximately 9 h. Although it has been believed that the time scale for this reaction is fast, we have shown that it can take days to reach completion.