<p>Paste and mortar specimens were prepared with sulfoaluminate cement (SAC), P·O 42.5 ordinary Portland cement (OPC), and standard sand, and mixed and cured with pure water and artificial seawater, respectively. The mechanical properties of mortar specimens were tested. Hydration and microstructure of paste specimens were also investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), and ²⁷Al nuclear magnetic resonance (NMR), respectively. The results indicate that SAC mortar samples mixed and cured by seawater have faster strength growth before 28 d and higher compressive strength than OPC mortar samples. Compared to curing in deionized water, the hydration products of SAC are somewhat coarser when cured in simulated seawater. The evolution of aluminum phase hydration products during the hydration process of SAC mixed and cured in simulated seawater is quite different from that of OPC. From 3 to 28 d, the content of each aluminum phase hydration product in SAC paste cured in simulated seawater changed little, while that in OPC paste changed significantly; for example, from 7 to 28 d, the content of ettringite (AFt) in OPC paste increased significantly. This type of AFt formed loosely, harming the mortar’s microstructure.</p>